1HTML::Mason::Devel(3) User Contributed Perl DocumentationHTML::Mason::Devel(3)
2
6 HTML::Mason::Devel - Mason Developer's Manual
7
9 This manual is written for content developers who know HTML and at
10 least a little Perl. The goal is to write, run, and debug Mason
11 components.
12 If you are the webmaster (or otherwise responsible for the Mason
14 installation), you should also read the administrator's manual. There
15 you will find information about site configuration, performance tuning,
16 component caching, and so on.
17 If you are a developer just interested in knowing more about Mason's
19 capabilities and implementation, then the administrator's manual is for
20 you too.
21 We strongly suggest that you have a working Mason to play with as you
23 work through these examples. Other component examples can be found in
24 the "samples/" directory.
25 While Mason can be used for tasks besides implementing a dynamic web
27 site, that is what most people want to do with Mason, and is thus the
28 focus of this manual.
29 If you are planning to use Mason outside of the web, this manual will
31 still be useful, of course. Also make sure to read the running outside
32 of mod_perl section of the administrator's manual.
33
35 If you are just learning Mason and want to get started quickly, we
36 recommend the following sections:
37 o What Are Components?
39 o In-Line Perl Sections
41 o Calling Components
43 o Top-Level Components
45 o Passing Parameters
47 o Initialization and Cleanup (mainly "<%init>")
49 o Web-Specific Features
51 o Common Traps
53
55 The component - a mix of Perl and HTML - is Mason's basic building
56 block and computational unit. Under Mason, web pages are formed by
57 combining the output from multiple components. An article page for a
58 news publication, for example, might call separate components for the
59 company masthead, ad banner, left table of contents, and article body.
60 Consider this layout sketch:
61 +---------+------------------+
63 |Masthead | Banner Ad |
64 +---------+------------------+
65 | | |
66 |+-------+|Text of Article ..|
67 || || |
68 ||Related||Text of Article ..|
69 ||Stories|| |
70 || ||Text of Article ..|
71 |+-------+| |
72 | +------------------+
73 | | Footer |
74 +---------+------------------+
75 The top level component decides the overall page layout, perhaps with
77 HTML tables. Individual cells are then filled by the output of
78 subordinate components, one for the Masthead, one for the Footer, etc.
79 In practice pages are built up from as few as one, to as many as twenty
80 or more components.
81 This component approach reaps many benefits in a web environment. The
83 first benefit is consistency: by embedding standard design elements in
84 components, you ensure a consistent look and make it possible to update
85 the entire site with just a few edits. The second benefit is
86 concurrency: in a multi-person environment, one person can edit the
87 masthead while another edits the table of contents. A last benefit is
88 reuseability: a component produced for one site might be useful on
89 another. You can develop a library of generally useful components to
90 employ on your sites and to share with others.
91 Most components emit chunks of HTML. "Top level" components, invoked
93 from a URL, represent an entire web page. Other, subordinate components
94 emit smaller bits of HTML destined for inclusion in top level
95 components.
96 Components receive form and query data from HTTP requests. When called
98 from another component, they can accept arbitrary parameter lists just
99 like a subroutine, and optionally return values. This enables a type
100 of component that does not print any HTML, but simply serves as a
101 function, computing and returning a result.
102 Mason actually compiles components down to Perl subroutines, so you can
104 debug and profile component-based web pages with standard Perl tools
105 that understand the subroutine concept, e.g. you can use the Perl
106 debugger to step through components, and Devel::DProf to profile their
107 performance.
108
110 Here is a simple component example:
111 <%perl>
113 my $noun = 'World';
114 my @time = localtime;
115 </%perl>
116 Hello <% $noun %>,
117 % if ( $time[2] < 12 ) {
118 good morning.
119 % } else {
120 good afternoon.
121 % }
122 After 12 pm, the output of this component is:
124 Hello World, good afternoon.
126 This short example demonstrates the three primary "in-line" Perl
128 sections. In-line sections are generally embedded within HTML and
129 execute in the order they appear. Other sections ("<%init>", "<%args>",
130 etc.) are tied to component events like initialization, cleanup, and
131 argument definition.
132 The parsing rules for these Perl sections are as follows:
134 1. Blocks of the form <% xxx %> are replaced with the result of
136 evaluating xxx as a single Perl expression. These are often used
137 for variable replacement. such as 'Hello, <% $name %>!'.
138 2. Lines beginning with a '%' character are treated as Perl.
140 3. Multiline blocks of Perl code can be inserted with the "<%perl>" ..
142 "</%perl>" tag. The enclosed text is executed as Perl and the
143 return value, if any, is discarded.
144 The "<%perl>" tag, like all block tags in Mason, is case-
146 insensitive. It may appear anywhere in the text, and may span any
147 number of lines.
148 Examples and Recommended Usage
150 % lines
151 Most useful for conditional and loop structures - if, while, foreach, ,
153 etc. - as well as side-effect commands like assignments. To improve
154 readability, always put a space after the '%'. Examples:
155 o Conditional code
157 % my $ua = $r->header_in('User-Agent');
159 % if ($ua =~ /msie/i) {
160 Welcome, Internet Explorer users
161 ...
162 % } elsif ($ua =~ /mozilla/i) {
163 Welcome, Netscape users
164 ...
165 % }
166 o HTML list formed from array
168 <ul>
170 % foreach $item (@list) {
171 <li><% $item %></li>
172 % }
173 </ul>
174 o HTML list formed from hash
176 <ul>
178 % while (my ($key,$value) = each(%ENV)) {
179 <li>
180 <b><% $key %></b>: <% $value %>
181 </li>
182 % }
183 </ul>
184 o HTML table formed from list of hashes
186 <table>
188 % foreach my $h (@loh) {
189 <tr>
190 <td><% $h->{foo} %></td>
191 <td bgcolor=#ee0000><% $h->{bar} %></td>
192 <td><% $h->{baz} %></td>
193 </tr>
194 % }
195 </table>
196 <% xxx %>
198 Most useful for printing out variables, as well as more complex
200 expressions. To improve readability, always separate the tag and
201 expression with spaces. Examples:
202 Dear <% $name %>: We will come to your house at <% $address %> in the
204 fair city of <% $city %> to deliver your $<% $amount %> dollar prize!
205 The answer is <% ($y+8) % 2 %>.
207 You are <% $age < 18 ? 'not' : '' %> permitted to enter this site.
209 <%perl> xxx </%perl>
211 Useful for Perl blocks of more than a few lines.
213
215 This section describes the various objects in the Mason universe. If
216 you're just starting out, all you need to worry about initially are the
217 request objects.
218 Request Objects
220 Two global per-request objects are available to all components: $r and
221 $m.
222 $r, the mod_perl request object, provides a Perl API to the current
224 Apache request. It is fully described in Apache.pod. Here is a
225 sampling of methods useful to component developers:
226 $r->uri # the HTTP request URI
228 $r->header_in(..) # get the named HTTP header line
229 $r->content_type # set or retrieve content-type
230 $r->header_out(..) # set or retrieve an outgoing header
231 $r->content # don't use this one! (see Tips and Traps)
233 $m, the Mason request object, provides an analogous API for Mason.
235 Almost all Mason features not activated by syntactic tags are accessed
236 via $m methods. You'll be introduced to these methods throughout this
237 document as they are needed. For a description of all methods see
238 HTML::Mason::Request.
239 Because these are always set inside components, you should not ever
241 define other variables with the same name, or else your code may fail
242 in strange and mysterious ways.
243 Component Objects
245 Mason provides an object API for components, allowing you to query a
246 component's various asociated files, arguments, etc. For a description
247 of all methods see HTML::Mason::Component. Typically you get a handle
248 on a component object from request methods like "$m->current_comp" and
249 "$m->fetch_comp".
250 Note that for many basic applications all you'll want to do with
252 components is call them, for which no object method is needed. See next
253 section.
254 System Objects
256 Many system objects share the work of serving requests in Mason:
257 HTML::Mason::Lexer, HTML::Mason::Compiler, HTML::Mason::Interp,
258 HTML::Mason::Resolver, and HTML::Mason::ApacheHandler are examples. The
259 administrator creates these objects and provides parameters that shape
260 Mason's behavior. As a pure component developer you shouldn't need to
261 worry about or access these objects, but occasionally we'll mention a
262 relevant parameter.
263
265 Mason pages often are built not from a single component, but from
266 multiple components that call each other in a hierarchical fashion.
267 Components that output HTML
269 To call one component from another, use the <& &> tag:
270 <& comp_path, [name=>value, ...] &>
272 comp_path:
274 The component path. With a leading '/', the path is relative to the
275 component root (comp_root). Otherwise, it is relative to the
276 location of the calling component.
277 name => value pairs:
279 Parameters are passed as one or more "name => value" pairs, e.g.
280 "player => 'M. Jordan'".
281 comp_path may be a literal string (quotes optional) or a Perl
283 expression that evaluates to a string. To eliminate the need for quotes
284 in most cases, Mason employs some magic parsing: If the first character
285 is one of "[\w/_.]", comp_path is assumed to be a literal string
286 running up to the first comma or &>. Otherwise, comp_path is evaluated
287 as an expression.
288 Here are some examples:
290 # relative component paths
292 <& topimage &>
293 <& tools/searchbox &>
294 # absolute component path
296 <& /shared/masthead, color=>'salmon' &>
297 # this component path MUST have quotes because it contains a comma
299 <& "sugar,eggs", mix=>1 &>
300 # variable component path
302 <& $comp &>
303 # variable component and arguments
305 <& $comp, %args &>
306 # you can use arbitrary expression for component path, but it cannot
308 # begin with a letter or number; delimit with () to remedy this
309 <& (int(rand(2)) ? 'thiscomp' : 'thatcomp'), id=>123 &>
310 Several request methods also exist for calling components. "$m->comp"
312 performs the equivalent action to <& &>:
313 $m->comp('/shared/masthead', color=>'salmon');
315 "$m->scomp" is like the sprintf version of "$m->comp": it returns the
317 component output, allowing the caller to examine and modify it before
318 printing:
319 my $masthead = $m->scomp('/shared/masthead', color=>'salmon');
321 $masthead =~ ...;
322 $m->print($masthead);
323 Component Calls with Content
325 Components can be used to filter part of the page's content using an
326 extended component syntax.
327 <&| /path/to/comp &> this is the content </&>
329 <&| comp, arg1 => 'hi' &> filters can take arguments </&>
330 <&| comp &> content can include <% "tags" %> of all kinds </&>
331 <&| comp1 &> nesting is also <&| comp2 &> OK </&> </&>
332 <&| SELF:method1 &> subcomponents can be filters </&>
333 The filtering component can be called in all the same ways a normal
335 component is called, with arguments and so forth. The only difference
336 between a filtering component and a normal component is that a
337 filtering component is expected to fetch the content by calling
338 $m->content and do something with it.
339 The ending tag may optionally contain the name of the component, and
341 Mason will verify that it matches the name in the starting tag. This
342 may be helpful when the tags are far apart or nested. To avoid
343 ambiguous situations, this is only allowed when the component name is
344 an unquoted literal (starting with "[\w/_.]"). For anything more
345 complicated, such as "<|& $var &>" or "<&| 'name' &>", the simple
346 "</&>" form must be used.
347 <&| "outer" &>
349 <&| /inner/comp, arg=>'this' &>
350 <&| .mycomp &>
351 Yada yada yada
352 </& .mycomp >
353 </& /inner/comp >
354 </&>
355 Here is an example of a component used for localization. Its content
357 is a series of strings in different languages, and it selects the
358 correct one based on a global $lang variable, which could be setup in a
359 site-level autohandler.
360 <&| /i18n/itext &>
362 <en>Hello, <% $name %> This is a string in English</en>
363 <de>Schoene Gruesse, <% $name %>, diese Worte sind auf Deutsch</de>
364 <pig>ellohay <% substr($name,2).substr($name,1,1).'ay' %>,
365 isthay isay igpay atinlay</pig>
366 </&>
367 Here is the /i18n/itext component:
369 <% $text %>
371 <%init>
373 # this assumes $lang is a global variable which has been set up earlier.
374 local $_ = $m->content;
375 my ($text) = m{<$lang>(.*?)</$lang>};
376 </%init>
377 You can explicitly check whether a component has passed content by
379 checking the boolean "$m->has_content". This allows you to write a
380 component that will do different things depending on whether it was
381 passed content. However, before overloading a component in this way,
382 consider whether splitting the behavior into two distinct components
383 would work as well.
384 If a normal component which does not call "$m->content" is called with
386 content, the content will not be output.
387 If you wrap a filtering component call around the entire component, the
389 result will be functionally similar to a "<%filter>" section. See also
390 Filtering.
391 Advanced Components Calls with Content
393 Internally "$m->content" is implemented with a closure containing the
394 part of the component which is the content. In English, that means
395 that any mason tags and perl code in the content are evaluated when
396 "$m->content" is called, and "$m->content" returns the text which would
397 have been output by mason. Because the contents are evaluated at the
398 time that "$m->content" is called, one can write components which act
399 as control structures or which output their contents multiple times
400 with different values for the variables (can you say taglibs?).
401 The tricky part of using filter components as control structures is
403 setting up variables which can be accessed from both the filter
404 component and the content, which is in the component which calls the
405 filter component. The content has access to all variables in the
406 surrounding component, but the filtering component does not. There are
407 two ways to do this: use global variables, or pass a reference to a
408 lexical variable to the filter component.
409 Here is a simple example using the second method:
411 % my $var;
413 <ol>
414 <&| list_items , list => \@items, var => \$var &>
415 <li> <% $var %></li>
416 </&>
417 </ol>
418 list_items component:
420 <%args>
422 @list
423 $var
424 </%args>
425 % foreach (@list) {
426 % $$var = $_; # $var is a reference
427 <% $m->content %>
428 % }
429 Using global variables can be somewhat simpler. Below is the same
431 example, with $var defined as a global variable. The site
432 administrator must make sure that $var is included in Mason's
433 allow_globals parameter. Local-izing $var within the filter component
434 will allow the list_items component to be nested.
435 <ol>
437 <&| list_items, list => \@items &>
438 <li> <% $var %></li>
439 </&>
440 </ol>
441 list_items component:
443 <%args>
445 @list
446 </%args>
447 % foreach (@list) {
448 % local $var = $_;
449 <% $m->content %>
450 % }
451 Besides remembering to include $var in allow_globals, the developers
453 should take care not to use that variable is other places where it
454 might conflict with usage by the filter component. Local-izing $var
455 will also provide some protection against using it in other places.
456 An even simpler method is to use the $_ variable. It is already
458 global, and is automatically local-ized by the foreach statement:
459 <ol>
461 <&| list_items, list => \@items &>
462 <li> <% $_ %> </li>
463 </&>
464 </ol>
465 list_items component:
467 <%args>
469 @list
470 </%args>
471 % foreach (@list) {
472 <% $m->content %>
473 % }
474 Components that Return Values
476 So far you have seen components used solely to output HTML. However,
477 components may also be used to return values.
478 While we will demonstrate how this is done, we strongly encourage you
480 to put code like this in modules instead. There are several reasons
481 why this is a good idea:
482 · You can re-use this code outside of Mason.
484 · It is easy to preload module code when running under mod_perl,
486 which can lower memory usage.
487 · Using Mason components as subroutines is slower than just using
489 modules to do the same thing.
490 · It's easier to regression test module code.
492 With that being said, there are times when you may want to write a
494 component which returns a value.
495 As an example, you might have a component "is_netscape" that analyzes
497 the user agent to determine whether it is a Netscape browser:
498 <%init>
500 my $ua = $r->header_in('User-Agent');
501 return ($ua =~ /Mozilla/i && $ua !~ /MSIE/i) ? 1 : 0;
502 </%init>
503 Because components are implemented underneath with Perl subroutines,
505 they can return values and even understand scalar/list context. e.g.
506 The result of wantarray() inside a component will reflect whether the
507 component was called in scalar or list context.
508 The <& &> notation only calls a component for its side-effect, and
510 discards its return value, if any. To get at the return value of a
511 component, use the "$m->comp" command:
512 % if ($m->comp('is_netscape')) {
514 Welcome, Netscape user!
515 % }
516 Mason adds a "return undef" to the bottom of each component to provide
518 an empty default return value. To return your own value from a
519 component, you must use an explicit "return" statement. You cannot rely
520 on the usual Perl trick of letting return values "fall through".
521 While it is possible for a component to generate output and return
523 values, there is very little reason for a component to do both. For
524 example, it would not be very friendly for "is_netscape" to output "hi
525 Mom" while it was computing its value, thereby surprising the "if"
526 statement! Conversely, any value returned by an output generating
527 component would typically be discarded by the <& &> tag that invoked
528 it.
529 Subrequests
531 You may sometimes want to have a component call go through all the
532 steps that the initial component call goes through, such as checking
533 for autohandlers and dhandlers. To do this, you need to execute a
534 subrequest.
535 A subrequest is simply a Mason Request object and has all of the
537 methods normally associated with one.
538 To create a subrequest you simply use the "$m->make_subrequest" method.
540 This method can take any parameters belonging to HTML::Mason::Request,
541 such as autoflush or out_method. Once you have a new request object
542 you simply call its "exec" method to execute it, which takes exactly
543 the same parameters as the "comp" method.
544 Since subrequests inherit their parent request's parameters, output
546 from a component called via a subrequest goes to the same desintation
547 as output from components called during the parent request. Of course,
548 you can change this.
549 Here are some examples:
551 <%perl>
553 my $req = $m->make_subrequest( comp => '/some/comp', args => [ id => 172 ] );
554 $req->exec;
555 </%perl>
556 If you want to capture the subrequest's output in a scalar, you can
558 simply pass an out_method parameter to "$m->make_subrequest":
559 <%perl>
561 my $buffer;
562 my $req =
563 $m->make_subrequest
564 ( comp => '/some/comp', args => [ id => 172 ], out_method => \$buffer );
565 $req->exec;
566 </%perl>
567 Now $buffer contains all the output from that call to /some/comp.
569 For convenience, Mason also provides an "$m->subexec" method. This
571 method takes the same arguments as "$m->comp" and internally calls
572 "$m->make_subrequest" and then "exec" on the created request, all in
573 one fell swoop. This is useful in cases where you have no need to
574 override any of the parent request object's attributes.
575 By default, output from a subrequest appears inline in the calling
577 component, at the point where it is executed. If you wish to do
578 something else, you will need to explicitly override the subrequest's
579 out_method parameter.
580 Mason Request objects are only designed to handle a single call to
582 "exec". If you wish to make multiple subrequests, you must create a
583 new subrequest object for each one.
584
586 The first component invoked for a page (the "top-level component")
587 resides within the DocumentRoot and is chosen based on the URL. For
588 example:
589 http://www.foo.com/mktg/prods.html?id=372
591 Mason converts this URL to a filename, e.g.
593 /usr/local/www/htdocs/mktg/prods.html. Mason loads and executes that
594 file as a component. In effect, Mason calls
595 $m->comp('/mktg/prods.html', id=>372)
597 This component might in turn call other components and execute some
599 Perl code, or it might contain nothing more than static HTML.
600 dhandlers
602 What happens when a user requests a component that doesn't exist? In
603 this case Mason scans backward through the URI, checking each directory
604 for a component named dhandler ("default handler"). If found, the
605 dhandler is invoked and is expected to use "$m->dhandler_arg" as the
606 parameter to some access function, perhaps a database lookup or
607 location in another filesystem. In a sense, dhandlers are similar in
608 spirit to Perl's AUTOLOAD feature; they are the "component of last
609 resort" when a URL points to a non-existent component.
610 Consider the following URL, in which newsfeeds/ exists but not the
612 subdirectory LocalNews nor the component Story1:
613 http://myserver/newsfeeds/LocalNews/Story1
615 In this case Mason constructs the following search path:
617 /newsfeeds/LocalNews/Story1 => no such thing
619 /newsfeeds/LocalNews/dhandler => no such thing
620 /newsfeeds/dhandler => found! (search ends)
621 /dhandler
622 The found dhandler would read "LocalNews/Story1" from
624 "$m->dhandler_arg" and use it as a retrieval key into a database of
625 stories.
626 Here's how a simple /newsfeeds/dhandler might look:
628 <& header &>
630 <b><% $headline %></b><p>
631 <% $body %>
632 <& footer &>
633 <%init>
635 my $arg = $m->dhandler_arg; # get rest of path
636 my ($section, $story) = split("/", $arg); # split out pieces
637 my $sth = $DBH->prepare
638 (qq{SELECT headline,body FROM news
639 WHERE section = ? AND story = ?);
640 $sth->execute($section, $story);
641 my ($headline, $body) = $sth->fetchrow_array;
642 return 404 if !$headline; # return "not found" if no such story
643 </%init>
644 By default dhandlers do not get a chance to handle requests to a
646 directory itself (e.g. /newsfeeds). These are automatically deferred to
647 Apache, which generates an index page or a FORBIDDEN error. Often this
648 is desirable, but if necessary the administrator can let in directory
649 requests as well; see the allowing directory requests section of the
650 administrator's manual.
651 A component or dhandler that does not want to handle a particular
653 request may defer control to the next dhandler by calling
654 "$m->decline".
655 When using dhandlers under mod_perl, you may find that sometimes Apache
657 will not set a content type for a response. This usually happens when
658 a dhandler handles a request for a non-existent file or directory. You
659 can add a "<Location>" or "<LocationMatch>" block containing a
660 "SetType" directive to your Apache config file, or you can just set the
661 content type dynamically by calling "$r->content_type".
662 The administrator can customize the file name used for dhandlers with
664 the dhandler_name parameter.
665 autohandlers
667 Autohandlers allow you to grab control and perform some action just
668 before Mason calls the top-level component. This might mean adding a
669 standard header and footer, applying an output filter, or setting up
670 global variables.
671 Autohandlers are directory based. When Mason determines the top-level
673 component, it checks that directory and all parent directories for a
674 component called autohandler. If found, the autohandler is called
675 first. After performing its actions, the autohandler typically calls
676 "$m->call_next" to transfer control to the original intended component.
677 "$m->call_next" works just like "$m->comp" except that the component
679 path and arguments are implicit. You can pass additional arguments to
680 "$m->call_next"; these are merged with the original arguments, taking
681 precedence in case of conflict. This allows you, for example, to
682 override arguments passed in the URL.
683 Here is an autohandler that adds a common header and footer to each
685 page underneath its directory:
686 <html>
688 <head><title>McHuffy Incorporated</title></head>
689 <body style="background-color: pink">
690 % $m->call_next;
692 <hr />
694 Copyright 1999 McHuffy Inc.
695 </body>
696 </html>
697 Same idea, using components for the header/footer:
699 <& /shared/header &>
701 % $m->call_next;
702 <& /shared/footer &>
703 The next autohandler applies a filter to its pages, adding an absolute
705 hostname to relative image URLs:
706 % $m->call_next;
708 <%filter>
710 s{(<img[^>]+src=\")/} {$1http://images.mysite.com/}ig;
711 </%filter>
712 Most of the time autohandler can simply call "$m->call_next" without
714 needing to know what the next component is. However, should you need
715 it, the component object is available from "$m->fetch_next". This is
716 useful for calling the component manually, e.g. if you want to suppress
717 some original arguments or if you want to use "$m->scomp" to store and
718 process the output.
719 If more than one autohandler applies to a page, each autohandler gets a
721 chance to run. The top-most autohandler runs first; each
722 "$m->call_next" transfers control to the next autohandler and finally
723 to the originally called component. This allows you, for example, to
724 combine general site-wide templates and more specific section-based
725 templates.
726 Autohandlers can be made even more powerful in conjunction with Mason's
728 object-oriented style features: methods, attributes, and inheritance.
729 In the interest of space these are discussed in a separate section,
730 Object-Oriented Techniques.
731 The administrator can customize the file name used for autohandlers
733 with the autohandler_name parameter.
734 dhandlers vs. autohandlers
736 dhandlers and autohandlers both provide a way to exert control over a
737 large set of URLs. However, each specializes in a very different
738 application. The key difference is that dhandlers are invoked only
739 when no appropriate component exists, while autohandlers are invoked
740 only in conjunction with a matching component.
741 As a rule of thumb: use an autohandler when you have a set of
743 components to handle your pages and you want to augment them with a
744 template/filter. Use a dhandler when you want to create a set of
745 "virtual URLs" that don't correspond to any actual components, or to
746 provide default behavior for a directory.
747 dhandlers and autohandlers can even be used in the same directory. For
749 example, you might have a mix of real URLs and virtual URLs to which
750 you would like to apply a common template/filter.
751
753 This section describes Mason's facilities for passing parameters to
754 components (either from HTTP requests or component calls) and for
755 accessing parameter values inside components.
756 In Component Calls
758 Any Perl data type can be passed in a component call:
759 <& /sales/header, s => 'dog', l => [2, 3, 4], h => {a => 7, b => 8} &>
761 This command passes a scalar ($s), a list (@l), and a hash (%h). The
763 list and hash must be passed as references, but they will be
764 automatically dereferenced in the called component.
765 In HTTP requests
767 Consider a CGI-style URL with a query string:
768 http://www.foo.com/mktg/prods.html?str=dog&lst=2&lst=3&lst=4
770 or an HTTP request with some POST content. Mason automatically parses
772 the GET/POST values and makes them available to the component as
773 parameters.
774 Accessing Parameters
776 Component parameters, whether they come from GET/POST or another
777 component, can be accessed in two ways.
778 1. Declared named arguments: Components can define an "<%args>"
780 section listing argument names, types, and default values. For example:
781 <%args>
783 $a
784 @b # a comment
785 %c
786 # another comment
788 $d => 5
789 $e => $d*2
790 @f => ('foo', 'baz')
791 %g => (joe => 1, bob => 2)
792 </%args>
793 Here, $a, @b, and %c are required arguments; the component generates an
795 error if the caller leaves them unspecified. $d, $e, @f and %g are
796 optional arguments; they are assigned the specified default values if
797 unspecified. All the arguments are available as lexically scoped
798 ("my") variables in the rest of the component.
799 Arguments are separated by one or more newlines. Comments may be used
801 at the end of a line or on their own line.
802 Default expressions are evaluated in top-to-bottom order, and one
804 expression may reference an earlier one (as $e references $d above).
805 Only valid Perl variable names may be used in "<%args>" sections.
807 Parameters with non-valid variable names cannot be pre-declared and
808 must be fetched manually out of the %ARGS hash (see below). One common
809 example of undeclarable parameters are the "button.x/button.y"
810 parameters sent for a form submit.
811 2. %ARGS hash: This variable, always available, contains all of the
813 parameters passed to the component (whether or not they were declared).
814 It is especially handy for dealing with large numbers of parameters,
815 dynamically named parameters, or parameters with non-valid variable
816 names. %ARGS can be used with or without an "<%args>" section, and its
817 contents are unrelated to what you have declared in "<%args>".
818 Here's how to pass all of a component's parameters to another
820 component:
821 <& template, %ARGS &>
823 Parameter Passing Examples
825 The following examples illustrate the different ways to pass and
826 receive parameters.
827 1. Passing a scalar id with value 5.
829 In a URL: /my/URL?id=5
831 In a component call: <& /my/comp, id => 5 &>
832 In the called component, if there is a declared argument named...
833 $id, then $id will equal 5
834 @id, then @id will equal (5)
835 %id, then an error occurs
836 In addition, $ARGS{id} will equal 5.
837 2. Passing a list colors with values red, blue, and green.
839 In a URL: /my/URL?colors=red&colors=blue&colors=green
841 In an component call: <& /my/comp, colors => ['red', 'blue', 'green'] &>
842 In the called component, if there is a declared argument named...
843 $colors, then $colors will equal ['red', 'blue', 'green']
844 @colors, then @colors will equal ('red', 'blue', 'green')
845 %colors, then an error occurs
846 In addition, $ARGS{colors} will equal ['red', 'blue', 'green'].
847 3. Passing a hash grades with pairs Alice => 92 and Bob => 87.
849 In a URL: /my/URL?grades=Alice&grades=92&grades=Bob&grades=87
851 In an component call: <& /my/comp, grades => {Alice => 92, Bob => 87} &>
852 In the called component, if there is a declared argument named...
853 @grades, then @grades will equal ('Alice', 92, 'Bob', 87)
854 %grades, then %grades will equal (Alice => 92, Bob => 87)
855 In addition, $grade and $ARGS{grades} will equal
856 ['Alice',92,'Bob',87] in the URL case, or {Alice => 92, Bob => 87}
857 in the component call case. (The discrepancy exists because, in a
858 query string, there is no detectable difference between a list or
859 hash.)
860 Using @_ instead
862 If you don't like named parameters, you can pass a traditional list of
863 ordered parameters:
864 <& /mktg/prods.html', 'dog', [2, 3, 4], {a => 7, b => 8} &>
866 and access them as usual through Perl's @_ array:
868 my ($scalar, $listref, $hashref) = @_;
870 In this case no "<%args>" section is necessary.
872 We generally recommend named parameters for the benefits of
874 readability, syntax checking, and default value automation. However
875 using @_ may be convenient for very small components, especially
876 subcomponents created with "<%def>".
877 Before Mason 1.21, @_ contained copies of the caller's arguments. In
879 Mason 1.21 and beyond, this unnecessary copying was eliminated and @_
880 now contains aliases to the caller's arguments, just as with regular
881 Perl subroutines. For example, if a component updates $_[0], the
882 corresponding argument is updated (or an error occurs if it is not
883 updatable).
884 Most users won't notice this change because "<%args>" variables and the
886 %ARGS hash always contain copies of arguments.
887 See perlsub for more information on @_ aliasing.
889
891 The following sections contain blocks of Perl to execute at specific
892 times.
893 <%init>
895 This section contains initialization code that executes as soon as the
896 component is called. For example: checking that a user is logged in;
897 selecting rows from a database into a list; parsing the contents of a
898 file into a data structure.
899 Technically an "<%init>" block is equivalent to a "<%perl>" block at
901 the beginning of the component. However, there is an aesthetic
902 advantage of placing this block at the end of the component rather than
903 the beginning.
904 We've found that the most readable components (especially for non-
906 programmers) contain HTML in one continuous block at the top, with
907 simple substitutions for dynamic elements but no distracting blocks of
908 Perl code. At the bottom an "<%init>" block sets up the substitution
909 variables. This organization allows non-programmers to work with the
910 HTML without getting distracted or discouraged by Perl code. For
911 example:
912 <html>
914 <head><title><% $headline %></title></head>
915 <body>
916 <h2><% $headline %></h2>
917 <p>By <% $author %>, <% $date %></p>
918 <% $body %>
920 </body>
922 </html>
923 <%init>
925 # Fetch article from database
926 my $dbh = DBI::connect ...;
927 my $sth = $dbh->prepare("select * from articles where id = ?");
928 $sth->execute($article_id);
929 my ($headline, $date, $author, $body) = $sth->fetchrow_array;
930 # Massage the fields
931 $headline = uc($headline);
932 my ($year, $month, $day) = split('-', $date);
933 $date = "$month/$day";
934 </%init>
935 <%args>
937 $article_id
938 </%args>
939 <%cleanup>
941 This section contains cleanup code that executes just before the
942 component exits. For example: closing a database connection or closing
943 a file handle.
944 A "<%cleanup>" block is equivalent to a "<%perl>" block at the end of
946 the component. This means it will NOT execute if the component
947 explicitly returns, or if an abort or error occurs in that component or
948 one of its children. Because of this limitation, and because Perl is
949 usually so good about cleaning up at the end of a lexical scope (e.g.
950 component), "<%cleanup>" sections are rarely needed.
951 If you need code that is guaranteed to run when the component or
953 request exits, consider using a mod_perl cleanup handler, or creating a
954 custom class with a DESTROY method.
955 <%once>
957 This code executes once when the component is loaded. Variables
958 declared in this section can be seen in all of a component's code and
959 persist for the lifetime of the component.
960 This section is useful for declaring persistent component-scoped
962 lexical variables (especially objects that are expensive to create),
963 declaring subroutines (both named and anonymous), and initializing
964 state.
965 This code does not run inside a request context. You cannot call
967 components or access $m or $r from this section. Also, do not attempt
968 to "return()" from a "<%once>" section; the current compiler cannot
969 properly handle it.
970 Normally this code will execute individually from every HTTP child that
972 uses the component. However, if the component is preloaded, this code
973 will only execute once in the parent. Unless you have total control
974 over what components will be preloaded, it is safest to avoid
975 initializing variables that can't survive a fork(), e.g. DBI handles.
976 Use code like this to initialize such variables in the "<%init>"
977 section:
978 <%once>
980 my $dbh; # declare but don't assign
981 ...
982 </%once>
983 <%init>
985 $dbh ||= DBI::connect ...
986 ...
987 </%init>
988 In addition, using $m or $r in this section will not work in a
990 preloaded component, because neither of those variable exist when a
991 component is preloaded.
992 <%shared>
994 As with "<%once>", lexical ("my") variables declared in this section
995 can be seen in all the rest of a component's code: the main body,
996 subcomponents, and methods. However, unlike "<%once>", the code runs
997 once per request (whenever the component is used) and its variables
998 last only until the end of the request.
999 A "<%shared>" section is useful for initializing variables needed in,
1001 say, the main body and one more subcomponents or methods. See Object-
1002 Oriented Techniques for an example of usage.
1003 It's important to realize that you do not have access to the %ARGS hash
1005 or variables created via an "<%args>" block inside a shared section.
1006 However, you can access arguments via $m->request_args.
1007 Additionally, you cannot call a components' own methods or
1009 subcomponents from inside a "<%shared>", though you can call other
1010 components.
1011 Avoid using "<%shared>" for side-effect code that needs to run at a
1013 predictable time during page generation. You may assume only that
1014 "<%shared>" runs just before the first code that needs it and runs at
1015 most once per request.
1016 In the current implementation, the scope sharing is done with closures,
1018 so variables will only be shared if they are visible at compile-time in
1019 the other parts of the component. In addition, you can't rely on the
1020 specific destruction time of the shared variables, because they may not
1021 be destroyed until the first time the "<%shared>" section executes in a
1022 future request. "<%init>" offers a more predictable execution and
1023 destruction time.
1024 Currently any component with a "<%shared>" section incurs an extra
1026 performance penalty, because Mason must recreate its anonymous
1027 subroutines the first time each new request uses the component. The
1028 exact penalty varies between systems and for most applications will be
1029 unnoticeable. However, one should avoid using "<%shared>" when patently
1030 unnecessary, e.g. when an "<%init>" would work just as well.
1031 Do not attempt to "return()" from a "<%shared>" section; the current
1033 compiler cannot properly handle it.
1034
1036 <%def name>
1037 Each instance of this section creates a subcomponent embedded inside
1038 the current component. Inside you may place anything that a regular
1039 component contains, with the exception of "<%def>", "<%method>",
1040 "<%once>", and "<%shared>" tags.
1041 The name consists of characters in the set "[\w._-]". To call a
1043 subcomponent simply use its name in <& &> or "$m->comp". A subcomponent
1044 can only be seen from the surrounding component.
1045 If you define a subcomponent with the same name as a file-based
1047 component in the current directory, the subcomponent takes precedence.
1048 You would need to use an absolute path to call the file-based
1049 component. To avoid this situation and for general clarity, we
1050 recommend that you pick a unique way to name all of your subcomponents
1051 that is unlikely to interfere with file-based components. A commonly
1052 accepted practice is to start subcomponent names with ".".
1053 While inside a subcomponent, you may use absolute or relative paths to
1055 call file-based components and also call any of your "sibling"
1056 subcomponents.
1057 The lexical scope of a subcomponent is separate from the main
1059 component. However a subcomponent can declare its own "<%args>"
1060 section and have relevant values passed in. You can also use a
1061 "<%shared>" section to declare variables visible from both scopes.
1062 In the following example, we create a ".link" subcomponent to produce a
1064 standardized hyperlink:
1065 <%def .link>
1067 <a href="http://www.<% $site %>.com"><% $label %></a>
1068 <%args>
1070 $site
1071 $label=>ucfirst($site)
1072 </%args>
1073 </%def>
1074 Visit these sites:
1076 <ul>
1077 <li><& .link, site=>'yahoo' &></li>
1078 <li><& .link, site=>'cmp', label=>'CMP Media' &></li>
1079 <li><& .link, site=>'excite' &></li>
1080 </ul>
1081 <%method name>
1083 Each instance of this section creates a method embedded inside the
1084 current component. Methods resemble subcomponents in terms of naming,
1085 contents, and scope. However, while subcomponents can only be seen from
1086 the parent component, methods are meant to be called from other
1087 components.
1088 There are two ways to call a method. First, via a path of the form
1090 "comp:method":
1091 <& /foo/bar:method1 &>
1093 $m->comp('/foo/bar:method1');
1095 Second, via the call_method component method:
1097 my $comp = $m->fetch_comp('/foo/bar');
1099 ...
1100 $comp->call_method('method1');
1101 Methods are commonly used in conjunction with autohandlers to make
1103 templates more flexible. See Object-Oriented Techniques for more
1104 information.
1105 You cannot create a subcomponent and method with the same name. This
1107 is mostly to prevent obfuscation and accidental errors.
1108
1110 The "<%flags>" and "<%attr>" sections consist of key/value pairs, one
1111 per line, joined by '=>'. In each pair, the key must be any valid Perl
1112 "bareword identifier" (made of letters, numbers, and the underscore
1113 character), and the value may be any scalar value, including
1114 references. An optional comment may follow each line.
1115 <%flags>
1117 Use this section to set official Mason flags that affect the current
1118 component's behavior.
1119 Currently there is only one flag, "inherit", which specifies the
1121 component's parent in the form of a relative or absolute component
1122 path. A component inherits methods and attributes from its parent; see
1123 Object-Oriented Techniques for examples.
1124 <%flags>
1126 inherit=>'/site_handler'
1127 </%flags>
1128 <%attr>
1130 Use this section to assign static key/value attributes that can be
1131 queried from other components.
1132 <%attr>
1134 color => 'blue'
1135 fonts => [qw(arial geneva helvetica)]
1136 </%attr>
1137 To query an attribute of a component, use the "attr" method:
1139 my $color = $comp->attr('color')
1141 where $comp is a component object.
1143 Mason evaluates attribute values once when loading the component. This
1145 makes them faster but less flexible than methods.
1146
1148 This section describes several ways to apply filtering functions over
1149 the results of the current component. By separating out and hiding a
1150 filter that, say, changes HTML in a complex way, we allow non-
1151 programmers to work in a cleaner HTML environment.
1152 <%filter> section
1154 The "<%filter>" section allows you to arbitrarily filter the output of
1155 the current component. Upon entry to this code, $_ contains the
1156 component output, and you are expected to modify it in place. The code
1157 has access to component arguments and can invoke subroutines, call
1158 other components, etc.
1159 This simple filter converts the component output to UPPERCASE:
1161 <%filter>
1163 tr/a-z/A-Z/
1164 </%filter>
1165 The following navigation bar uses a filter to "unlink" and highlight
1167 the item corresponding to the current page:
1168 <a href="/">Home</a> | <a href="/products/">Products</a> |
1170 <a href="/bg.html">Background</a> | <a href="/finance/">Financials</a> |
1171 <a href="/support/">Tech Support</a> | <a href="/contact.html">Contact Us</a>
1172 <%filter>
1174 my $uri = $r->uri;
1175 s{<a href="$uri/?">(.*?)</a>} {<b>$1</b>}i;
1176 </%filter>
1177 This allows a designer to code such a navigation bar intuitively
1179 without "if" statements surrounding each link! Note that the regular
1180 expression need not be very robust as long as you have control over
1181 what will appear in the body.
1182 A filter block does not have access to variables declared in a
1184 component's "<%init>" section, though variables declared in the
1185 "<%args>", "<%once>" or "<%shared>" blocks are usable in a filter.
1186 It should be noted that a filter cannot rely on receiving all of a
1188 component's output at once, and so may be called multiple times with
1189 different chunks of output. This can happen if autoflush is on, or if
1190 a filter-containing component, or the components it calls, call the
1191 "$m->flush_buffer()" method.
1192 You should never call Perl's "return()" function inside a filter
1194 section, or you will not see any output at all.
1195 You can use Component Calls with Content if you want to filter specific
1197 parts of a component rather than the entire component.
1198
1200 There are several ways to place comments in components, i.e. arbitrary
1201 text that is ignored by the parser.
1202 <%doc>
1204 Text in this section is treated as a comment and ignored. Most useful
1205 for a component's main documentation. One can easily write a program
1206 to sift through a set of components and pull out their "<%doc>" blocks
1207 to form a reference page.
1208 <% # comment... %>
1210 A "<% %>" tag is considered a comment if all of its lines are either
1211 whitespace, or begin with a '#' optionally preceded by whitespace. For
1212 example,
1213 <% # This is a single-line comment %>
1215 <%
1217 # This is a
1218 # multi-line comment
1219 %>
1220 %# comment
1222 Because a line beginning with "%" is treated as Perl, "%#"
1223 automatically works as a comment. However we prefer the "<% # comment
1224 %>" form over "%#", because it stands out a little more as a comment
1225 and because it is more flexible with regards to preceding whitespace.
1226 % if (0) { }
1228 Anything between these two lines
1229 % if (0) {
1231 ...
1232 % }
1233 will be skipped by Mason, including component calls. While we don't
1235 recomend this for comments per se, it is a useful notation for
1236 "commenting out" code that you don't want to run.
1237 HTML/XML/... comments
1239 HTML and other markup languages will have their own comment markers,
1240 for example "<!-- -->". Note two important differences with these
1241 comments versus the above comments:
1242 · They will be sent to the client and appear in the source of the
1244 page.
1245 · They do not block component calls and other code from running, so
1247 don't try to use them to comment out code!
1248 <!-- Oops, the code below will still run
1250 <& /shared/expensive.mhtml &>
1251 -->
1252
1254 <%text>
1255 Text in this section is printed as-is with all Mason syntax ignored.
1256 This is useful, for example, when documenting Mason itself from a
1257 component:
1258 <%text>
1260 % This is an example of a Perl line.
1261 <% This is an example of an expression block. %>
1262 </%text>
1263 This works for almost everything, but doesn't let you output "</%text>"
1265 itself! When all else fails, use "$m->print":
1266 % $m->print('The tags are <%text> and </%text>.');
1268 Escaping expressions
1270 Mason has facilities for escaping the output from "<% %>" tags, on
1271 either a site-wide or a per-expression basis.
1272 Any "<% %>" expression may be terminated by a '|' and one or more
1274 escape flags (plus arbitrary whitespace), separated by commas:
1275 <% $file_data |h %>
1277 The current valid flags are:
1279 · h
1281 Escape HTML ('<' => '<', etc.) using "HTML::Entities::encode()".
1283 Before Perl 5.8.0 this module assumes that text is in the
1284 ISO-8859-1 character set; see the next section for how to override
1285 this escaping. After 5.8.0, the encoding assumes that text is in
1286 Unicode.
1287 · u
1289 Escape a URL query string (':' => '%3A', etc.) - all but
1291 [a-zA-Z0-9_.-]
1292 · n
1294 This is a special flag indicating that the default escape flags
1296 should not be used for this substitution.
1297 The administrator may specify a set of default escape flags via the
1299 default_escape_flags parameter. For example, if the administrator sets
1300 default_escape_flags to "['h']", then all <% %> expressions will
1301 automatically be HTML-escaped. In this case you would use the "n" flag
1302 to turn off HTML-escaping for a specific expression:
1303 <% $html_block |n %>
1305 Multiple escapes can be specified as a comma-separated list:
1307 <% $uri | u, n %>
1309 The old pre-defined escapes, 'h', 'u', and 'n', can be used without
1311 commas, so that this is legal:
1312 <% $uri | un %>
1314 However, this only works for these three escapes, and no others. If
1316 you are using user-defined escapes as well, you must use a comma:
1317 <% $uri | u, add_session %>
1319 User-defined Escapes
1321 Besides the default escapes mentioned above, it is possible for the
1323 user to define their own escapes or to override the built-in 'h' and
1324 'u' escapes.
1325 This is done via the Interp object's escape_flags parameter or
1327 set_escape() method. Escape names may be any number of characters as
1328 long as it matches the regex "/^[\w-]+$/". The one exception is that
1329 you cannot override the 'n' flag.
1330 Each escape flag is associated with a subroutine reference. The
1332 subroutine should expect to receive a scalar reference, which should be
1333 manipulated in place. Any return value from this subroutine is
1334 ignored.
1335 Escapes can be defined at any time but using an escape that is not
1337 defined will cause an error when executing that component.
1338 A common use for this feature is to override the built-in HTML
1340 escaping, which will not work with non-ISO-8559-1 encodings. If you
1341 are using such an encoding and want to switch the 'h' flag to do escape
1342 just the minimal set of characters ("<", ">", "&", """), put this in
1343 your Apache configuration:
1344 PerlSetVar MasonEscapeFlags "h => \&HTML::Mason::Escapes::basic_html_escape"
1346 Or, in a top-level autohandler:
1348 $m->interp->set_escape( h => \&HTML::Mason::Escapes::basic_html_escape );
1350 Or you could write your own escape function for a particular encoding:
1352 $ah->interp->set_escape( h => \&my_html_escape );
1354 And of course this can be used for all sorts of other things, like a
1356 naughty words filter for the easily offended:
1357 $interp->set_escape( 'no-naughty' => \&remove_naughty_words );
1359 Manually applying escapes
1361 You can manually apply one or more escapes to text using the Interp
1363 object's "apply_escapes()" method. e.g.
1364 $m->interp->apply_escapes( 'some html content', 'h' );
1366 Backslash at end of line
1368 A backslash (\) at the end of a line suppresses the newline. In HTML
1369 components, this is mostly useful for fixed width areas like "<pre>"
1370 tags, since browsers ignore white space for the most part. An example:
1371 <pre>
1373 foo
1374 % if (1) {
1375 bar
1376 % }
1377 baz
1378 </pre>
1379 outputs
1381 foo
1383 bar
1384 baz
1385 because of the newlines on lines 2 and 4. (Lines 3 and 5 do not
1387 generate a newline because the entire line is taken by Perl.) To
1388 suppress the newlines:
1389 <pre>
1391 foo\
1392 % if (1) {
1393 bar\
1394 % }
1395 baz
1396 </pre>
1397 which prints
1399 foobarbaz
1401
1403 Mason's data caching interface allows components to cache the results
1404 of computation for improved performance. Anything may be cached, from
1405 a block of HTML to a complex data structure.
1406 Each component gets its own private, persistent data cache. Except
1408 under special circumstances, one component does not access another
1409 component's cache. Each cached value may be set to expire at a certain
1410 time.
1411 Data caching is implemented on top of one of two external caching APIs:
1413 "Cache::Cache" or its newer sibling "CHI". You control which one Mason
1414 uses with the data_cache_api parameter. Currently "Cache::Cache" is the
1415 default, but we will switch to "CHI" once it is more mature. The APIs
1416 are very similar for Mason users, so that most of the information below
1417 applies to both; any differences are noted.
1418 Basic Usage
1420 The "$m->cache" method returns a cache object representing the cache
1421 for this component. Here's the typical usage of "$m->cache":
1422 my $result = $m->cache->get('key');
1424 if (!defined($result)) {
1425 ... compute $result ...
1426 $m->cache->set('key', $result);
1427 }
1428 "$m->cache->get" attempts to retrieve this component's cache value. If
1430 the value is available it is placed in $result. If the value is not
1431 available, $result is computed and stored in the cache by
1432 "$m->cache->set".
1433 Multiple Keys/Values
1435 A cache can store multiple key/value pairs. A value can be anything
1436 serializable by "Storable", from a simple scalar to an arbitrary
1437 complex list or hash reference:
1438 $m->cache->set(name => $string);
1440 $m->cache->set(friends => \@list);
1441 $m->cache->set(map => \%hash);
1442 You can fetch all the keys in a cache with
1444 my @idents = $m->cache->get_keys;
1446 It should be noted that Mason reserves all keys beginning with
1448 "__mason" for its own use.
1449 Expiration
1451 You can pass an optional third argument to "$m->cache->set" indicating
1452 when the item should expire:
1453 $m->cache->set('name1', $string1, '5 min'); # Expire in 5 minutes
1455 $m->cache->set('name2', $string2, '3h'); # Expire in 3 hours
1456 To change the expiration time for a piece of data, call "set" again
1458 with the new expiration. To expire an item immediately, use
1459 "$m->cache->remove".
1460 You can also specify an expiration condition when you fetch the item,
1462 using the expire_if option:
1463 my $result = $m->cache->get('key',
1465 expire_if=>sub { $_[0]->get_created_at < (stat($file))[9] });
1466 expire_if takes an anonymous subroutine, which is called with the cache
1468 object as its only parameter. If the subroutine returns a true value,
1469 the item is expired. In the example above, we expire the item whenever
1470 a certain file changes.
1471 Finally, you can expire a cache item from an external script; see
1473 Accessing a Cache Externally below.
1474 Avoiding Concurrent Recomputation
1476 The code shown in "Basic Usage" above,
1477 my $result = $m->cache->get('key');
1479 if (!defined($result)) {
1480 ... compute $result ...
1481 $m->cache->set('key', $result);
1482 }
1483 can suffer from a kind of race condition for caches that are accessed
1485 frequently and take a long time to recompute.
1486 Suppose that a particular cache value is accessed five times a second
1488 and takes three seconds to recompute. When the cache expires, the
1489 first process comes in, sees that it is expired, and starts to
1490 recompute the value. The second process comes in and does the same
1491 thing. This sequence continues until the first process finishes and
1492 stores the new value. On average, the value will be recomputed and
1493 written to the cache 15 times!
1494 One solution is the busy_lock flag:
1496 my $result = $m->cache->get('key', busy_lock=>'30 sec');
1498 In this case, when the value cannot be retrieved, "get()" sets the
1500 expiration time of the value 30 seconds in the future before returning
1501 "undef". This tells the first process to compute the new value while
1502 causing subsequent processes to use the old value for 30 seconds.
1503 Should the 30 seconds expire before the first process is done, a second
1505 process will start computing the new value while setting the expiration
1506 time yet another 30 seconds in the future, and so on.
1507 The disadvantage of this solution is that multiple writes to the cache
1509 will be performed for each "set()".
1510 Another solution, available only if you are using "CHI", is
1512 "expires_variance" which will create a variable time window during
1513 which expiration may occur. See the "CHI" documentation for details.
1514 Caching All Output
1516 Occasionally you will need to cache the complete output of a component.
1517 For this purpose, Mason offers the "$m->cache_self" method. This
1518 method causes Mason to check to see if this component has already been
1519 run and its output cached. If this is the case, this output is simply
1520 sent as output. Otherwise, the component run normally and its output
1521 and return value cached.
1522 It is typically used right at the top of an "<%init>" section:
1524 <%init>
1526 return if $m->cache_self(key => 'fookey', expires_in => '3 hours',
1527 ... <other cache options> ...);
1528 ... <rest of init> ...
1529 </%init>
1530 A full list of parameters and examples are available in the cache_self
1532 section of the Request manual.
1533 Cache Object
1535 "$m->cache->get_object" returns a "Cache::Object" or "CHI::CacheObject"
1536 associated with a particular key. You can use this to retrieve useful
1537 meta-data:
1538 my $co = $m->cache->get_object('name1');
1540 $co->get_created_at(); # when was object stored in cache
1541 $co->get_expires_at(); # when does object expire
1542 Choosing a Cache Subclass - with Cache::Cache
1544 The "Cache::Cache" API is implemented by a variety of backend
1545 subclasses. For example, "FileCache" implements the interface with a
1546 set of directories and files, "MemoryCache" implements the interface in
1547 process memory, and "SharedMemoryCache" implements the interface in
1548 shared memory.
1549 By default "$m->cache" uses "FileCache", but you can override this with
1551 the cache_class keyword. The value must be the name of a "Cache::Cache"
1552 subclass; the prefix "Cache::" need not be included. For example:
1553 my $result = $m->cache(cache_class => 'MemoryCache')->get('key');
1555 $m->cache(cache_class => 'MemoryCache')->set(key => $result);
1556 You can even specify different subclasses for different keys in the
1558 same component. Just make sure the correct value is passed to all calls
1559 to "$m->cache"; Mason does not remember which subclass you have used
1560 for a given component or key.
1561 The administrator can set the default cache subclass used by all
1563 components with the data_cache_defaults parameter.
1564 Choosing a Cache Subclass - with CHI
1566 The "CHI" API is implemented by a variety of drivers, for example
1567 "CHI::Driver::File", "CHI::Driver::FastMmap", and
1568 "CHI::Driver::Memcached".
1569 "CHI::Driver::File" is the default, but you can override this with the
1571 driver keyword. The value must be the name of a "CHI::Driver" subclass;
1572 the prefix "CHI::Driver::" need not be included. For example:
1573 my $cache = $m->cache(driver => 'Memcached', servers => [ ... ]);
1575 my $result = $cache->get('key');
1576 $cache->set(key => $result);
1577 You can even specify different subclasses for different keys in the
1579 same component. Just make sure the correct value is passed to all calls
1580 to "$m->cache"; Mason does not remember which subclass you have used
1581 for a given component or key.
1582 The administrator can set the default cache subclass used by all
1584 components with the data_cache_defaults parameter.
1585 Accessing a Cache Externally
1587 To access a component's cache from outside the component (e.g. in an
1588 external Perl script), you'll need have the following information:
1589 · the namespace associated with the component. For "Cache::Cache",
1591 the function "HTML::Mason::Utils::data_cache_namespace", given a
1592 component id (usually just the component path), returns the
1593 namespace. For "CHI", the component id/path itself is the
1594 namespace.
1595 · the cache_root, for file-based caches only. Defaults to the "cache"
1597 subdirectory under the Mason data directory.
1598 Given this information you can get a handle on the component's cache.
1600 For example, the following code removes a cache item for component
1601 /foo/bar, assuming the data directory is /usr/local/www/mason and you
1602 are using the default file backend:
1603 use HTML::Mason::Utils qw(data_cache_namespace);
1605 # With Cache::Cache
1607 my $cache = new Cache::FileCache
1608 ( { namespace => data_cache_namespace("/foo/bar"),
1609 cache_root => "/usr/local/www/mason/cache" } );
1610 # With CHI
1612 my $cache = CHI->new
1613 ( driver => 'File',
1614 namespace => "/foo/bar",
1615 cache_root => "/usr/local/www/mason/cache" );
1616 # Remove one key
1618 $cache->remove('key1');
1619 # Remove all keys
1621 $cache->clear;
1622 Mason 1.0x Cache API
1624 For users upgrading from 1.0x and earlier, any existing $m->cache code
1625 will be incompatible with the new API. However, if you wish to continue
1626 using the 1.0x cache API for a while, you (or your administrator) can
1627 set data_cache_api to '1.0'. All of the $m->cache options with the
1628 exception of "tie_class" should be supported.
1629 The "access_data_cache" function is no longer available; this will need
1631 to be converted to use "Cache::Cache" directly, as described in the
1632 previous section.
1633
1635 Sending HTTP Headers
1636 Mason automatically sends HTTP headers via "$r->send_http_header" but
1637 it will not send headers if they've already been sent manually.
1638 To determine the exact header behavior on your system, you need to know
1640 whether your server's default is to have autoflush on or off. Your
1641 administrator should have this information. If your administrator
1642 doesn't know then it is probably off, the default.
1643 With autoflush off the header situation is extremely simple: Mason
1645 waits until the very end of the request to send headers. Any component
1646 can modify or augment the headers.
1647 With autoflush on the header situation is more complex. Mason will
1649 send headers just before sending the first output. This means that if
1650 you want to affect the headers with autoflush on, you must do so before
1651 any component sends any output. Generally this takes place in an
1652 "<%init>" section.
1653 For example, the following top-level component calls another component
1655 to see whether the user has a cookie; if not, it inserts a new cookie
1656 into the header.
1657 <%init>
1659 my $cookie = $m->comp('/shared/get_user_cookie');
1660 if (!$cookie) {
1661 $cookie = new CGI::Cookie (...);
1662 $r->header_out('Set-cookie' => $cookie);
1663 }
1664 ...
1665 </%init>
1666 With autoflush off this code will always work. Turn autoflush on and
1668 this code will only work as long as /shared/get_user_cookie doesn't
1669 output anything (given its functional nature, it shouldn't).
1670 The administrator can turn off automatic header sending via the
1672 auto_send_headers parameter. You can also turn it off on individual
1673 pages with
1674 $m->auto_send_headers(0);
1676 Returning HTTP Status
1678 The value returned from the top-most component becomes the status code
1679 of the request. If no value is explicitly returned, it defaults to OK
1680 (0).
1681 Simply returning an error status (such as 404) from the top-most
1683 component has two problems in practice. First, the decision to return
1684 an error status often resides further down in the component stack.
1685 Second, you may have generated some content by the time this decision
1686 is made. (Both of these are more likely to be true when using
1687 autohandlers.)
1688 Thus the safer way to generate an error status is
1690 $m->clear_buffer;
1692 $m->abort($status);
1693 "$m->abort" bypasses the component stack and ensures that $status is
1695 returned from the top-most component. It works by throwing an
1696 exception. If you wrapped this code (directly or indirectly) in an
1697 eval, you must take care to rethrow the exception, or the status will
1698 not make it out:
1699 eval { $m->comp('...') };
1701 if (my $err = $@) {
1702 if ($m->aborted) {
1703 die $err;
1704 } else {
1705 # deal with non-abort exceptions
1706 }
1707 }
1708 Filters and $m->abort
1710 A filter section will still be called after a component aborts with
1712 "$m->abort". You can always check "$m->aborted" in your "<%filter>"
1713 block if you don't want to run the filter after an abort.
1714 <%filter>
1716 unless ( $m->aborted ) {
1717 $_ .= ' filter stuff';
1718 }
1719 </%filter>
1720 External Redirects
1722 Because it is so commonly needed, Mason 1.1x and on provides an
1723 external redirect method:
1724 $m->redirect($url); # Redirects with 302 status
1726 This method uses the clear_buffer/abort technique mentioned above, so
1728 the same warnings apply regarding evals.
1729 Also, if you generate any output after calling "$m->redirect", then
1731 this output will be sent, and will break the redirect. For example:
1732 % eval { $m->comp('redirect', ...) };
1734 % die $@ if $@;
1736 The blank line between the two Perl lines is new output generated after
1738 the redirect. Either remove it or call "$m->clear_buffer" immediately
1739 before calling "die()".
1740 Internal Redirects
1742 There are two ways to perform redirects that are invisible to the
1743 client.
1744 First, you can use a Mason subrequest (see "Subrequests"). This only
1746 works if you are redirecting to another Mason page.
1747 Second, you can use Apache's internal_redirect method, which works
1749 whether or not the new URL will be handled by Mason. Use it this way:
1750 $r->internal_redirect($url);
1752 $m->auto_send_headers(0);
1753 $m->clear_buffer;
1754 $m->abort;
1755 The last three lines prevent the original request from accidentally
1757 generating extra headers or content.
1758
1760 You can use the perl debugger in conjunction with a live mod_perl/Mason
1761 server with the help of Apache::DB, available from CPAN. Refer to the
1762 Apache::DB documentation for details.
1763 The only tricky thing about debugging Mason pages is that components
1765 are implemented by anonymous subroutines, which are not easily
1766 breakpoint'able. To remedy this, Mason calls the dummy subroutine
1767 "debug_hook" at the beginning of each component. You can breakpoint
1768 this subroutine like so:
1769 b HTML::Mason::Request::debug_hook
1771 debug_hook is called with two parameters: the current Request object
1773 and the full component path. Thus you can breakpoint specific
1774 components using a conditional on $_[1]:
1775 b HTML::Mason::Request::debug_hook $_[1] =~ /component name/
1777 You can avoid all that typing by adding the following to your ~/.perldb
1779 file:
1780 # Perl debugger aliases for Mason
1782 $DB::alias{mb} = 's/^mb\b/b HTML::Mason::Request::debug_hook/';
1783 which reduces the previous examples to just:
1785 mb
1787 mb $_[1] =~ /component name/
1788 Mason normally inserts '#line' directives into compiled components so
1790 that line numbers are reported relative to the source file. Depending
1791 on your task, this can be a help or a hindrance when using the
1792 debugger. The administrator can turn off '#line' directives with the
1793 use_source_line_numbers parameter.
1794
1796 Earlier you learned how to assign a common template to an entire
1797 hierarchy of pages using autohandlers. The basic template looks like:
1798 header HTML
1800 % $m->call_next;
1801 footer HTML
1802 However, sometimes you'll want a more flexible template that adjusts to
1804 the requested page. You might want to allow each page or subsection to
1805 specify a title, background color, or logo image while leaving the rest
1806 of the template intact. You might want some pages or subsections to use
1807 a different template, or to ignore templates entirely.
1808 These issues can be addressed with the object-oriented style primitives
1810 introduced in Mason 0.85.
1811 Note: we use the term object-oriented loosely. Mason borrows concepts
1813 like inheritance, methods, and attributes from object methodology but
1814 implements them in a shallow way to solve a particular set of problems.
1815 Future redesigns may incorporate a deeper object architecture if the
1816 current prototype proves successful.
1817 Determining inheritance
1819 Every component may have a single parent. The default parent is a
1820 component named "autohandler" in the closest parent directory. This
1821 rule applies to autohandlers too: an autohandler may not have itself as
1822 a parent but may have an autohandler further up the tree as its parent.
1823 You can use the "inherit" flag to override a component's parent:
1825 <%flags>
1827 inherit => '/foo/bar'
1828 </%flags>
1829 If you specify undef as the parent, then the component inherits from no
1831 one. This is how to suppress templates.
1832 Currently there is no way to specify a parent dynamically at run-time,
1834 or to specify multiple parents.
1835 Content wrapping
1837 At page execution time, Mason builds a chain of components from the
1838 called component, its parent, its parent's parent, and so on. Execution
1839 begins with the top-most component; calling "$m->call_next" passes
1840 control to the next component in the chain. This is the familiar
1841 autohandler "wrapping" behavior, generalized for any number of
1842 arbitrarily named templates.
1843 Accessing methods and attributes
1845 A template can access methods and/or attributes of the requested page.
1846 First, use "$m->request_comp" to get a handle on the appropriate
1847 component:
1848 my $self = $m->request_comp;
1850 $self now refers to the component corresponding to the requested page
1852 (the component at the end of the chain).
1853 To access a method for the page, use "call_method":
1855 $self->call_method('header');
1857 This looks for a method named 'header' in the page component. If no
1859 such method exists, the chain of parents is searched upwards, until
1860 ultimately a "method not found" error occurs. Use 'method_exists' to
1861 avoid this error for questionable method calls:
1862 if ($self->method_exists('header')) { ...
1864 The component returned by the "$m->request_comp" method never changes
1866 during request execution. In contrast, the component returned by
1867 "$m->base_comp" may change several times during request execution.
1868 When execution starts, the base component is the same as the requested
1870 component. Whenever a component call is executed, the base component
1871 may become the component that was called. The base component will
1872 change for all component calls except in the following cases:
1873 · A component is called via its component object rather than its
1875 path, for example:
1876 <& $m->fetch_comp('/some/comp'), foo => 1 &>
1878 · A subcomponent (defined with "<%def>") is called.
1880 · A method is called via the use of "SELF:", "PARENT:", or
1882 "REQUEST:". These are covered in more detail below.
1883 In all other cases, the base component is the called component or the
1885 called component's owner component if that called component is a
1886 method.
1887 As hinted at above, Mason provides a shortcut syntax for method calls.
1889 If a component call path starts with "SELF:", then Mason will start
1891 looking for the method (the portion of the call after "SELF:"), in the
1892 base component.
1893 <& SELF:header &>
1895 $m->comp('SELF:header')
1896 If the call path starts with "PARENT:", then Mason will start looking
1898 in the current component's parent for the named method.
1899 <& PARENT:header &>
1901 $m->comp('PARENT:header')
1902 In the context of a component path, PARENT is shorthand for
1904 "$m->current_comp->parent".
1905 If the call path begins with "REQUEST:", then Mason looks for the
1907 method in the requested component. REQUEST is shorthand for
1908 "$m->request_comp".
1909 The rules for attributes are similar. To access an attribute for the
1911 page, use "attr":
1912 my $color = $self->attr('color')
1914 This looks for an attribute named 'color' in the $self component. If no
1916 such attribute exists, the chain of parents is searched upwards, until
1917 ultimately an "attribute not found" error occurs. Use "attr_exists" or
1918 "attr_if_exist" to avoid this error for questionable attributes:
1919 if ($self->attr_exists('color')) { ...
1921 my $color = $self->attr_if_exists('color'); # if it doesn't exist $color is undef
1923 Sharing data
1925 A component's main body and its methods occupy separate lexical scopes.
1926 Variables declared, say, in the "<%init>" section of the main component
1927 cannot be seen from methods.
1928 To share variables, declare them either in the "<%once>" or "<%shared>"
1930 section. Both sections have an all-inclusive scope. The "<%once>"
1931 section runs once when the component loads; its variables are
1932 persistent for the lifetime of the component. The "<%shared>" section
1933 runs once per request (when needed), just before any code in the
1934 component runs; its variables last only til the end of the request.
1935 In the following example, various sections of code require information
1937 about the logged-in user. We use a "<%shared>" section to fetch these
1938 in a single request.
1939 <%attr>
1941 title=>sub { "Account for $full_name" }
1942 </%attr>
1943 <%method lefttoc>
1945 <i><% $full_name %></i>
1946 (<a href="logout.html">Log out</a>)<br />
1947 ...
1948 </%method>
1949 Welcome, <% $fname %>. Here are your options:
1951 <%shared>
1953 my $dbh = DBI::connect ...;
1954 my $user = $r->connection->user;
1955 my $sth = $dbh->prepare("select lname,fname, from users where user_id = ?");
1956 $sth->execute($user);
1957 my ($lname, $fname) = $sth->fetchrow_array;
1958 my $full_name = "$first $last";
1959 </%shared>
1960 "<%shared>" presents a good alternative to "<%init>" when data is
1962 needed across multiple scopes. Outside these situations, "<%init>" is
1963 preferred for its slightly greater speed and predictable execution
1964 model.
1965 Example
1967 Let's say we have three components:
1968 /autohandler
1970 /products/autohandler
1971 /products/index.html
1972 and that a request comes in for /products/index.html.
1974 /autohandler contains a general template for the site, referring to a
1976 number of standard methods and attributes for each page:
1977 <head>
1979 <title><& SELF:title &></title>
1980 </head>
1981 <body style="<% $self->attr('body_style') %>">
1982 <& SELF:header &>
1983 <div id="main">
1985 % $m->call_next;
1986 </div>
1987 <& SELF:footer &>
1989 </body>
1990 <%init>
1992 my $self = $m->base_comp;
1993 ...
1994 </%init>
1995 <%attr>
1997 body_style => 'standard'
1998 </%attr>
1999 <%method title>
2001 McGuffey Inc.
2002 </%method>
2003 <%method header>
2005 <h2><& SELF:title &></h2>
2006 </%method>
2007 <%method footer>
2009 </%method>
2010 Notice how we provide defaults for each method and attribute, even if
2012 blank.
2013 /products/autohandler overrides some attributes and methods for the
2015 /products section of the site.
2016 <%attr>
2018 body_style => 'plain'
2019 </%attr>
2020 <%method title>
2021 McGuffey Inc.: Products
2022 </%method>
2023 % $m->call_next;
2025 Note that this component, though it only defines attributes and
2027 methods, must call "$m->call_next" if it wants the rest of the chain to
2028 run.
2029 /products/index.html might override a few attributes, but mainly
2031 provides a primary section for the body.
2032
2034 Do not call $r->content or "new CGI"
2035 Mason calls "$r->content" itself to read request input, emptying
2036 the input buffer and leaving a trap for the unwary: subsequent
2037 calls to "$r->content" hang the server. This is a mod_perl
2038 "feature" that may be fixed in an upcoming release.
2039 For the same reason you should not create a CGI object like
2041 my $query = new CGI;
2043 when handling a POST; the CGI module will try to reread request
2045 input and hang. Instead, create an empty object:
2046 my $query = new CGI ("");
2048 such an object can still be used for all of CGI's useful HTML
2050 output functions. Or, if you really want to use CGI's input
2051 functions, initialize the object from %ARGS:
2052 my $query = new CGI (\%ARGS);
2054
2056 Modules which work as source filters, such as "Switch.pm", will only
2057 work when you are using object files. This is because of how source
2058 filters are implemented, and cannot be changed by the Mason authors.
2059
2061 Jonathan Swartz <swartz@pobox.com>, Dave Rolsky <autarch@urth.org>, Ken
2062 Williams <ken@mathforum.org>
2063
2065 HTML::Mason, HTML::Mason::Admin, HTML::Mason::Request
2066perl v5.12.0 2010-05-03 HTML::Mason::Devel(3)