1Dancer2::Tutorial(3) User Contributed Perl Documentation Dancer2::Tutorial(3)
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6 Dancer2::Tutorial - An example to get you dancing
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9 version 0.300004
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12 This tutorial is has three parts. Since they build on one another, each
13 part is meant to be gone through in sequential order.
14 Part I, the longest part of this tutorial, will focus on the basics of
16 Dancer2 development by building a simple yet functional blog app,
17 called "dancr", that you can use to impress your friends, mates, and
18 family.
19 In Part II, you'll learn about the preferred way to get your own web
21 apps up and running by using the "dancer2" utility. We will take the
22 script written in Part I and convert it into a proper Dancer2 app,
23 called "Dancr2", to help you gain an understanding of what the
24 "dancer2" utility does for you.
25 Finally, in Part III, we give you a taste of the power of plugins that
27 other developers have written and will show you how to modify the
28 "Dancr2" app to use a database plugin.
29 This tutorial assumes you have some familiarity with Perl and that you
31 know how to create and execute a Perl script on your computer. Some
32 experience with web development is also greatly helpful but not
33 entirely necessary. This tutorial is mostly geared toward developers
34 but website designers can get something out of it as well since the
35 basics of templating are covered plus it might be good for a designer
36 to have a decent idea of how Dancer2 works.
37
39 Part I covers many of the basic concepts you'll need to know to lay a
40 good foundation for your future development work with Dancer2 by
41 building a simple micro-blogging app.
42 What is Dancer2?
44 Dancer2 is a micro-web framework, written in the Perl programming
45 language, and is modeled after a Ruby web application framework called
46 Sinatra <http://www.sinatrarb.com>.
47 When we say "micro" framework, we mean that Dancer2 aims to maximize
49 your freedom and control by getting out of your way. "Micro" doesn't
50 mean Dancer2 is only good for creating small apps. Instead, it means
51 that Dancer2's primary focus is on taking care of a lot of the boring,
52 technical details of your app for you and by creating an easy, clean
53 routing layer on top of your app's code. It also means you have almost
54 total control over the app's functionality and how you create and
55 present your content. You will not confined to someone else's approach
56 to creating a website or app.
57 With Dancer2, you can build anything from a specialized content
59 management system to providing a simple API for querying a database
60 over the web. But you don't have to reinvent the wheel, either. Dancer2
61 has hundreds of plugins that you can take advantage of. You can add
62 only the capabilities your app needs to keep complexity to a minimum.
63 As a framework, Dancer2 provides you with the tools and infrastructure
65 you can leverage to deliver content on the web quickly, easily and
66 securely. The tools, Dancer2 provides, called "keywords," are commands
67 that you use to build your app, access the data inside of it, and
68 deliver it on the internet in many different formats.
69 Dancer2's keywords provide what is called a Domain Specific Language
71 (DSL) designed specifically for the task of building apps. But don't
72 let the technical jargon scare you off. Things will become clearer in
73 our first code example which we will look at shortly.
74 Getting Dancer2 installed
76 First, we need to make sure you have Dancer2 installed. Typically, you
78 will do that with one of the following two commands:
79 cpan Dancer2 # requires the cpan command to be installed and configured
81 cpanm Dancer2 # requires you have cpanminus installed
82 If you aren't familiar with installing Perl modules on your machine,
84 you should read this guide <https://www.cpan.org/modules/INSTALL.html>.
85 You may also want to consult your OS's documentation or a knowledgeable
86 expert. And, of course, your search engine of choice is always there
87 for you, as well.
88 Your first Dancer2 "Hello World!" app
90 Now that you have Dancer2 installed, open up your favorite text editor
92 and copy and paste the following lines of Perl code into it and save it
93 to a file called "dancr.pl":
94 #!/usr/bin/env perl
96 use Dancer2;
97 get '/' => sub {
99 return 'Hello World!';
100 };
101 start;
103 If you make this script executable and run it, it will fire up a
105 simple, standalone web server that will display "Hello World!" when you
106 point your browser to <http://localhost:3000>. Cool!
107 Important note: We want to emphasize that writing a script file like
109 this with a "start" command is not how you would typically begin
110 writing a Dancer2 app. Part II of this tutorial will show you the
111 recommended approach using the "dancer2" utility. For now, we want to
112 stay focused on the fundamentals.
113 So, though our example app is very simple, there is a lot going on
115 under the hood when we invoke "use Dancer2;" in our first line of code.
116 We won't go into the gory details of how it all works. For now, it's
117 enough for you to know that the Dancer2 module infuses your script with
118 the ability to use Dancer2 keywords for building apps. Getting
119 comfortable with the concept of keywords is probably the most important
120 step you can take as a budding Dancer2 developer and this tutorial will
121 do its best to help foster your understanding of them.
122 The next line of code in our example (which spans three lines to make
124 it more readable) is the route handler. Let's examine this line
125 closely, because route handlers are at the core of how to build an app
126 with Dancer2.
127 The syntax of a Dancer2 "route handler" has three parts:
129 · an http method or http verb; in this example, we use the "get"
131 keyword to tell Dancer2 that this route should apply to GET http
132 requests. "get" is the first of many keywords that Dancer2 provides
133 that we will cover in this tutorial. Those familiar with web
134 development will know that a GET request is what we use to fetch
135 information from a website.
136 · the route pattern; this is the bit of code that appears immediately
138 after our "get" keyword. In this example it is a forward slash
139 ("/"), wrapped in single quotes, and it represents the pattern we
140 wish to match against the URL that the browser, or client, has
141 requested. Web developers will immediately recognize that the
142 forward slash symbolizes the root directory of our website.
143 Experienced Perl programmers will pick up on the fact that the
144 route pattern is nothing more than an argument for our "get"
145 keyword.
146 · the route action; this is the subroutine that returns our data.
148 More precisely, it is a subroutine reference. The route action in
149 our example returns a simple string, "Hello World!". Like the route
150 pattern, the route action is nothing more than an argument to our
151 "get" keyword.
152 Note that convention has us use the fat comma ("=>") operator
154 between the route pattern and the action to to make our code more
155 readable. But we could just as well have used a regular old comma
156 to separate these argument to our "get" method. Gotta love Perl for
157 its flexibility.
158 So to put our route pattern in the example into plain English, we are
160 telling our app, "If the root directory is requested with the GET http
161 method, send the string 'Hello World!' back in our response." Of
162 course, since this is a web app, we also have to send back headers with
163 our response. This is quitely taken care of for us by Dancer2 so we
164 don't have to think about it.
165 The syntax for route handlers might seem a bit foreign for newer Perl
167 developers. But rest assured there is nothing magical about it and it
168 is all just plain old Perl under the hood. If you keep in mind that the
169 keyword is a subroutine (or more precisely, a method) and that the
170 pattern and action are arguments to the keyword, you'll pick it up in
171 no time. Thinking of these keywords as "built-ins" to the Dancer2
172 framework might also eliminate any initial confusion about them.
173 The most important takeaway here is that we build our app by adding
175 route handlers which are nothing more than a collection of, HTTP verbs,
176 URL patterns, and actions.
177 How about a little more involved example?
179 While investigating some Python web frameworks like Flask
180 <http://flask.pocoo.org/> or Bottle <https://bottlepy.org/docs/dev/>, I
181 enjoyed the way they explained step-by-step how to build an example
182 application which was a little more involved than a trivial example.
183 This tutorial is modeled after them.
184 Using the Flaskr <https://github.com/pallets/flask> sample application
186 as my inspiration (OK, shamelessly plagiarised) I translated that
187 application to the Dancer2 framework so I could better understand how
188 Dancer2 worked. (I'm learning it too!)
189 So "dancr" was born.
191 dancr is a simple "micro" blog which uses the SQLite
193 <http://www.sqlite.org> database engine for simplicity's sake. You'll
194 need to install sqlite on your server if you don't have it installed
195 already. Consult your OS documentation for getting SQLite installed on
196 your machine.
197 Required Perl modules
199 Obviously you need Dancer2 installed. You'll also need the Template
201 Toolkit, File::Slurper, and DBD::SQLite modules. These all can be
202 installed using your CPAN client with the following command:
203 cpan Template File::Slurper DBD::SQLite
205 The database code
207 We're not going to spend a lot of time on the database, as it's not
208 really the point of this particular tutorial. Try not to dwell on this
209 section too much if you don't understand all of it.
210 Open your favorite text editor <http://www.vim.org> and create a schema
212 definition called 'schema.sql' with the following content:
213 create table if not exists entries (
215 id integer primary key autoincrement,
216 title string not null,
217 text string not null
218 );
219 Here we have a single table with three columns: id, title, and text.
221 The 'id' field is the primary key and will automatically get an ID
222 assigned by the database engine when a row is inserted.
223 We want our application to initialize the database automatically for us
225 when we start it. So, let's edit the 'dancr.pl' file we created earlier
226 and give it the ability to talk to our database with the following
227 subroutines: (Or, if you prefer, you can copy and paste the finished
228 dancr.pl script, found near the end of Part I in this tutorial, into
229 the file all at once and then just follow along with the tutorial.)
230 sub connect_db {
232 my $dbh = DBI->connect("dbi:SQLite:dbname=".setting('database'))
233 or die $DBI::errstr;
234 return $dbh;
236 }
237 sub init_db {
239 my $db = connect_db();
240 my $schema = read_text('./schema.sql');
241 $db->do($schema)
242 or die $db->errstr;
243 }
244 Nothing too fancy in here, I hope. It's standard DBI except for the
246 "setting('database')" thing, more on that in a bit. For now, just
247 assume that the expression evaluates to the location of the database
248 file.
249 In Part III of the tutorial, we will show you how to use the
251 Dancer2::Plugin::Database module for an easier way to configure and
252 manage database connections for your Dancer2 apps.
253 Our first route handler
255 Ok, let's get back to the business of learning Dancer2 by creating our
256 app's first route handler for the root URL. Replace the route handler
257 in our simple example above with this one:
258 get '/' => sub {
260 my $db = connect_db();
261 my $sql = 'select id, title, text from entries order by id desc';
262 my $sth = $db->prepare($sql)
264 or die $db->errstr;
265 $sth->execute
267 or die $sth->errstr;
268 template 'show_entries.tt', {
270 msg => get_flash(),
271 add_entry_url => uri_for('/add'),
272 entries => $sth->fetchall_hashref('id'),
273 };
274 };
275 Our new route handler is the same as the one in our first example
277 except that our route action does a lot more work.
278 Something you might not have noticed right away is the semicolon at the
280 end of the route handler. This might confuse newer Perl coders and is a
281 source of bugs for more experienced ones who forget to add it. We need
282 the semicolon there because we are creating a reference to a subroutine
283 and because that's just what the Perl compiler demands and we must obey
284 if we want our code to run.
285 Alright, let's take a closer look at this route's action. The first few
287 lines are standard DBI. The important bit related to Dancer2 is the
288 "template" keyword at the end of the action. That tells Dancer2 to
289 process the output through one of its templating engines. There are
290 many template engines available for use with Dancer2. In this tutorial,
291 we're using Template Toolkit which offers a lot more flexibility than
292 the simple default Dancer2 template engine.
293 Templates all go into a "views/" directory which located in the same
295 directory as our dancr.pl script. Optionally, you can create a "layout"
296 template which provides a consistent look and feel for all of your
297 views. We'll construct our own layout template, cleverly named main.tt,
298 a little later in this tutorial.
299 So what's going on with the hashref as the second argument to the
301 template directive? Those are all of the parameters we want to pass
302 into our template. We have a "msg" field which displays a message to
303 the user when an event happens like a new entry is posted, or the user
304 logs in or out. It's called a "flash" message because we only want to
305 display it one time, not every time the "/" URL is rendered.
306 The "uri_for" directive tells Dancer2 to provide a URI for that
308 specific route, in this case, it is the route to post a new entry into
309 the database. You might ask why we don't simply hardcode the "/add"
310 URI in our application or templates. The best reason not to do that is
311 because it removes a layer of flexibility as to where to "mount" the
312 web application. Although the application is coded to use the root URL
313 "/" it might be better in the future to locate it under its own URL
314 route (maybe "/dancr"?) - at that point we'd have to go through our
315 application and the templates and update the URLs and hope we didn't
316 miss any of them. By using the "uri_for" Dancer2 method, we can easily
317 load the application wherever we like and not have to modify the
318 application at all.
319 Finally, the "entries" field contains a hashref with the results from
321 our database query. Those results will be rendered in the template
322 itself, so we just pass them in.
323 So what does the show_entries.tt template look like? This:
325 [% IF session.logged_in %]
327 <form action="[% add_entry_url %]" method=post class=add-entry>
328 <dl>
329 <dt>Title:
330 <dd><input type=text size=30 name=title>
331 <dt>Text:
332 <dd><textarea name=text rows=5 cols=40></textarea>
333 <dd><input type=submit value=Share>
334 </dl>
335 </form>
336 [% END %]
337 <ul class=entries>
338 [% IF entries.size %]
339 [% FOREACH id IN entries.keys.nsort %]
340 <li><h2>[% entries.$id.title | html %]</h2>[% entries.$id.text | html %]
341 [% END %]
342 [% ELSE %]
343 <li><em>Unbelievable. No entries here so far</em>
344 [% END %]
345 </ul>
346 Go ahead and create a "views/" directory in the same directory as the
348 script and add this file to it.
349 Again, since this isn't a tutorial about Template Toolkit, we'll gloss
351 over the syntax here and just point out the section which starts with
352 "<ul class=entries>". This is the section where the database query
353 results are displayed. You can also see at the very top some discussion
354 about a session, more on that soon.
355 The only other Template Toolkit related thing that has to be mentioned
357 here is the "| html" in "[% entries.$id.title | html %]". That's a
358 filter <http://www.template-
359 toolkit.org/docs/manual/Filters.html#section_html> to convert
360 characters like "<" and ">" to "<" and ">". This way they will be
361 displayed by the browser as content on the page rather than just
362 included. If we did not do this, the browser might interpret content as
363 part of the page, and a malicious user could smuggle in all kinds of
364 bad code that would then run in another user's browser. This is called
365 Cross Site Scripting <https://en.wikipedia.org/wiki/Cross-
366 site_scripting> or XSS and you should make sure to avoid it by always
367 filtering data that came in from the web when you display it in a
368 template.
369 Other HTTP verbs
371 There are 8 defined HTTP verbs defined in RFC 2616
372 <http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9>: OPTIONS,
373 GET, HEAD, POST, PUT, DELETE, TRACE, CONNECT. Of these, the majority
374 of web applications focus on the verbs which closely map to the CRUD
375 (Create, Retrieve, Update, Delete) operations most database-driven
376 applications need to implement.
377 In addition, the "PATCH" verb was defined in RFC5789
379 <http://tools.ietf.org/html/rfc5789>, and is intended as a "partial
380 PUT", sending just the changes required to the entity in question. How
381 this would be handled is down to your app, it will vary depending on
382 the type of entity in question and the serialization in use.
383 Dancer2's keywords currently supports GET, PUT/PATCH, POST, DELETE,
385 OPTIONS which map to Retrieve, Update, Create, Delete respectively.
386 Let's take a look now at the "/add" route handler which handles a POST
387 operation.
388 post '/add' => sub {
390 if ( not session('logged_in') ) {
391 send_error("Not logged in", 401);
392 }
393 my $db = connect_db();
395 my $sql = 'insert into entries (title, text) values (?, ?)';
396 my $sth = $db->prepare($sql)
397 or die $db->errstr;
398 $sth->execute(
400 body_parameters->get('title'),
401 body_parameters->get('text')
402 ) or die $sth->errstr;
403 set_flash('New entry posted!');
405 redirect '/';
406 };
407 As before, the HTTP verb begins the handler, followed by the route, and
409 a subroutine to do something; in this case it will insert a new entry
410 into the database.
411 The first check in the subroutine is to make sure the user sending the
413 data is logged in. If not, the application returns an error and stops
414 processing. Otherwise, we have standard DBI stuff. Let me insert (heh,
415 heh) a blatant plug here for always, always using parameterized INSERTs
416 in your application SQL statements. It's the only way to be sure your
417 application won't be vulnerable to SQL injection. (See
418 <http://www.bobby-tables.com> for correct INSERT examples in multiple
419 languages.) Here we're using the "body_parameters" convenience method
420 to pull in the parameters in the current HTTP request. (You can see the
421 'title' and 'text' form parameters in the show_entries.tt template
422 above.) Those values are inserted into the database, then we set a
423 flash message for the user and redirect her back to the root URL.
424 It's worth mentioning that the "flash message" is not part of Dancer2,
426 but a part of this specific application. We need to implement it
427 ourself.
428 Logins and sessions
430 Dancer2 comes with a simple in-memory session manager out of the box.
431 It supports a bunch of other session engines including YAML, memcached,
432 browser cookies and others. We'll just stick with the in-memory model
433 which works great for development and tutorials, but won't persist
434 across server restarts or scale very well in "real world" production
435 scenarios.
436 Configuration options
438 To use sessions in our application, we have to tell Dancer2 to activate
440 the session handler and initialize a session manager. To do that, we
441 add some configuration directives toward the top of our 'dancr.pl'
442 file. But there are more options than just the session engine we want
443 to set.
444 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
446 set 'session' => 'Simple';
447 set 'template' => 'template_toolkit';
448 set 'logger' => 'console';
449 set 'log' => 'debug';
450 set 'show_errors' => 1;
451 set 'startup_info' => 1;
452 set 'warnings' => 1;
453 Hopefully these are fairly self-explanatory. We want the Simple session
455 engine, the Template Toolkit template engine, logging enabled (at the
456 'debug' level with output to the console instead of a file), we want to
457 show errors to the web browser, log access attempts and log Dancer2
458 warnings (instead of silently ignoring them).
459 Dancer2 doesn't impose any limits on what parameters you can set using
461 the "set" syntax. For this application we're going to embed our single
462 username and password into the application itself:
463 set 'username' => 'admin';
465 set 'password' => 'password';
466 Hopefully no one will ever guess our clever password! Obviously, you
468 will want a more sophisticated user authentication scheme in any sort
469 of non-tutorial application but this is good enough for our purposes.
470 In Part II of our tutorial, we will show you how to use Dancer2's
472 configuration files to manage these options and set up different
473 environments for your app using different configuration files. For now,
474 we're going to keep it simple and leave that discussion for later.
475 Logging in
477 Now that dancr is configured to handle sessions, let's take a look at
479 the URL handler for the "/login" route.
480 any ['get', 'post'] => '/login' => sub {
482 my $err;
483 if ( request->method() eq "POST" ) {
485 # process form input
486 if ( body_parameters->get('username') ne setting('username') ) {
487 $err = "Invalid username";
488 }
489 elsif ( body_parameters->get('password') ne setting('password') ) {
490 $err = "Invalid password";
491 }
492 else {
493 session 'logged_in' => true;
494 set_flash('You are logged in.');
495 return redirect '/';
496 }
497 }
498 # display login form
500 template 'login.tt', {
501 err => $err,
502 };
503 };
504 This is the first handler which accepts two different verb types, a GET
506 for a human browsing to the URL and a POST for the browser to submit
507 the user's input to the web application. Since we're handling two
508 different verbs, we check to see what verb is in the request. If it's
509 not a POST, we drop down to the "template" directive and display the
510 login.tt template:
511 <h2>Login</h2>
513 [% IF err %]<p class=error><strong>Error:</strong> [% err %][% END %]
514 <form action="[% login_url %]" method=post>
515 <dl>
516 <dt>Username:
517 <dd><input type=text name=username>
518 <dt>Password:
519 <dd><input type=password name=password>
520 <dd><input type=submit value=Login>
521 </dl>
522 </form>
523 This is even simpler than our show_entries.tt template–but wait–
525 there's a "login_url" template parameter and we're only passing in the
526 "err" parameter. Where's the missing parameter? It's being generated
527 and sent to the template in a "before_template_render" directive, we'll
528 come back to that in a moment or two.
529 So the user fills out the login.tt template and submits it back to the
531 "/login" route handler. We now check the user input against our
532 application settings and if the input is incorrect, we alert the user,
533 otherwise the application starts a session and sets the "logged_in"
534 session parameter to the "true()" value. Dancer2 exports both a
535 "true()" and "false()" convenience method which we use here. After
536 that, it's another flash message and back to the root URL handler.
537 Logging out
539 And finally, we need a way to clear our user's session with the
541 customary logout procedure.
542 get '/logout' => sub {
544 app->destroy_session;
545 set_flash('You are logged out.');
546 redirect '/';
547 };
548 "app->destroy_session;" is Dancer2's way to remove a stored session.
550 We notify the user she is logged out and route her back to the root URL
551 once again.
552 You might wonder how we can then set a value in the session in
554 "set_flash", because we just destroyed the session.
555 Destroying the session has removed the data from the persistence layer
557 (which is the memory of our running application, because we are using
558 the "simple" session engine). If we write to the session now, it will
559 actually create a completely new session for our user. This new, empty
560 session will have a new session ID, which Dancer2 tells the user's
561 browser about in the response. When the browser requests the root URL,
562 it will send this new session ID to our application.
563 Layout and static files
565 We still have a missing puzzle piece or two. First, how can we use
566 Dancer2 to serve our CSS stylesheet? Second, where are flash messages
567 displayed? Third, what about the "before_template_render" directive?
568 Serving static files
570 In Dancer2, static files should go into the "public/" directory, but in
572 the application itself be sure to omit the "public/" element from the
573 path. For example, the stylesheet for dancr lives in
574 "dancr/public/css/style.css" but is served from
575 <http://localhost:3000/css/style.css>.
576 If you wanted to build a mostly static web site you could simply write
578 route handlers like this one:
579 get '/' => sub {
581 send_file 'index.html';
582 };
583 where index.html would live in your "public/" directory.
585 "send_file" does exactly what it says: it loads a static file, then
587 sends the contents of that file to the user.
588 Let's go ahead and create our style sheet. In the same directory as
590 your dancr.pl script, issue the following commands:
591 mkdir public && mkdir public/css && touch public/css/style.css
593 Next add the following css to the "public/css/style.css" file you just
595 created:
596 body { font-family: sans-serif; background: #eee; }
598 a, h1, h2 { color: #377ba8; }
599 h1, h2 { font-family: 'Georgia', serif; margin: 0; }
600 h1 { border-bottom: 2px solid #eee; }
601 h2 { font-size: 1.2em; }
602 .page { margin: 2em auto; width: 35em; border: 5px solid #ccc;
604 padding: 0.8em; background: white; }
605 .entries { list-style: none; margin: 0; padding: 0; }
606 .entries li { margin: 0.8em 1.2em; }
607 .entries li h2 { margin-left: -1em; }
608 .add-entry { font-size: 0.9em; border-bottom: 1px solid #ccc; }
609 .add-entry dl { font-weight: bold; }
610 .metanav { text-align: right; font-size: 0.8em; padding: 0.3em;
611 margin-bottom: 1em; background: #fafafa; }
612 .flash { background: #cee5F5; padding: 0.5em;
613 border: 1px solid #aacbe2; }
614 .error { background: #f0d6d6; padding: 0.5em; }
615 Be sure to save the file.
617 Layouts
619 I mentioned earlier in the tutorial that it is possible to create a
621 "layout" template. In dancr, that layout is called "main" and it's set
622 up by putting in a directive like this:
623 set layout => 'main';
625 near the top of your web application. This tells Dancer2's template
627 engine that it should look for a file called main.tt in
628 "views/layouts/" and insert the calls from the "template" directive
629 into a template parameter called "content".
630 Here is the simple layout file we will use for this web application. Go
632 ahead and add this the main.tt file to the "views/layouts/" directory.
633 <!doctype html>
635 <html>
636 <head>
637 <title>dancr</title>
638 <link rel=stylesheet type=text/css href="[% css_url %]">
639 </head>
640 <body>
641 <div class=page>
642 <h1>dancr</h1>
643 <div class=metanav>
644 [% IF not session.logged_in %]
645 <a href="[% login_url %]">log in</a>
646 [% ELSE %]
647 <a href="[% logout_url %]">log out</a>
648 [% END %]
649 </div>
650 [% IF msg %]
651 <div class=flash> [% msg %] </div>
652 [% END %]
653 [% content %]
654 </div>
655 </body>
656 </html>
657 Aha! You now see where the flash message "msg" parameter gets rendered.
659 You can also see where the content from the specific route handlers is
660 inserted (the fourth line from the bottom in the "content" template
661 parameter).
662 But what about all those other *_url template parameters?
664 Using "before_template_render"
666 Dancer2 has a way to manipulate the template parameters before they're
668 passed to the engine for processing. It's "before_template_render".
669 Using this keyword, you can generate and set the URIs for the "/login"
670 and "/logout" route handlers and the URI for the stylesheet. This is
671 handy for situations like this where there are values which are re-used
672 consistently across all (or most) templates. This cuts down on code-
673 duplication and makes your app easier to maintain over time since you
674 only need to update the values in this one place instead of everywhere
675 you render a template.
676 hook before_template_render => sub {
678 my $tokens = shift;
679 $tokens->{'css_url'} = request->base . 'css/style.css';
681 $tokens->{'login_url'} = uri_for('/login');
682 $tokens->{'logout_url'} = uri_for('/logout');
683 };
684 Here again I'm using "uri_for" instead of hardcoding the routes. This
686 code block is executed before any of the templates are processed so
687 that the template parameters have the appropriate values before being
688 rendered.
689 Putting it all together
691 Here's the complete 'dancr.pl' script from start to finish.
692 use Dancer2;
694 use DBI;
695 use File::Spec;
696 use File::Slurper qw/ read_text /;
697 use Template;
698 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
700 set 'session' => 'Simple';
701 set 'template' => 'template_toolkit';
702 set 'logger' => 'console';
703 set 'log' => 'debug';
704 set 'show_errors' => 1;
705 set 'startup_info' => 1;
706 set 'warnings' => 1;
707 set 'username' => 'admin';
708 set 'password' => 'password';
709 set 'layout' => 'main';
710 sub set_flash {
712 my $message = shift;
713 session flash => $message;
715 }
716 sub get_flash {
718 my $msg = session('flash');
719 session->delete('flash');
720 return $msg;
722 }
723 sub connect_db {
725 my $dbh = DBI->connect("dbi:SQLite:dbname=".setting('database'))
726 or die $DBI::errstr;
727 return $dbh;
729 }
730 sub init_db {
732 my $db = connect_db();
733 my $schema = read_text('./schema.sql');
734 $db->do($schema)
735 or die $db->errstr;
736 }
737 hook before_template_render => sub {
739 my $tokens = shift;
740 $tokens->{'css_url'} = request->base . 'css/style.css';
742 $tokens->{'login_url'} = uri_for('/login');
743 $tokens->{'logout_url'} = uri_for('/logout');
744 };
745 get '/' => sub {
747 my $db = connect_db();
748 my $sql = 'select id, title, text from entries order by id desc';
749 my $sth = $db->prepare($sql)
751 or die $db->errstr;
752 $sth->execute
754 or die $sth->errstr;
755 template 'show_entries.tt', {
757 msg => get_flash(),
758 add_entry_url => uri_for('/add'),
759 entries => $sth->fetchall_hashref('id'),
760 };
761 };
762 post '/add' => sub {
764 if ( not session('logged_in') ) {
765 send_error("Not logged in", 401);
766 }
767 my $db = connect_db();
769 my $sql = 'insert into entries (title, text) values (?, ?)';
770 my $sth = $db->prepare($sql)
772 or die $db->errstr;
773 $sth->execute(
775 body_parameters->get('title'),
776 body_parameters->get('text')
777 ) or die $sth->errstr;
778 set_flash('New entry posted!');
780 redirect '/';
781 };
782 any ['get', 'post'] => '/login' => sub {
784 my $err;
785 if ( request->method() eq "POST" ) {
787 # process form input
788 if ( body_parameters->get('username') ne setting('username') ) {
789 $err = "Invalid username";
790 }
791 elsif ( body_parameters->get('password') ne setting('password') ) {
792 $err = "Invalid password";
793 }
794 else {
795 session 'logged_in' => true;
796 set_flash('You are logged in.');
797 return redirect '/';
798 }
799 }
800 # display login form
802 template 'login.tt', {
803 err => $err,
804 };
805 };
807 get '/logout' => sub {
809 app->destroy_session;
810 set_flash('You are logged out.');
811 redirect '/';
812 };
813 init_db();
815 start;
816 Advanced route moves
818 There's a lot more to route matching than shown here. For example, you
820 can match routes with regular expressions, or you can match pieces of a
821 route like "/hello/:name" where the ":name" piece magically turns into
822 a named parameter in your handler for manipulation.
823 You can explore this and other advanced concepts by reading the
825 Dancer2::Manual.
826
828 In Part I, we took an ordinary Perl script and turned it into a simple
829 web app to teach you basic Dancer2 concepts. While starting with a
830 simple script like this helped make it easier to teach these concepts,
831 it did not demonstrate how a typical app is built by a Dancer2
832 developer. So let's show you how things really get done.
833 Creating a new app
835 So now that you have a better idea of what goes into building an app
836 with Dancer2, it's time to cha-cha with the "dancer2" utility which
837 will save you a lot of time and effort by setting up directories,
838 files, and default configuration settings for you.
839 The "dancer2" utility was installed on your machine when you installed
841 the Dancer2 distribution. Hop over to the command line into a directory
842 you have permission to write to and issue the following command:
843 dancer2 -a Dancr2
845 That command should output something like the following to the console:
847 + Dancr2
849 + Dancr2/config.yml
850 + Dancr2/Makefile.PL
851 + Dancr2/MANIFEST.SKIP
852 + Dancr2/.dancer
853 + Dancr2/cpanfile
854 + Dancr2/bin
855 + Dancr2/bin/app.psgi
856 + Dancr2/environments
857 + Dancr2/environments/development.yml
858 + Dancr2/environments/production.yml
859 + Dancr2/lib
860 + Dancr2/lib/Dancr2.pm
861 + Dancr2/public
862 + Dancr2/public/favicon.ico
863 + Dancr2/public/500.html
864 + Dancr2/public/dispatch.cgi
865 + Dancr2/public/404.html
866 + Dancr2/public/dispatch.fcgi
867 + Dancr2/public/css
868 + Dancr2/public/css/error.css
869 + Dancr2/public/css/style.css
870 + Dancr2/public/images
871 + Dancr2/public/images/perldancer.jpg
872 + Dancr2/public/images/perldancer-bg.jpg
873 + Dancr2/public/javascripts
874 + Dancr2/public/javascripts/jquery.js
875 + Dancr2/t
876 + Dancr2/t/001_base.t
877 + Dancr2/t/002_index_route.t
878 + Dancr2/views
879 + Dancr2/views/index.tt
880 + Dancr2/views/layouts
881 + Dancr2/views/layouts/main.tt
882 What you just did was create a fully functional app in Dancer2 with
884 just one command! The new app, named "Dancr2," won't do anything
885 particularly useful until you add your own routes to it, but it does
886 take care of many of the tedious tasks of setting up an app for you.
887 The files and folders that were generated and that you see listed above
889 provide a convenient scaffolding, or skeleton, upon which you can build
890 your app. The default skelelton provides you with basic error pages,
891 css, javascript, graphics, tests, templates and other files which you
892 are free to modify and customize to your liking.
893 If you don't like the default skeleton provided to you by Dancer, the
895 "dancer2" command allows you to generate your own custom skeletons.
896 Consult "BOOTSTRAPPING_A_NEW_APP" in Dancer2::Manual for further
897 details on this and other capabilities of the "dancer2") utility.
898 Getting the new app up and running with Plack
900 In Part I, we used the "start" command in our script to launch a server
901 to serve our app. Things are a little different when using "dancer2",
902 however. You'll notice that the "dancer2" utility created a "bin/"
903 directory with a file in it called "app.psgi". This is the file we use
904 to get our app up and running.
905 Let's see how to to do that by first changing into the Dancr2 directory
907 and then starting the server using the "plackup" command:
908 cd Dancr2;
910 plackup -p 5000 bin/app.psgi
911 If all went well, you'll be able to see the Dancr2 home page by
913 visiting:
914 http://localhost:5000
916 The web page you see there gives you some very basic advice for tuning
918 and modifying your app and where you can go for more information to
919 learn about developing apps with Dancer2 (like this handy tutorial!).
920 Our Dancr2 app is served on a simple web server provided by Plack.
922 Plack is PSGI compliant software, hence the "psgi" extension for our
923 file in the "bin/" directory. Plack and PSGI is beyond the scope of
924 this tutorial but you can learn more by visiting the Plack website
925 <http://plackperl.org/>.
926 For now, all you need to know is that if you are deploying an app for
928 use by just yourself or a handful of people on a local network, Plack
929 alone may do the trick. More typically, you would use Plack in
930 conjunction with other server software to make your app much more
931 robust. But in the early stages of your app's development, a simple
932 Plack server is more than likely all you need.
933 To learn more about the different ways for deploying your app, see the
935 Dancer2 Deployment Manual
936 Porting dancr.pl over to the new Dancr2 app
938 Ok, so now that we've got our new Dancr2 app up and running, it's time
939 to learn how to take advantage of what the "dancer2" utility set up for
940 us by porting our dancr.pl script created in Part I into Dancr2.
941 The "lib/" directory
943 The "lib/" directory in our Dancr2 app is where our "app.psgi" file
945 will expect our code to live. So let's take a peek at the file
946 generated for us in there:
947 cat lib/Dancr2.pm
949 You'll see something like the following bit of code which provides a
951 single route to our app's home page and loads the index template:
952 package Dancr2;
954 use Dancer2;
955 our $VERSION = '0.1';
957 get '/' => sub {
959 template 'index' => { title => 'Dancr2' };
960 };
961 true;
963 The first thing you'll notice is that instead of a script, we are using
965 a module, "Dancr2" to package our code. Modules make it easer to pull
966 off many powerful tricks like packaging our app across several discrete
967 modules. We'll let the manual explain this more advanced technique.
968 Updating the Dancr2 module
970 Now that we know where to put our code, let's update the "Dancr2.pm"
972 module with our original "dancr.pl" code. Remove the existing sample
973 route in "Dancr2.pm" and replace it with the code from our "dancr.pl"
974 file. You'll have to make a couple of adjustments to the "dancr.pl"
975 code like removing the "use Dancer2;" line since it's already provided
976 by our module. You'll also want to be sure to remove the "start;" line
977 as well from the end of the file.
978 When you're done, "Dancr2.pm" should look something close to this:
980 package Dancr2;
982 use Dancer2;
983 our $VERSION = '0.1';
985 # Our original dancr.pl code with some minor tweaks
987 use DBI;
988 use File::Spec;
989 use File::Slurper qw/ read_text /;
990 use Template;
991 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
993 set 'session' => 'YAML';
994 ...
995 <snip> # The rest of the stuff </snip>
997 ...
999 sub init_db {
1001 my $schema = read_text('./schema.sql');
1002 $db->do($schema)
1003 or die $db->errstr;
1004 }
1005 get '/logout' => sub {
1007 app->destroy_session;
1008 set_flash('You are logged out.');
1009 redirect '/';
1010 };
1011 init_db();
1013 Finally, to avoid getting an error in the "init_db") subroutine when it
1015 tries to load our schema file, copy over the "schema.db" file to the
1016 root directory of the Dancr2 app:
1017 cp /path/to/dancr.pl/schema.db /path/to/Dancr2;
1019 Ok, now that we've got the code moved over, let's move the assets from
1021 dancr.pl to our new app.
1022 The "public/" directory
1024 As mentioned in Part I, our static assets go into our "public/"
1026 directory. If you followed along with the tutorial in Part I, you
1027 should have a "public/" directory with a "public/css" subdirectory and
1028 a file called "style.css" within that.
1029 Dancer2 has conveniently generated the "public/css" directory for us
1031 which has a default css file. Let's copy the style sheet from our
1032 original app so our new app can use it:
1033 # Note: This command overwrites the default style sheet. Move it or copy
1035 # it if you wish to preserve it.
1036 cp /path/to/dancr.pl/public/css/style.css /path/to/Dancr2/public/css;
1038 The "views" directory
1040 Along with our "public/" directory, Dancer has also provided a "views/"
1042 directory, which as we covered, serves as the a home for our templates.
1043 Let's get those copied over now:
1044 # NOTE: This command will overwrite the default main.tt tempalte file. Move
1046 # it or copy it if you wish to preserve it.
1047 cp -r /path/to/dancr.pl/views/* /path/to/Dancr2/views;
1049 Does it work?
1051 If you followed the instructions here closely, your Dancr2 app should
1053 be working. Shut down any running Plack servers and then issue the
1054 same plackup command to see if it runs:
1055 cd /path/to/Dancr2
1057 plackup -p 5000 bin/app.psgi
1058 If you see any errors, get them resolved until the app loads.
1060 Configuring Your App
1062 In Part I, you configured your app with a series of "set" statements
1063 near the top of your file. Now we will show you a better way to
1064 configure your app using Dancer2's configuration files.
1065 Your skeleton provides your app with three different configuration
1067 files. The first two files we'll discuss, found in the "environments/"
1068 folder of your app, are "development.yml" and "production.yml". As you
1069 can probably guess, the "development.yml" file has settings intended to
1070 be used while developing the app. The "production.yml" file has
1071 settings more appropriate for running your app when used by others. The
1072 third configuration file is found in the root directory of your app and
1073 is named "config.yml". This file has the settings that are common to
1074 all environments but that can be overridden by the environment
1075 configuration files. You can still override any configuration file
1076 settings in your modules using the "set" command.
1077 We will take a look at the "development.yml" file first. Open that file
1079 in your text editor and take a look inside. It has a bunch of helpful
1080 comments and the following five settings sprinkled throughout:
1081 logger: "console"
1083 log: "core"
1084 warnings: 1
1085 show_errors: 1
1086 startup_info: 1
1087 The first four settings duplicate many of the settings in our new
1089 Dancr2 app. So in the spirit of DRY (don't repeat yourself), edit your
1090 Dancr2 module and delete the four lines that correspond to these four
1091 settings.
1092 Then, in the configuration file, be sure to change the value for the
1094 "log" setting from "core" to "debug" so it matches the value we had in
1095 our module.
1096 We will leave it up to you what you want to do with the fifth setting,
1098 "startup_info". You can read about that setting, along with all the
1099 other settings, in the configuration manual.
1100 Finally, let's add a new setting to the configuration file for
1102 "session" with the following line:
1103 session: "Simple"
1105 Then delete the corresponding setting from your Dancr2 module.
1107 Alright, our Dancr2 app is a little leaner and meaner. Now open the
1109 main "config.yml" file and look for the settings in there that are also
1110 duplicated in our app's module. There are two:
1111 layout: "main"
1113 template: "simple"
1114 Leave "layout" as is but change the template setting to
1116 "template_toolkit". Then edit your Dancr2 module file and delete these
1117 two settings.
1118 Finally, add the following configuration settings to the .yml file:
1120 username: "admin"
1122 password: "password"
1123 Then you delete these two settings from the Dancr2 module, as well.
1125 So, if you have been following along, you now have only the following
1127 "set" command in your Dancr2 module, related to the database
1128 configuration:
1129 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
1131 We will get rid of this setting in Part III of the tutorial. All the
1133 rest of the settings have been transferred to our configuration files.
1134 Nice!
1135 We still have a little more cleanup we can do. Now that Dancer2 knows
1137 we are using Template::Toolkit, we can delete the "use Template;" line
1138 from our module.
1139 Now start the app "plackup" command and check to see that everything
1141 works. By default, Dancer2 will load the development environment
1142 configuration. When it comes time to put your app into production, you
1143 can load the "production.yml" file configuration with plackup's "--env"
1144 switch like so:
1145 plackup -p 5000 --env production bin/app.psgi
1147 Keep on Dancing!
1149 This concludes Part II of our tutorial where we showed you how to take
1150 advantage of the "dancer2" utility to set up a app skeleton to make it
1151 really easy to get started developing your own apps.
1152 Part III will refine our app a little further by showing you how to use
1154 plugins so you can start capitalizing on all the great work contributed
1155 by other Dancer2 developers.
1156
1158 Dancer2 takes advantage of the open source software revolution by
1159 making it exceedingly easy to use plugins that you can mix into your
1160 app to give it new functionality. In Part III of this tutorial, we will
1161 update our new Dancr2 app to use the Dancer2::Plugin::Database to give
1162 you enough skills to go out and explore other plugins on your own.
1163 Installing plugins
1165 Like Dancer2 itself, Dancer2 plugins can be found on the CPAN. Use your
1166 favorite method for downloading and installing the
1167 Dancer2::Plugin::Database module on your machine. We recommend using
1168 "cpanminus" like so:
1169 cpanm Dancer2::Plugin::Database
1171 Using plugins
1173 Using a plugin couldn't be easier. Simply add the following line to
1174 your Dancr2 module below the "use Dancer2;" line in your module:
1175 use Dancer2::Plugin::Database;
1177 Configuring plugins
1179 Plugins can be configured with the YAML configuration files mentioned
1180 in Part II of this tutorial. Let's edit the "development.yml" file and
1181 add our database configuration there. Below the last line in that file,
1182 add the following lines, being careful to keep the indentation as you
1183 see it here:
1184 plugins: ; all plugin configuration settings go in this section
1186 Database: ; the name of our plugin
1187 driver: "SQLite" ; driver we want to use
1188 database: "dancr.db" ; where the database will go in our app
1189 ; run a query when connecting to the datbase:
1190 on_connect_do: [ "create table if not exists entries (id integer primary key autoincrement, title string not null, text string not null)" ]
1191 Here, we direct our database plugin to use the "SQLite" driver and to
1193 place the database in the root directory of our Dancr2. The
1194 "on_connect_db" setting tells the plugin to run an SQL query when it
1195 connects with the database to create a table for us if it doesn't
1196 already exist.
1197 Modifying our database code in the Dancr2 module
1199 Now it's time to modify our Dancr2 module so it will use the plugin to
1200 query the database instead of our own code. There are a few things to
1201 do. First, we will delete the code we no longer need.
1202 Since our configuration file tells the plugin where our database is, we
1204 can delete this line:
1205 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
1207 And since the database plugin will create our database connection and
1209 initialize our database for us, we can scrap the following two
1210 subroutines and line from our module:
1211 sub connect_db {
1213 my $dbh = DBI->connect("dbi:SQLite:dbname=".setting('database'))
1214 or die $DBI::errstr;
1215 return $dbh;
1217 }
1218 sub init_db {
1220 my $db = connect_db();
1221 my $schema = read_text('./schema.sql');
1222 $db->do($schema)
1223 or die $db->errstr;
1224 }
1225 init_db(); # Found at the bottom of our file
1227 With that done, let's now take advantage of a hook the plugin provides
1229 us that we can use to handle certain events by adding the following
1230 command to our module to handle database errors:
1231 hook 'database_error' => sub {
1233 my $error = shift;
1234 die $error;
1235 };
1236 Now let's make a few adjustments to the bits of code that make the
1238 database queries. In our "get '/'" route, change all instances of $db
1239 with "database" and remove all the "die" calls since we now have a hook
1240 to handle the errors for us. When you are done, your route should look
1241 something like this:
1242 get '/' => sub {
1244 my $sql = 'select id, title, text from entries order by id desc';
1245 my $sth = database->prepare($sql);
1246 $sth->execute;
1247 template 'show_entries.tt', {
1248 msg => get_flash(),
1249 add_entry_url => uri_for('/add'),
1250 entries => $sth->fetchall_hashref('id'),
1251 };
1252 };
1253 Make the same changes to the "post '/add'" route to transform it into
1255 this:
1256 post '/add' => sub {
1258 if ( not session('logged_in') ) {
1259 send_error("Not logged in", 401);
1260 }
1261 my $sql = 'insert into entries (title, text) values (?, ?)';
1263 my $sth = database->prepare($sql);
1264 $sth->execute(
1265 body_parameters->get('title'),
1266 body_parameters->get('text')
1267 );
1268 set_flash('New entry posted!');
1270 redirect '/';
1271 };
1272 Our last step is to get rid of the following lines which we no longer
1274 need, thanks to our plugin:
1275 use DBI;
1277 use File::Spec;
1278 use File::Slurper qw/ read_text /;
1279 That's it! Now start your app with "plackup" to make sure you don't get
1281 any errors and then point your browser to test the app to make sure it
1282 works as expected. If it doesn't, double and triple check your
1283 configuration settings and your module's code which should now look
1284 like this:
1285 package Dancr2;
1287 use Dancer2;
1288 use Dancer2::Plugin::Database;
1289 our $VERSION = '0.1';
1291 my $flash;
1293 sub set_flash {
1295 my $message = shift;
1296 $flash = $message;
1298 }
1299 sub get_flash {
1301 my $msg = $flash;
1302 $flash = "";
1303 return $msg;
1305 }
1306 hook before_template_render => sub {
1308 my $tokens = shift;
1309 $tokens->{'css_url'} = request->base . 'css/style.css';
1311 $tokens->{'login_url'} = uri_for('/login');
1312 $tokens->{'logout_url'} = uri_for('/logout');
1313 };
1314 get '/' => sub {
1316 my $sql = 'select id, title, text from entries order by id desc';
1317 my $sth = database->prepare($sql);
1318 $sth->execute;
1319 template 'show_entries.tt', {
1320 msg => get_flash(),
1321 add_entry_url => uri_for('/add'),
1322 entries => $sth->fetchall_hashref('id'),
1323 };
1324 };
1325 post '/add' => sub {
1327 if ( not session('logged_in') ) {
1328 send_error("Not logged in", 401);
1329 }
1330 my $sql = 'insert into entries (title, text) values (?, ?)';
1332 my $sth = database->prepare($sql);
1333 $sth->execute(
1334 body_parameters->get('title'),
1335 body_parameters->get('text')
1336 );
1337 set_flash('New entry posted!');
1339 redirect '/';
1340 };
1341 any ['get', 'post'] => '/login' => sub {
1343 my $err;
1344 if ( request->method() eq "POST" ) {
1346 # process form input
1347 if ( params->{'username'} ne setting('username') ) {
1348 $err = "Invalid username";
1349 }
1350 elsif ( params->{'password'} ne setting('password') ) {
1351 $err = "Invalid password";
1352 }
1353 else {
1354 session 'logged_in' => true;
1355 set_flash('You are logged in.');
1356 return redirect '/';
1357 }
1358 }
1359 # display login form
1361 template 'login.tt', {
1362 err => $err,
1363 };
1364 };
1366 get '/logout' => sub {
1368 app->destroy_session;
1369 set_flash('You are logged out.');
1370 redirect '/';
1371 };
1372 true;
1374 Next steps
1376 Congrats! You are now using the database plugin like a boss. The
1377 database plugin does a lot more than what we showed you here. We'll
1378 leave it up to you to consult the Dancer2::Plugin::Database to unlock
1379 its full potential.
1380 There are many more plugins for you to explore. You now know enough to
1382 install and experiment with them. Some of the more popular and useful
1383 plugins are listed at Dancer2::Plugins. You can also search CPAN with
1384 "Dancer2::Plugin" for a more comprehensive listing.
1385 If you are feeling really inspired, you can learn how to extend Dancer2
1387 with your own plugins by reading Dancer2::Plugin.
1388
1390 I hope these tutorials have been helpful and interesting enough to get
1391 you exploring Dancer2 on your own. The framework is still under
1392 development but it's definitely mature enough to use in a production
1393 project.
1394 Happy dancing!
1396 One more thing: Test!
1398 Before we go, we want to mention that Dancer2 makes it very easy to run
1399 automated tests on your app to help you find bugs. If you are new to
1400 testing, we encourage you to start learning how. Your future self will
1401 thank you. The effort you put into creating tests for your app will
1402 save you many hours of frustration in the long run. Unfortunately,
1403 until we get Part IV of this tutorial written, you'll have to consult
1404 the Dancer2 testing documentation for more details on how to test your
1405 app.
1406 Enjoy!
1408
1410 · <http://perldancer.org>
1411 · <http://github.com/PerlDancer/Dancer2>
1413 · Dancer2::Plugins
1415
1417 The CSS stylesheet is copied verbatim from the Flaskr example
1418 application and is subject to their license:
1419 Copyright (c) 2010, 2013 by Armin Ronacher and contributors.
1421 Some rights reserved.
1423 Redistribution and use in source and binary forms of the software as
1425 well as documentation, with or without modification, are permitted
1426 provided that the following conditions are met:
1427 · Redistributions of source code must retain the above copyright
1429 notice, this list of conditions and the following disclaimer.
1430 · Redistributions in binary form must reproduce the above copyright
1432 notice, this list of conditions and the following disclaimer in the
1433 documentation and/or other materials provided with the
1434 distribution.
1435 · The names of the contributors may not be used to endorse or promote
1437 products derived from this software without specific prior written
1438 permission.
1439
1441 Dancer Core Developers
1442
1444 This software is copyright (c) 2020 by Alexis Sukrieh.
1445 This is free software; you can redistribute it and/or modify it under
1447 the same terms as the Perl 5 programming language system itself.
1448perl v5.32.0 2020-07-28 Dancer2::Tutorial(3)