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 0.400001
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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 assume
247 that the expression evaluates to the location of the database file.
248 In Part III of the tutorial, we will show you how to use the
250 Dancer2::Plugin::Database module for an easier way to configure and
251 manage database connections for your Dancer2 apps.
252 Our first route handler
254 Ok, let's get back to the business of learning Dancer2 by creating our
255 app's first route handler for the root URL. Replace the route handler
256 in our simple example above with this one:
257 get '/' => sub {
259 my $db = connect_db();
260 my $sql = 'select id, title, text from entries order by id desc';
261 my $sth = $db->prepare($sql)
263 or die $db->errstr;
264 $sth->execute
266 or die $sth->errstr;
267 template 'show_entries.tt', {
269 msg => get_flash(),
270 add_entry_url => uri_for('/add'),
271 entries => $sth->fetchall_hashref('id'),
272 };
273 };
274 Our new route handler is the same as the one in our first example
276 except that our route action does a lot more work.
277 Something you might not have noticed right away is the semicolon at the
279 end of the route handler. This might confuse newer Perl coders and is a
280 source of bugs for more experienced ones who forget to add it. We need
281 the semicolon there because we are creating a reference to a subroutine
282 and because that's just what the Perl compiler demands and we must obey
283 if we want our code to run.
284 Alright, let's take a closer look at this route's action. The first few
286 lines are standard DBI. The important bit related to Dancer2 is the
287 "template" keyword at the end of the action. That tells Dancer2 to
288 process the output through one of its templating engines. There are
289 many template engines available for use with Dancer2. In this tutorial,
290 we're using Template Toolkit which offers a lot more flexibility than
291 the simple default Dancer2 template engine.
292 Templates all go into a "views/" directory which located in the same
294 directory as our dancr.pl script. Optionally, you can create a "layout"
295 template which provides a consistent look and feel for all of your
296 views. We'll construct our own layout template, cleverly named main.tt,
297 a little later in this tutorial.
298 So what's going on with the hashref as the second argument to the
300 template directive? Those are all of the parameters we want to pass
301 into our template. We have a "msg" field which displays a message to
302 the user when an event happens like a new entry is posted, or the user
303 logs in or out. It's called a "flash" message because we only want to
304 display it one time, not every time the "/" URL is rendered.
305 The "uri_for" directive tells Dancer2 to provide a URI for that
307 specific route, in this case, it is the route to post a new entry into
308 the database. You might ask why we don't simply hardcode the "/add"
309 URI in our application or templates. The best reason not to do that is
310 because it removes a layer of flexibility as to where to "mount" the
311 web application. Although the application is coded to use the root URL
312 "/" it might be better in the future to locate it under its own URL
313 route (maybe "/dancr"?) - at that point we'd have to go through our
314 application and the templates and update the URLs and hope we didn't
315 miss any of them. By using the "uri_for" Dancer2 method, we can easily
316 load the application wherever we like and not have to modify the
317 application at all.
318 Finally, the "entries" field contains a hashref with the results from
320 our database query. Those results will be rendered in the template
321 itself, so we just pass them in.
322 So what does the show_entries.tt template look like? This:
324 [% IF session.logged_in %]
326 <form action="[% add_entry_url %]" method=post class=add-entry>
327 <dl>
328 <dt>Title:
329 <dd><input type=text size=30 name=title>
330 <dt>Text:
331 <dd><textarea name=text rows=5 cols=40></textarea>
332 <dd><input type=submit value=Share>
333 </dl>
334 </form>
335 [% END %]
336 <ul class=entries>
337 [% IF entries.size %]
338 [% FOREACH id IN entries.keys.nsort %]
339 <li><h2>[% entries.$id.title | html %]</h2>[% entries.$id.text | html %]
340 [% END %]
341 [% ELSE %]
342 <li><em>Unbelievable. No entries here so far</em>
343 [% END %]
344 </ul>
345 Go ahead and create a "views/" directory in the same directory as the
347 script and add this file to it.
348 Again, since this isn't a tutorial about Template Toolkit, we'll gloss
350 over the syntax here and just point out the section which starts with
351 "<ul class=entries>". This is the section where the database query
352 results are displayed. You can also see at the very top some discussion
353 about a session, more on that soon.
354 The only other Template Toolkit related thing that has to be mentioned
356 here is the "| html" in "[% entries.$id.title | html %]". That's a
357 filter <http://www.template-
358 toolkit.org/docs/manual/Filters.html#section_html> to convert
359 characters like "<" and ">" to "<" and ">". This way they will be
360 displayed by the browser as content on the page rather than just
361 included. If we did not do this, the browser might interpret content as
362 part of the page, and a malicious user could smuggle in all kinds of
363 bad code that would then run in another user's browser. This is called
364 Cross Site Scripting <https://en.wikipedia.org/wiki/Cross-
365 site_scripting> or XSS and you should make sure to avoid it by always
366 filtering data that came in from the web when you display it in a
367 template.
368 Other HTTP verbs
370 There are 8 defined HTTP verbs defined in RFC 2616
371 <http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9>: OPTIONS,
372 GET, HEAD, POST, PUT, DELETE, TRACE, CONNECT. Of these, the majority
373 of web applications focus on the verbs which closely map to the CRUD
374 (Create, Retrieve, Update, Delete) operations most database-driven
375 applications need to implement.
376 In addition, the "PATCH" verb was defined in RFC5789
378 <http://tools.ietf.org/html/rfc5789>, and is intended as a "partial
379 PUT", sending just the changes required to the entity in question. How
380 this would be handled is down to your app, it will vary depending on
381 the type of entity in question and the serialization in use.
382 Dancer2's keywords currently supports GET, PUT/PATCH, POST, DELETE,
384 OPTIONS which map to Retrieve, Update, Create, Delete respectively.
385 Let's take a look now at the "/add" route handler which handles a POST
386 operation.
387 post '/add' => sub {
389 if ( not session('logged_in') ) {
390 send_error("Not logged in", 401);
391 }
392 my $db = connect_db();
394 my $sql = 'insert into entries (title, text) values (?, ?)';
395 my $sth = $db->prepare($sql)
396 or die $db->errstr;
397 $sth->execute(
399 body_parameters->get('title'),
400 body_parameters->get('text')
401 ) or die $sth->errstr;
402 set_flash('New entry posted!');
404 redirect '/';
405 };
406 As before, the HTTP verb begins the handler, followed by the route, and
408 a subroutine to do something; in this case it will insert a new entry
409 into the database.
410 The first check in the subroutine is to make sure the user sending the
412 data is logged in. If not, the application returns an error and stops
413 processing. Otherwise, we have standard DBI stuff. Let me insert (heh,
414 heh) a blatant plug here for always, always using parameterized INSERTs
415 in your application SQL statements. It's the only way to be sure your
416 application won't be vulnerable to SQL injection. (See
417 <http://www.bobby-tables.com> for correct INSERT examples in multiple
418 languages.) Here we're using the "body_parameters" convenience method
419 to pull in the parameters in the current HTTP request. (You can see the
420 'title' and 'text' form parameters in the show_entries.tt template
421 above.) Those values are inserted into the database, then we set a
422 flash message for the user and redirect her back to the root URL.
423 It's worth mentioning that the "flash message" is not part of Dancer2,
425 but a part of this specific application. We need to implement it
426 ourself.
427 Logins and sessions
429 Dancer2 comes with a simple in-memory session manager out of the box.
430 It supports a bunch of other session engines including YAML, memcached,
431 browser cookies and others. We'll just stick with the in-memory model
432 which works great for development and tutorials, but won't persist
433 across server restarts or scale very well in "real world" production
434 scenarios.
435 Configuration options
437 To use sessions in our application, we have to tell Dancer2 to activate
439 the session handler and initialize a session manager. To do that, we
440 add some configuration directives toward the top of our 'dancr.pl'
441 file. But there are more options than just the session engine we want
442 to set.
443 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
445 set 'session' => 'Simple';
446 set 'template' => 'template_toolkit';
447 set 'logger' => 'console';
448 set 'log' => 'debug';
449 set 'show_stacktrace' => 1;
450 set 'startup_info' => 1;
451 Hopefully these are fairly self-explanatory. We want the Simple session
453 engine, the Template Toolkit template engine, logging enabled (at the
454 'debug' level with output to the console instead of a file), we want to
455 show errors to the web browser and prints a banner at the server start
456 with information such as versions and the environment.
457 Dancer2 doesn't impose any limits on what parameters you can set using
459 the "set" syntax. For this application we're going to embed our single
460 username and password into the application itself:
461 set 'username' => 'admin';
463 set 'password' => 'password';
464 Hopefully no one will ever guess our clever password! Obviously, you
466 will want a more sophisticated user authentication scheme in any sort
467 of non-tutorial application but this is good enough for our purposes.
468 In Part II of our tutorial, we will show you how to use Dancer2's
470 configuration files to manage these options and set up different
471 environments for your app using different configuration files. For now,
472 we're going to keep it simple and leave that discussion for later.
473 Logging in
475 Now that dancr is configured to handle sessions, let's take a look at
477 the URL handler for the "/login" route.
478 any ['get', 'post'] => '/login' => sub {
480 my $err;
481 if ( request->method() eq "POST" ) {
483 # process form input
484 if ( body_parameters->get('username') ne setting('username') ) {
485 $err = "Invalid username";
486 }
487 elsif ( body_parameters->get('password') ne setting('password') ) {
488 $err = "Invalid password";
489 }
490 else {
491 session 'logged_in' => true;
492 set_flash('You are logged in.');
493 return redirect '/';
494 }
495 }
496 # display login form
498 template 'login.tt', {
499 err => $err,
500 };
501 };
502 This is the first handler which accepts two different verb types, a GET
504 for a human browsing to the URL and a POST for the browser to submit
505 the user's input to the web application. Since we're handling two
506 different verbs, we check to see what verb is in the request. If it's
507 not a POST, we drop down to the "template" directive and display the
508 login.tt template:
509 <h2>Login</h2>
511 [% IF err %]<p class=error><strong>Error:</strong> [% err %][% END %]
512 <form action="[% login_url %]" method=post>
513 <dl>
514 <dt>Username:
515 <dd><input type=text name=username>
516 <dt>Password:
517 <dd><input type=password name=password>
518 <dd><input type=submit value=Login>
519 </dl>
520 </form>
521 This is even simpler than our show_entries.tt template–but wait–
523 there's a "login_url" template parameter and we're only passing in the
524 "err" parameter. Where's the missing parameter? It's being generated
525 and sent to the template in a "before_template_render" directive, we'll
526 come back to that in a moment or two.
527 So the user fills out the login.tt template and submits it back to the
529 "/login" route handler. We now check the user input against our
530 application settings and if the input is incorrect, we alert the user,
531 otherwise the application starts a session and sets the "logged_in"
532 session parameter to the true() value. Dancer2 exports both a true()
533 and false() convenience method which we use here. After that, it's
534 another flash message and back to the root URL handler.
535 Logging out
537 And finally, we need a way to clear our user's session with the
539 customary logout procedure.
540 get '/logout' => sub {
542 app->destroy_session;
543 set_flash('You are logged out.');
544 redirect '/';
545 };
546 "app->destroy_session;" is Dancer2's way to remove a stored session.
548 We notify the user she is logged out and route her back to the root URL
549 once again.
550 You might wonder how we can then set a value in the session in
552 "set_flash", because we just destroyed the session.
553 Destroying the session has removed the data from the persistence layer
555 (which is the memory of our running application, because we are using
556 the "simple" session engine). If we write to the session now, it will
557 actually create a completely new session for our user. This new, empty
558 session will have a new session ID, which Dancer2 tells the user's
559 browser about in the response. When the browser requests the root URL,
560 it will send this new session ID to our application.
561 Layout and static files
563 We still have a missing puzzle piece or two. First, how can we use
564 Dancer2 to serve our CSS stylesheet? Second, where are flash messages
565 displayed? Third, what about the "before_template_render" directive?
566 Serving static files
568 In Dancer2, static files should go into the "public/" directory, but in
570 the application itself be sure to omit the "public/" element from the
571 path. For example, the stylesheet for dancr lives in
572 "dancr/public/css/style.css" but is served from
573 <http://localhost:3000/css/style.css>.
574 If you wanted to build a mostly static web site you could simply write
576 route handlers like this one:
577 get '/' => sub {
579 send_file 'index.html';
580 };
581 where index.html would live in your "public/" directory.
583 "send_file" does exactly what it says: it loads a static file, then
585 sends the contents of that file to the user.
586 Let's go ahead and create our style sheet. In the same directory as
588 your dancr.pl script, issue the following commands:
589 mkdir public && mkdir public/css && touch public/css/style.css
591 Next add the following css to the "public/css/style.css" file you just
593 created:
594 body { font-family: sans-serif; background: #eee; }
596 a, h1, h2 { color: #377ba8; }
597 h1, h2 { font-family: 'Georgia', serif; margin: 0; }
598 h1 { border-bottom: 2px solid #eee; }
599 h2 { font-size: 1.2em; }
600 .page { margin: 2em auto; width: 35em; border: 5px solid #ccc;
602 padding: 0.8em; background: white; }
603 .entries { list-style: none; margin: 0; padding: 0; }
604 .entries li { margin: 0.8em 1.2em; }
605 .entries li h2 { margin-left: -1em; }
606 .add-entry { font-size: 0.9em; border-bottom: 1px solid #ccc; }
607 .add-entry dl { font-weight: bold; }
608 .metanav { text-align: right; font-size: 0.8em; padding: 0.3em;
609 margin-bottom: 1em; background: #fafafa; }
610 .flash { background: #cee5F5; padding: 0.5em;
611 border: 1px solid #aacbe2; }
612 .error { background: #f0d6d6; padding: 0.5em; }
613 Be sure to save the file.
615 Layouts
617 I mentioned earlier in the tutorial that it is possible to create a
619 "layout" template. In dancr, that layout is called "main" and it's set
620 up by putting in a directive like this:
621 set layout => 'main';
623 near the top of your web application. This tells Dancer2's template
625 engine that it should look for a file called main.tt in
626 "views/layouts/" and insert the calls from the "template" directive
627 into a template parameter called "content".
628 Here is the simple layout file we will use for this web application. Go
630 ahead and add this the main.tt file to the "views/layouts/" directory.
631 <!doctype html>
633 <html>
634 <head>
635 <title>dancr</title>
636 <link rel=stylesheet type=text/css href="[% css_url %]">
637 </head>
638 <body>
639 <div class=page>
640 <h1>dancr</h1>
641 <div class=metanav>
642 [% IF not session.logged_in %]
643 <a href="[% login_url %]">log in</a>
644 [% ELSE %]
645 <a href="[% logout_url %]">log out</a>
646 [% END %]
647 </div>
648 [% IF msg %]
649 <div class=flash> [% msg %] </div>
650 [% END %]
651 [% content %]
652 </div>
653 </body>
654 </html>
655 Aha! You now see where the flash message "msg" parameter gets rendered.
657 You can also see where the content from the specific route handlers is
658 inserted (the fourth line from the bottom in the "content" template
659 parameter).
660 But what about all those other *_url template parameters?
662 Using "before_template_render"
664 Dancer2 has a way to manipulate the template parameters before they're
666 passed to the engine for processing. It's "before_template_render".
667 Using this keyword, you can generate and set the URIs for the "/login"
668 and "/logout" route handlers and the URI for the stylesheet. This is
669 handy for situations like this where there are values which are re-used
670 consistently across all (or most) templates. This cuts down on code-
671 duplication and makes your app easier to maintain over time since you
672 only need to update the values in this one place instead of everywhere
673 you render a template.
674 hook before_template_render => sub {
676 my $tokens = shift;
677 $tokens->{'css_url'} = request->base . 'css/style.css';
679 $tokens->{'login_url'} = uri_for('/login');
680 $tokens->{'logout_url'} = uri_for('/logout');
681 };
682 Here again I'm using "uri_for" instead of hardcoding the routes. This
684 code block is executed before any of the templates are processed so
685 that the template parameters have the appropriate values before being
686 rendered.
687 Putting it all together
689 Here's the complete 'dancr.pl' script from start to finish.
690 use Dancer2;
692 use DBI;
693 use File::Spec;
694 use File::Slurper qw/ read_text /;
695 use Template;
696 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
698 set 'session' => 'Simple';
699 set 'template' => 'template_toolkit';
700 set 'logger' => 'console';
701 set 'log' => 'debug';
702 set 'show_stacktrace' => 1;
703 set 'startup_info' => 1;
704 set 'username' => 'admin';
705 set 'password' => 'password';
706 set 'layout' => 'main';
707 sub set_flash {
709 my $message = shift;
710 session flash => $message;
712 }
713 sub get_flash {
715 my $msg = session('flash');
716 session->delete('flash');
717 return $msg;
719 }
720 sub connect_db {
722 my $dbh = DBI->connect("dbi:SQLite:dbname=".setting('database'))
723 or die $DBI::errstr;
724 return $dbh;
726 }
727 sub init_db {
729 my $db = connect_db();
730 my $schema = read_text('./schema.sql');
731 $db->do($schema)
732 or die $db->errstr;
733 }
734 hook before_template_render => sub {
736 my $tokens = shift;
737 $tokens->{'css_url'} = request->base . 'css/style.css';
739 $tokens->{'login_url'} = uri_for('/login');
740 $tokens->{'logout_url'} = uri_for('/logout');
741 };
742 get '/' => sub {
744 my $db = connect_db();
745 my $sql = 'select id, title, text from entries order by id desc';
746 my $sth = $db->prepare($sql)
748 or die $db->errstr;
749 $sth->execute
751 or die $sth->errstr;
752 template 'show_entries.tt', {
754 msg => get_flash(),
755 add_entry_url => uri_for('/add'),
756 entries => $sth->fetchall_hashref('id'),
757 };
758 };
759 post '/add' => sub {
761 if ( not session('logged_in') ) {
762 send_error("Not logged in", 401);
763 }
764 my $db = connect_db();
766 my $sql = 'insert into entries (title, text) values (?, ?)';
767 my $sth = $db->prepare($sql)
769 or die $db->errstr;
770 $sth->execute(
772 body_parameters->get('title'),
773 body_parameters->get('text')
774 ) or die $sth->errstr;
775 set_flash('New entry posted!');
777 redirect '/';
778 };
779 any ['get', 'post'] => '/login' => sub {
781 my $err;
782 if ( request->method() eq "POST" ) {
784 # process form input
785 if ( body_parameters->get('username') ne setting('username') ) {
786 $err = "Invalid username";
787 }
788 elsif ( body_parameters->get('password') ne setting('password') ) {
789 $err = "Invalid password";
790 }
791 else {
792 session 'logged_in' => true;
793 set_flash('You are logged in.');
794 return redirect '/';
795 }
796 }
797 # display login form
799 template 'login.tt', {
800 err => $err,
801 };
802 };
804 get '/logout' => sub {
806 app->destroy_session;
807 set_flash('You are logged out.');
808 redirect '/';
809 };
810 init_db();
812 start;
813 Advanced route moves
815 There's a lot more to route matching than shown here. For example, you
817 can match routes with regular expressions, or you can match pieces of a
818 route like "/hello/:name" where the ":name" piece magically turns into
819 a named parameter in your handler for manipulation.
820 You can explore this and other advanced concepts by reading the
822 Dancer2::Manual.
823
825 In Part I, we took an ordinary Perl script and turned it into a simple
826 web app to teach you basic Dancer2 concepts. While starting with a
827 simple script like this helped make it easier to teach these concepts,
828 it did not demonstrate how a typical app is built by a Dancer2
829 developer. So let's show you how things really get done.
830 Creating a new app
832 So now that you have a better idea of what goes into building an app
833 with Dancer2, it's time to cha-cha with the "dancer2" utility which
834 will save you a lot of time and effort by setting up directories,
835 files, and default configuration settings for you.
836 The "dancer2" utility was installed on your machine when you installed
838 the Dancer2 distribution. Hop over to the command line into a directory
839 you have permission to write to and issue the following command:
840 dancer2 gen -a Dancr2
842 That command should output something like the following to the console:
844 + Dancr2
846 + Dancr2/config.yml
847 + Dancr2/Makefile.PL
848 + Dancr2/MANIFEST.SKIP
849 + Dancr2/.dancer
850 + Dancr2/cpanfile
851 + Dancr2/bin
852 + Dancr2/bin/app.psgi
853 + Dancr2/environments
854 + Dancr2/environments/development.yml
855 + Dancr2/environments/production.yml
856 + Dancr2/lib
857 + Dancr2/lib/Dancr2.pm
858 + Dancr2/public
859 + Dancr2/public/favicon.ico
860 + Dancr2/public/500.html
861 + Dancr2/public/dispatch.cgi
862 + Dancr2/public/404.html
863 + Dancr2/public/dispatch.fcgi
864 + Dancr2/public/css
865 + Dancr2/public/css/error.css
866 + Dancr2/public/css/style.css
867 + Dancr2/public/images
868 + Dancr2/public/images/perldancer.jpg
869 + Dancr2/public/images/perldancer-bg.jpg
870 + Dancr2/public/javascripts
871 + Dancr2/public/javascripts/jquery.js
872 + Dancr2/t
873 + Dancr2/t/001_base.t
874 + Dancr2/t/002_index_route.t
875 + Dancr2/views
876 + Dancr2/views/index.tt
877 + Dancr2/views/layouts
878 + Dancr2/views/layouts/main.tt
879 What you just did was create a fully functional app in Dancer2 with
881 just one command! The new app, named "Dancr2," won't do anything
882 particularly useful until you add your own routes to it, but it does
883 take care of many of the tedious tasks of setting up an app for you.
884 The files and folders that were generated and that you see listed above
886 provide a convenient scaffolding, or skeleton, upon which you can build
887 your app. The default skelelton provides you with basic error pages,
888 css, javascript, graphics, tests, templates and other files which you
889 are free to modify and customize to your liking.
890 If you don't like the default skeleton provided to you by Dancer, the
892 "dancer2" command allows you to generate your own custom skeletons.
893 Consult "BOOTSTRAPPING-A-NEW-APP" in Dancer2::Manual for further
894 details on this and other capabilities of the "dancer2") utility.
895 Getting the new app up and running with Plack
897 In Part I, we used the "start" command in our script to launch a server
898 to serve our app. Things are a little different when using "dancer2",
899 however. You'll notice that the "dancer2" utility created a "bin/"
900 directory with a file in it called "app.psgi". This is the file we use
901 to get our app up and running.
902 Let's see how to to do that by first changing into the Dancr2 directory
904 and then starting the server using the "plackup" command:
905 cd Dancr2;
907 plackup -p 5000 bin/app.psgi
908 If all went well, you'll be able to see the Dancr2 home page by
910 visiting:
911 http://localhost:5000
913 The web page you see there gives you some very basic advice for tuning
915 and modifying your app and where you can go for more information to
916 learn about developing apps with Dancer2 (like this handy tutorial!).
917 Our Dancr2 app is served on a simple web server provided by Plack.
919 Plack is PSGI compliant software, hence the "psgi" extension for our
920 file in the "bin/" directory. Plack and PSGI is beyond the scope of
921 this tutorial but you can learn more by visiting the Plack website
922 <http://plackperl.org/>.
923 For now, all you need to know is that if you are deploying an app for
925 use by just yourself or a handful of people on a local network, Plack
926 alone may do the trick. More typically, you would use Plack in
927 conjunction with other server software to make your app much more
928 robust. But in the early stages of your app's development, a simple
929 Plack server is more than likely all you need.
930 To learn more about the different ways for deploying your app, see the
932 Dancer2 Deployment Manual
933 Porting dancr.pl over to the new Dancr2 app
935 Ok, so now that we've got our new Dancr2 app up and running, it's time
936 to learn how to take advantage of what the "dancer2" utility set up for
937 us by porting our dancr.pl script created in Part I into Dancr2.
938 The "lib/" directory
940 The "lib/" directory in our Dancr2 app is where our "app.psgi" file
942 will expect our code to live. So let's take a peek at the file
943 generated for us in there:
944 cat lib/Dancr2.pm
946 You'll see something like the following bit of code which provides a
948 single route to our app's home page and loads the index template:
949 package Dancr2;
951 use Dancer2;
952 our $VERSION = '0.1';
954 get '/' => sub {
956 template 'index' => { title => 'Dancr2' };
957 };
958 true;
960 The first thing you'll notice is that instead of a script, we are using
962 a module, "Dancr2" to package our code. Modules make it easer to pull
963 off many powerful tricks like packaging our app across several discrete
964 modules. We'll let the manual explain this more advanced technique.
965 Updating the Dancr2 module
967 Now that we know where to put our code, let's update the "Dancr2.pm"
969 module with our original "dancr.pl" code. Remove the existing sample
970 route in "Dancr2.pm" and replace it with the code from our "dancr.pl"
971 file. You'll have to make a couple of adjustments to the "dancr.pl"
972 code like removing the "use Dancer2;" line since it's already provided
973 by our module. You'll also want to be sure to remove the "start;" line
974 as well from the end of the file.
975 When you're done, "Dancr2.pm" should look something close to this:
977 package Dancr2;
979 use Dancer2;
980 our $VERSION = '0.1';
982 # Our original dancr.pl code with some minor tweaks
984 use DBI;
985 use File::Spec;
986 use File::Slurper qw/ read_text /;
987 use Template;
988 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
990 set 'session' => 'YAML';
991 ...
992 <snip> # The rest of the stuff </snip>
994 ...
996 sub init_db {
998 my $schema = read_text('./schema.sql');
999 $db->do($schema)
1000 or die $db->errstr;
1001 }
1002 get '/logout' => sub {
1004 app->destroy_session;
1005 set_flash('You are logged out.');
1006 redirect '/';
1007 };
1008 init_db();
1010 Finally, to avoid getting an error in the "init_db") subroutine when it
1012 tries to load our schema file, copy over the "schema.db" file to the
1013 root directory of the Dancr2 app:
1014 cp /path/to/dancr.pl/schema.db /path/to/Dancr2;
1016 Ok, now that we've got the code moved over, let's move the assets from
1018 dancr.pl to our new app.
1019 The "public/" directory
1021 As mentioned in Part I, our static assets go into our "public/"
1023 directory. If you followed along with the tutorial in Part I, you
1024 should have a "public/" directory with a "public/css" subdirectory and
1025 a file called "style.css" within that.
1026 Dancer2 has conveniently generated the "public/css" directory for us
1028 which has a default css file. Let's copy the style sheet from our
1029 original app so our new app can use it:
1030 # Note: This command overwrites the default style sheet. Move it or copy
1032 # it if you wish to preserve it.
1033 cp /path/to/dancr.pl/public/css/style.css /path/to/Dancr2/public/css;
1035 The "views" directory
1037 Along with our "public/" directory, Dancer has also provided a "views/"
1039 directory, which as we covered, serves as the a home for our templates.
1040 Let's get those copied over now:
1041 # NOTE: This command will overwrite the default main.tt tempalte file. Move
1043 # it or copy it if you wish to preserve it.
1044 cp -r /path/to/dancr.pl/views/* /path/to/Dancr2/views;
1046 Does it work?
1048 If you followed the instructions here closely, your Dancr2 app should
1050 be working. Shut down any running Plack servers and then issue the
1051 same plackup command to see if it runs:
1052 cd /path/to/Dancr2
1054 plackup -p 5000 bin/app.psgi
1055 If you see any errors, get them resolved until the app loads.
1057 Configuring Your App
1059 In Part I, you configured your app with a series of "set" statements
1060 near the top of your file. Now we will show you a better way to
1061 configure your app using Dancer2's configuration files.
1062 Your skeleton provides your app with three different configuration
1064 files. The first two files we'll discuss, found in the "environments/"
1065 folder of your app, are "development.yml" and "production.yml". As you
1066 can probably guess, the "development.yml" file has settings intended to
1067 be used while developing the app. The "production.yml" file has
1068 settings more appropriate for running your app when used by others. The
1069 third configuration file is found in the root directory of your app and
1070 is named "config.yml". This file has the settings that are common to
1071 all environments but that can be overridden by the environment
1072 configuration files. You can still override any configuration file
1073 settings in your modules using the "set" command.
1074 We will take a look at the "development.yml" file first. Open that file
1076 in your text editor and take a look inside. It has a bunch of helpful
1077 comments and the following five settings sprinkled throughout:
1078 logger: "console"
1080 log: "core"
1081 show_stacktrace: 1
1082 startup_info: 1
1083 The first four settings duplicate many of the settings in our new
1085 Dancr2 app. So in the spirit of DRY (don't repeat yourself), edit your
1086 Dancr2 module and delete the four lines that correspond to these four
1087 settings.
1088 Then, in the configuration file, be sure to change the value for the
1090 "log" setting from "core" to "debug" so it matches the value we had in
1091 our module.
1092 We will leave it up to you what you want to do with the fourth setting,
1094 "startup_info". You can read about that setting, along with all the
1095 other settings, in the configuration manual.
1096 Finally, let's add a new setting to the configuration file for
1098 "session" with the following line:
1099 session: "Simple"
1101 Then delete the corresponding setting from your Dancr2 module.
1103 Alright, our Dancr2 app is a little leaner and meaner. Now open the
1105 main "config.yml" file and look for the settings in there that are also
1106 duplicated in our app's module. There are two:
1107 layout: "main"
1109 template: "simple"
1110 Leave "layout" as is but change the template setting to
1112 "template_toolkit". Then edit your Dancr2 module file and delete these
1113 two settings.
1114 Finally, add the following configuration settings to the .yml file:
1116 username: "admin"
1118 password: "password"
1119 Then you delete these two settings from the Dancr2 module, as well.
1121 So, if you have been following along, you now have only the following
1123 "set" command in your Dancr2 module, related to the database
1124 configuration:
1125 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
1127 We will get rid of this setting in Part III of the tutorial. All the
1129 rest of the settings have been transferred to our configuration files.
1130 Nice!
1131 We still have a little more cleanup we can do. Now that Dancer2 knows
1133 we are using Template::Toolkit, we can delete the "use Template;" line
1134 from our module.
1135 Now start the app "plackup" command and check to see that everything
1137 works. By default, Dancer2 will load the development environment
1138 configuration. When it comes time to put your app into production, you
1139 can load the "production.yml" file configuration with plackup's "--env"
1140 switch like so:
1141 plackup -p 5000 --env production bin/app.psgi
1143 Keep on Dancing!
1145 This concludes Part II of our tutorial where we showed you how to take
1146 advantage of the "dancer2" utility to set up a app skeleton to make it
1147 really easy to get started developing your own apps.
1148 Part III will refine our app a little further by showing you how to use
1150 plugins so you can start capitalizing on all the great work contributed
1151 by other Dancer2 developers.
1152
1154 Dancer2 takes advantage of the open source software revolution by
1155 making it exceedingly easy to use plugins that you can mix into your
1156 app to give it new functionality. In Part III of this tutorial, we will
1157 update our new Dancr2 app to use the Dancer2::Plugin::Database to give
1158 you enough skills to go out and explore other plugins on your own.
1159 Installing plugins
1161 Like Dancer2 itself, Dancer2 plugins can be found on the CPAN. Use your
1162 favorite method for downloading and installing the
1163 Dancer2::Plugin::Database module on your machine. We recommend using
1164 "cpanminus" like so:
1165 cpanm Dancer2::Plugin::Database
1167 Using plugins
1169 Using a plugin couldn't be easier. Simply add the following line to
1170 your Dancr2 module below the "use Dancer2;" line in your module:
1171 use Dancer2::Plugin::Database;
1173 Configuring plugins
1175 Plugins can be configured with the YAML configuration files mentioned
1176 in Part II of this tutorial. Let's edit the "development.yml" file and
1177 add our database configuration there. Below the last line in that file,
1178 add the following lines, being careful to keep the indentation as you
1179 see it here:
1180 plugins: # all plugin configuration settings go in this section
1182 Database: # the name of our plugin
1183 driver: "SQLite" # driver we want to use
1184 database: "dancr.db" # where the database will go in our app
1185 # run a query when connecting to the datbase:
1186 on_connect_do: [ "create table if not exists entries (id integer primary key autoincrement, title string not null, text string not null)" ]
1187 Here, we direct our database plugin to use the "SQLite" driver and to
1189 place the database in the root directory of our Dancr2. The
1190 "on_connect_db" setting tells the plugin to run an SQL query when it
1191 connects with the database to create a table for us if it doesn't
1192 already exist.
1193 Modifying our database code in the Dancr2 module
1195 Now it's time to modify our Dancr2 module so it will use the plugin to
1196 query the database instead of our own code. There are a few things to
1197 do. First, we will delete the code we no longer need.
1198 Since our configuration file tells the plugin where our database is, we
1200 can delete this line:
1201 set 'database' => File::Spec->catfile(File::Spec->tmpdir(), 'dancr.db');
1203 And since the database plugin will create our database connection and
1205 initialize our database for us, we can scrap the following two
1206 subroutines and line from our module:
1207 sub connect_db {
1209 my $dbh = DBI->connect("dbi:SQLite:dbname=".setting('database'))
1210 or die $DBI::errstr;
1211 return $dbh;
1213 }
1214 sub init_db {
1216 my $db = connect_db();
1217 my $schema = read_text('./schema.sql');
1218 $db->do($schema)
1219 or die $db->errstr;
1220 }
1221 init_db(); # Found at the bottom of our file
1223 With that done, let's now take advantage of a hook the plugin provides
1225 us that we can use to handle certain events by adding the following
1226 command to our module to handle database errors:
1227 hook 'database_error' => sub {
1229 my $error = shift;
1230 die $error;
1231 };
1232 Now let's make a few adjustments to the bits of code that make the
1234 database queries. In our "get '/'" route, change all instances of $db
1235 with "database" and remove all the "die" calls since we now have a hook
1236 to handle the errors for us. When you are done, your route should look
1237 something like this:
1238 get '/' => sub {
1240 my $sql = 'select id, title, text from entries order by id desc';
1241 my $sth = database->prepare($sql);
1242 $sth->execute;
1243 template 'show_entries.tt', {
1244 msg => get_flash(),
1245 add_entry_url => uri_for('/add'),
1246 entries => $sth->fetchall_hashref('id'),
1247 };
1248 };
1249 Make the same changes to the "post '/add'" route to transform it into
1251 this:
1252 post '/add' => sub {
1254 if ( not session('logged_in') ) {
1255 send_error("Not logged in", 401);
1256 }
1257 my $sql = 'insert into entries (title, text) values (?, ?)';
1259 my $sth = database->prepare($sql);
1260 $sth->execute(
1261 body_parameters->get('title'),
1262 body_parameters->get('text')
1263 );
1264 set_flash('New entry posted!');
1266 redirect '/';
1267 };
1268 Our last step is to get rid of the following lines which we no longer
1270 need, thanks to our plugin:
1271 use DBI;
1273 use File::Spec;
1274 use File::Slurper qw/ read_text /;
1275 That's it! Now start your app with "plackup" to make sure you don't get
1277 any errors and then point your browser to test the app to make sure it
1278 works as expected. If it doesn't, double and triple check your
1279 configuration settings and your module's code which should now look
1280 like this:
1281 package Dancr2;
1283 use Dancer2;
1284 use Dancer2::Plugin::Database;
1285 our $VERSION = '0.1';
1287 my $flash;
1289 sub set_flash {
1291 my $message = shift;
1292 $flash = $message;
1294 }
1295 sub get_flash {
1297 my $msg = $flash;
1298 $flash = "";
1299 return $msg;
1301 }
1302 hook before_template_render => sub {
1304 my $tokens = shift;
1305 $tokens->{'css_url'} = request->base . 'css/style.css';
1307 $tokens->{'login_url'} = uri_for('/login');
1308 $tokens->{'logout_url'} = uri_for('/logout');
1309 };
1310 hook 'database_error' => sub {
1312 my $error = shift;
1313 die $error;
1314 };
1315 get '/' => sub {
1317 my $sql = 'select id, title, text from entries order by id desc';
1318 my $sth = database->prepare($sql);
1319 $sth->execute;
1320 template 'show_entries.tt', {
1321 msg => get_flash(),
1322 add_entry_url => uri_for('/add'),
1323 entries => $sth->fetchall_hashref('id'),
1324 };
1325 };
1326 post '/add' => sub {
1328 if ( not session('logged_in') ) {
1329 send_error("Not logged in", 401);
1330 }
1331 my $sql = 'insert into entries (title, text) values (?, ?)';
1333 my $sth = database->prepare($sql);
1334 $sth->execute(
1335 body_parameters->get('title'),
1336 body_parameters->get('text')
1337 );
1338 set_flash('New entry posted!');
1340 redirect '/';
1341 };
1342 any ['get', 'post'] => '/login' => sub {
1344 my $err;
1345 if ( request->method() eq "POST" ) {
1347 # process form input
1348 if ( params->{'username'} ne setting('username') ) {
1349 $err = "Invalid username";
1350 }
1351 elsif ( params->{'password'} ne setting('password') ) {
1352 $err = "Invalid password";
1353 }
1354 else {
1355 session 'logged_in' => true;
1356 set_flash('You are logged in.');
1357 return redirect '/';
1358 }
1359 }
1360 # display login form
1362 template 'login.tt', {
1363 err => $err,
1364 };
1365 };
1367 get '/logout' => sub {
1369 app->destroy_session;
1370 set_flash('You are logged out.');
1371 redirect '/';
1372 };
1373 true;
1375 Next steps
1377 Congrats! You are now using the database plugin like a boss. The
1378 database plugin does a lot more than what we showed you here. We'll
1379 leave it up to you to consult the Dancer2::Plugin::Database to unlock
1380 its full potential.
1381 There are many more plugins for you to explore. You now know enough to
1383 install and experiment with them. Some of the more popular and useful
1384 plugins are listed at Dancer2::Plugins. You can also search CPAN with
1385 "Dancer2::Plugin" for a more comprehensive listing.
1386 If you are feeling really inspired, you can learn how to extend Dancer2
1388 with your own plugins by reading Dancer2::Plugin.
1389
1391 I hope these tutorials have been helpful and interesting enough to get
1392 you exploring Dancer2 on your own. The framework is still under
1393 development but it's definitely mature enough to use in a production
1394 project.
1395 Happy dancing!
1397 One more thing: Test!
1399 Before we go, we want to mention that Dancer2 makes it very easy to run
1400 automated tests on your app to help you find bugs. If you are new to
1401 testing, we encourage you to start learning how. Your future self will
1402 thank you. The effort you put into creating tests for your app will
1403 save you many hours of frustration in the long run. Unfortunately,
1404 until we get Part IV of this tutorial written, you'll have to consult
1405 the Dancer2 testing documentation for more details on how to test your
1406 app.
1407 Enjoy!
1409
1411 • <http://perldancer.org>
1412 • <http://github.com/PerlDancer/Dancer2>
1414 • Dancer2::Plugins
1416
1418 The CSS stylesheet is copied verbatim from the Flaskr example
1419 application and is subject to their license:
1420 Copyright (c) 2010, 2013 by Armin Ronacher and contributors.
1422 Some rights reserved.
1424 Redistribution and use in source and binary forms of the software as
1426 well as documentation, with or without modification, are permitted
1427 provided that the following conditions are met:
1428 • Redistributions of source code must retain the above copyright
1430 notice, this list of conditions and the following disclaimer.
1431 • Redistributions in binary form must reproduce the above copyright
1433 notice, this list of conditions and the following disclaimer in the
1434 documentation and/or other materials provided with the
1435 distribution.
1436 • The names of the contributors may not be used to endorse or promote
1438 products derived from this software without specific prior written
1439 permission.
1440
1442 Dancer Core Developers
1443
1445 This software is copyright (c) 2023 by Alexis Sukrieh.
1446 This is free software; you can redistribute it and/or modify it under
1448 the same terms as the Perl 5 programming language system itself.
1449perl v5.38.0 2023-07-20 Dancer2::Tutorial(3)