Dancer::Cookbook(3) User Contributed Perl Documentation Dancer::Cookbook(3)NAMEDancer::Cookbook - a quick-start guide to the Dancer web framework
DESCRIPTION
A quick-start guide with examples to get you up and running with the
Dancer web framework.
BEGINNER'S DANCE
Your first Dancer web app
Dancer has been designed to be easy to work with - it's trivial to
write a simple web app, but still has the power to work with larger
projects. To start with, let's make an incredibly simple "Hello World"
example:
#!/usr/bin/perl
use Dancer;
get '/hello/:name' => sub {
return "Why, hello there " . params->{name};
};
dance;
Yes - the above is a fully-functioning web app; running that script
will launch a webserver listening on the default port (3000); now you
can make a request
$ curl http://localhost:3000/hello/Bob
Why, hello there Bob
(or the name of the machine you ran it on, if it's not your local
system), and it will say hello. The ":name" part is a named parameter
within the route specification, whose value is made available through
"params" - more on that later.
Note that you don't need to use the "strict" and "warnings" pragma,
they are already loaded by Dancer.
Starting a Dancer project
The first simple example is fine for trivial projects, but for anything
more complex, you'll want a more maintainable solution - enter the
"dancer" helper script, which will build the framework of your
application with a single command:
$ dancer -a mywebapp
+ mywebapp
+ mywebapp/config.yml
+ mywebapp/environments
+ mywebapp/environments/development.yml
+ mywebapp/environments/production.yml
+ mywebapp/views
+ mywebapp/views/index.tt
+ mywebapp/views/layouts
+ mywebapp/views/layouts/main.tt
+ mywebapp/mywebapp.pl
+ mywebapp/lib
+ mywebapp/lib/mywebapp.pm
+ mywebapp/public
+ mywebapp/public/css
+ mywebapp/public/css/style.css
+ mywebapp/public/css/error.css
+ mywebapp/public/images
+ mywebapp/public/404.html
+ mywebapp/public/dispatch.fcgi
+ mywebapp/public/dispatch.cgi
+ mywebapp/public/500.html
+ mywebapp/Makefile.PL
+ mywebapp/t
+ mywebapp/t/002_index_route.t
+ mywebapp/t/001_base.t
As you can see, it creates a directory named after the name of the app,
along with a configuration file, a views directory (where your
templates and layouts will live), an environments directory (where
environment-specific settings live), a module containing the actual
guts of your application, a script to start it - or to run your web app
via Plack/PSGI - more on that later.
DANCE ROUTINES: ROUTES
Declaring routes
To control what happens when a web request is received by your webapp,
you'll need to declare "routes". A route declaration indicates which
HTTP method(s) it is valid for, the path it matches (e.g. /foo/bar),
and a coderef to execute, which returns the response.
get '/hello/:name' => sub {
return "Hi there " . params->{name};
};
The above route specifies that, for GET requests to '/hello/...', the
code block provided should be executed.
Handling multiple HTTP request methods
Routes can use "any" to match all, or a specified list of HTTP methods.
The following will match any HTTP request to the path /myaction:
any '/myaction' => sub {
# code
}
The following will match GET or POST requests to /myaction:
any ['get', 'post'] => '/myaction' => sub {
# code
};
For convenience, any route which matches GET requests will also match
HEAD requests.
Retrieving request parameters
The "params" method returns a hashref of request parameters; these will
be parameters supplied on the query string, within the path itself
(with named placeholders), and, for HTTTP POST requests, the content of
the POST body.
Named parameters in route path declarations
As seen above, you can use ":somename" in a route's path to capture
part of the path; this will become available by calling "params".
So, for a web app where you want to display information on a company,
you might use something like:
get '/company/view/:companyid' => sub {
my $company_id = params->{companyid};
# Look up the company and return appropriate page
};
Wildcard path matching and splat
You can also declare wildcards in a path, and retrieve the values they
matched with "splat":
get '/*/*' => sub {
my ($action, $id) = splat;
if (my $action eq 'view') {
return display_item($id);
} elsif ($action eq 'delete') {
return delete_item($id);
} else {
status 'not_found';
return "What?";
}
};
Before filters - processed before a request
A "before" filter declares code which should be handled before a
request is passed to the appropriate route.
before sub {
var note => 'Hi there';
request->path('/foo/oversee')
};
get '/foo/*' => sub {
my ($match) = splat; # 'oversee';
vars->{note}; # 'Hi there'
};
The above declares a before filter which uses "var" to set a variable
which will later be available within the route handler, then amends the
path of the request to "/foo/oversee"; this means that, whatever path
was requested, it will be treated as though the path requested was
"/foo/oversee".
Default route
In case you want to avoid a 404 error, or handle multiple routes in the
same way and you don't feel like configuring all of them, you can set
up a default route handler.
The default route handler will handle any request that doesn't get
served by any other route.
All you need to do is set up the following route as the last route:
any qr{.*} => sub {
status 'not_found';
template 'special_404', { path => request->path };
};
Then you can set up the template as such:
You tried to reach <% path %>, but it is unavailable at the moment.
Please try again or contact us at our email at <...>.
Using the auto_page feature for automatic route creation
For simple "static" pages, you can simply enable the "auto_page" config
setting; this means that you need not declare a route handler for those
pages; if a request is for "/foo/bar", Dancer will check for a matching
view (e.g. "/foo/bar.tt" and render it with the default layout etc if
found. For full details, see the documentation for the auto_page
setting.
Why should I use the Ajax plugin
As an Ajax query is just a HTTP query, it's similar to a GET or POST
route. You may ask yourself why you may want to use the "ajax" keyword
(from the Dancer::Plugin::Ajax plugin) instead of a simple "get".
Let's say you have a path like '/user/:user' in your application. You
may want to be able to serve this page, with a layout and HTML content.
But you may also want to be able to call this same url from a
javascript query using Ajax.
So, instead of having the following code:
get '/user/:user' => sub {
if (request->is_ajax) {
# create xml, set headers to text/xml, blablabla
header('Content-Type' => 'text/xml');
header('Cache-Control' => 'no-store, no-cache, must-revalidate');
to_xml({...})
}else{
template users, {....}
}
};
you can have
get '/user/:user' => sub {
template users, {...}
}
and
ajax '/user/:user' => sub {
to_xml({...}, RootName => undef);
}
Because it's an ajax query, you know you need to return a xml content,
so the content type of the response is set for you.
Using the prefix feature to split your application
For better maintainability, you may want to separate some of your
application components to different packages. Let's say we have a
simple web app with an admin section, and want to maintain this in a
different package:
package myapp;
use Dancer ':syntax';
use myapp::admin;
prefix undef;
get '/' => sub {...};
1;
package myapp::admin;
use Dancer ':syntax';
prefix '/admin';
get '/' => sub {...};
1;
The following routes will be generated for us:
- get /
- get /admin/
- head /
- head /admin/
MUSCLE MEMORY: STORING DATA
Handling sessions
It's common to want to use sessions to give your web applications
state; for instance, allowing a user to log in, creating a session, and
checking that session on subsequent requests.
To make use of sessions, you must first enable the session engine -
pick the session engine you want to use, then declare it in your config
file: config file, add:
session: Simple
The Dancer::Session::Simple backend implements very simple in-memory
session storage. This will be fast and useful for testing, but
sessions do not persist between restarts of your app.
You can also use the Dancer::Session::YAML backend included with
Dancer, which stores session data on disc in YAML files (since YAML is
a nice human-readable format, it makes inspecting the contents of
sessions a breeze):
session: YAML
Or, to enable session support from within your code,
set session => 'YAML';
(Controlling settings is best done from your config file, though).
'YAML' in the example is the session backend to use; this is shorthand
for Dancer::Session::YAML. There are other session backends you may
wish to use, for instance Dancer::Session::Memcache, but the YAML
backend is a simple and easy to use example which stores session data
in a YAML file in sessions).
Storing data in the session
Storing data in the session is as easy as:
session varname => 'value';
Retrieving data from the session
Retrieving data from the session is as easy as:
session('varname')
Or, alternatively,
session->{varname}
Controlling where sessions are stored
For disc-based session back ends like Dancer::Session::YAML,
Dancer::Session::Storable etc, session files are written to the session
dir specified by the "session_dir" setting, which defaults to
"appdir/sessions" if not specifically set.
If you need to control where session files are created, you can do so
quickly and easily within your config file, for example:
session_dir: /tmp/dancer-sessions
If the directory you specify does not exist, Dancer will attempt to
create it for you.
Destroying a session
When you're done with your session, you can destroy it:
session->destroy
Sessions and logging in
A common requirement is to check the user is logged in, and, if not,
require them to log in before continuing.
This can easily be handled with a before filter to check their session:
before sub {
if (! session('user') && request->path_info !~ m{^/login}) {
var requested_path => request->path_info;
request->path_info('/login');
}
};
get '/login' => sub {
# Display a login page; the original URL they requested is available as
# vars->{requested_path}, so could be put in a hidden field in the form
template 'login', { path => vars->{requested_path} };
};
post '/login' => sub {
# Validate the username and password they supplied
if (params->{user} eq 'bob' && params->{pass} eq 'letmein') {
session user => params->{user};
redirect params->{path} || '/';
} else {
redirect '/login?failed=1';
}
};
In your login page template, you'll want a text field named user, a
password field named pass, and a hidden field named path, which will be
populated with the path originally requested, so that it's sent back in
the POST submission, and can be used by the post route to redirect
onwards to the page originally requested once you're logged in.
Of course, you'll probably want to validate your users against a
database table, or maybe via IMAP/LDAP/SSH/POP3/local system accounts
via PAM etc. Authen::Simple is probably a good starting point here!
A simple working example of handling authentication against a database
table yourself (using Dancer::Plugin::Database which provides the
"database" keyword, and Crypt::SaltedHash to handle salted hashed
passwords (well, you wouldn't store your users passwords in the clear,
would you?)) follows:
post '/login' => sub {
my $user = database->quick_select('users',
{ username => params->{user} }
);
if (!$user) {
warning "Failed login for unrecognised user " . params->{user};
redirect '/login?failed=1';
} else {
if (Crypt::SaltedHash->validate($user->{password}, params->{pass}))
{
debug "Password correct";
# Logged in successfully
session user => $user;
redirect params->{path} || '/';
} else {
debug("Login failed - password incorrect for " . params->{user});
redirect '/login?failed=1';
}
}
};
APPEARANCE
Using templates - views and layouts
Returning plain content is all well and good for examples or trivial
apps, but soon you'll want to use templates to maintain separation
between your code and your content. Dancer makes this easy.
Views
It's possible to render the action's content with a template, this is
called a view. The `appdir/views' directory is the place where views
are located.
You can change this location by changing the setting 'views'.
By default, the internal template engine Dancer::Template::Simple is
used, but you may want to upgrade to Template::Toolkit. If you do so,
you have to enable this engine in your settings as explained in
Dancer::Template::TemplateToolkit. If you do so, you'll also have to
import the Template module in your application code.
Note that, by default, Dancer configures the Template::Toolkit engine
to use "<% %"> brackets instead of its default "[% %]" brackets. You
can change this by using the following in your config file:
template: template_toolkit
engines:
template_toolkit:
start_tag: '[%'
stop_tag: '%]'
All views must have a '.tt' extension. This may change in the future.
In order to render a view, just call the "template" keyword at the end
of the action by giving the view name and the HASHREF of tokens to
interpolate in the view (note that for convenience, the request,
session, params and vars are automatically accessible in the view,
named request, session, params and vars) - for example:
before => sub { var time => localtime() };
get '/hello/:name' => sub {
my $name = params->{name};
template 'hello.tt', { name => $name };
};
The template 'hello.tt' could contain, for example:
<p>Hi there, <% name %>!</p>
<p>You're using <% request.user_agent %></p>
<% IF session.username %>
<p>You're logged in as <% session.username %>
<% END %>
It's currently <% vars.time %>
For a full list of the tokens automatically added to your template
(like "session", "request" and "vars", refer to
Dancer::Template::Abstract).
Layouts
A layout is a special view, located in the 'layouts' directory (inside
the views directory) which must have a token named 'content'. That
token marks the place where to render the action view. This lets you
define a global layout for your actions, and have each individual view
contain only the specific content. This is a good thing to avoid lots
of needless duplication of HTML :)
Here is an example of a layout: "views/layouts/main.tt" :
<html>
<head>...</head>
<body>
<div id="header">
...
</div>
<div id="content">
<% content %>
</div>
</body>
</html>
You can tell your app which layout to use with "layout: name" in the
config file, or within your code:
set layout => 'main';
You can control which layout to use (or whether to use a layout at all)
for a specific request without altering the layout setting by passing
an options hashref as the third param to the template keyword:
template 'index.tt', {}, { layout => undef };
If your application is not mounted under root (/), you can use a
before_template instead of hardcoding the path to your application for
your css, images and javascript:
before_template sub {
my $tokens = shift;
$tokens->{uri_base} = request->base->path;
};
THen in your layout, modify your css inclusion as follows:
<link rel="stylesheet" href="<% uri_base %>/css/style.css" />
From now on, you can mount your application wherever you want, without
any further modification of the css inclusion
template and unicode
If you use Plack and have some unicode problem with your Dancer
application, don't forget to check if you have set your template engine
to use unicode, and set the default charset to UTF-8. So, if you are
using template toolkit, your config.yml will look like this:
charset: UTF-8
engines:
template_toolkit:
ENCODING: utf8
TT's WRAPPER directive in Dancer (META variables, SETs)
Dancer already provides a WRAPPER-like ability, which we call a
"layout". The reason we do not use TT's WRAPPER (which also makes it
incompatible with it) is because not all template systems support it.
Actually, most don't.
However, you might want to use it, and be able to define META variables
and regular Template::Toolkit variables.
These few steps will get you there:
· Disable the layout in Dancer
You can do this by simply commenting (or removing) the "layout"
configuration in the config.yml file.
· Use Template Toolkit template engine
Change the configuration of the template to Template Toolkit:
# in config.yml
template: "template_toolkit"
· Tell the Template Toolkit engine who's your wrapper
# in config.yml
# ...
engines:
template_toolkit:
WRAPPER: layouts/main.tt
Done! Everything will work fine out of the box, including variables and
META variables.
SETTING THE STAGE: CONFIGURATION AND LOGGING
Configuration and environments
Configuring a Dancer application can be done in many ways. The easiest
one (and maybe the the dirtiest) is to put all your settings statements
at the top of your script, before calling the dance() method.
Other ways are possible, you can define all your settings in the file
`appdir/config.yml'. For this, you must have installed the YAML module,
and of course, write the config file in YAML.
That's better than the first option, but it's still not perfect as you
can't switch easily from an environment to another without rewriting
the config.yml file.
The better way is to have one config.yml file with default global
settings, like the following:
# appdir/config.yml
logger: 'file'
layout: 'main'
And then write as many environment files as you like in
"appdir/environments". That way, the appropriate environment config
file will be loaded according to the running environment (if none is
specified, it will be 'development').
Note that you can change the running environment using the
"--environment" commandline switch.
Typically, you'll want to set the following values in a development
config file:
# appdir/environments/development.yml
log: 'debug'
startup_info: 1
show_errors: 1
And in a production one:
# appdir/environments/production.yml
log: 'warning'
startup_info: 0
show_errors: 0
Accessing configuration information from your app
A Dancer application can use the 'config' keyword to easily access the
settings within its config file, for instance:
get '/appname' => sub {
return "This is " . config->{appname};
};
This makes keeping your application's settings all in one place simple
and easy - you shouldn't need to worry about implementing all that
yourself :)
Accessing configuration information from a separate script
You may well want to access your webapp's configuration from outside
your webapp. You could, of course, use the YAML module of your choice
and load your webapps's config.yml, but chances are that this is not
convenient.
Use Dancer instead. Without any ado, magic or too big jumps, you can
use the values from config.yml and some additional default values:
# bin/script1.pl
use Dancer ':syntax';
print "template:".config->{template}."\n"; #simple
print "log:".config->{log}."\n"; #undef
Note that config->{log} should result undef error on a default scaffold
since you did not load the environment and in the default scaffold log
is defined in the environment and not in config.yml. Hence undef.
If you want to load an environment you need to tell Dancer where to
look for it. One way to do so, is to tell Dancer where the webapp
lives. From there Dancer deducts where the config.yml file is
(typically $webapp/config.yml).
# bin/script2.pl
use FindBin;
use Cwd qw/realpath/;
use Dancer ':syntax';
#tell the Dancer where the app lives
my $appdir=realpath( "$FindBin::Bin/..");
Dancer::Config::setting('appdir',$appdir);
Dancer::Config::load();
#getter
print "environment:".config->{environment}."\n"; #development
print "log:".config->{log}."\n"; #value from development environment
By default Dancer loads development environment (typically
$webapp/environment/development.yml). In contrast to the example
before, you do have a value from the development environment
(environment/development.yml) now. Also note that in the above example
Cwd and FindBin are used. They are likely to be already loaded by
Dancer anyways, so it's not a big overhead. You could just as well hand
over a simple path for the app if you like that better, e.g.:
Dancer::Config::setting('appdir','/path/to/app/dir');
If you want to load an environment other than the default, try this:
# bin/script2.pl
use Dancer ':syntax';
#tell the Dancer where the app lives
Dancer::Config::setting('appdir','/path/to/app/dir');
#which environment to load
config->{environment}='production';
Dancer::Config::load();
#getter
print "log:".config->{log}."\n"; #has value from production environment
By the way, you not only get values, you can also set values
straightforward like we do above with
config->{environment}='production'. Of course, this value does not get
written in any file; it only lives in memory and your webapp doesn't
have access to it, but you can use it inside your script.
Logging
Configuring logging
It's possible to log messages generated by the application and by
Dancer itself.
To start logging, select the logging engine you wish to use with the
"logger" setting; Dancer includes built-in log engines named "file" and
"console", which log to a logfile and to the console respectively.
To enable logging to a file, add the following to your config.yml:
logger: 'file'
Then you can choose which kind of messages you want to actually log:
log: 'core' # will log debug, warning and errors, and messages from
# Dancer itself
log: 'debug' # will log debug, warning and errors
log: 'warning' # will log warning and errors
log: 'error' # will log only errors
If you're using the "file" logging engine, a directory "appdir/logs"
will be created and will host one logfile per environment. The log
message contains the time it was written, the PID of the current
process, the message and the caller information (file and line).
Logging your own messages
Just call "debug", "warning" or "error" with your message:
debug "This is a debug message from my app.";
RESTING
Writing a REST application
With Dancer, it's easy to write REST applications. Dancer provides
helpers to serialize and deserialize for the following data formats:
JSON
YAML
XML
Data::Dumper
To activate this feature, you only have to set the "serializer" setting
to the format you require, for instance in your config.yml:
serializer: JSON
Or right in your code:
set serializer => 'JSON';
From now, all hash ref or array ref returned by a route will be
serialized to the format you chose, and all data received from POST or
PUT requests will be automatically deserialized.
get '/hello/:name' => sub {
# this structure will be returned to the client as
# {"name":"$name"}
return {name => params->{name}};
};
It's possible to let the client choose which serializer he want to use.
For this, use the mutable serializer, and an appropriate serializer
will be chosen from the Content-Type header.
It's also possible to return custom error, using the "send_error"
function. When you don't use a serializer, the "send_error" function
will take a string as first parameter (the message), and an optional
HTTP code. When using a serializer, the message can be a string, an
arrayref or a hashref:
get '/hello/:name' => sub {
if (...) {
send_error("you can't do that");
# or
send_error({reason => 'access denied', message => "no"});
}
};
The content of the error will be serialized using the appropriate
serializer.
Deploying your Dancer applications
For examples on deploying your Dancer applications (including
standalone, behind proxy/load-balancing software, and using common web
servers including Apache to run via CGI/FastCGI etc, see
Dancer::Deployment.
DANCER ON THE STAGE: DEPLOYMENT
Plack middlewares
If you deploy with Plack and use some Plack middlewares, you can enable
them directly from Dancer's configuration files.
Generic middlewares
To enable middlewares in Dancer, you just have to set the
plack_middlewares setting like the following:
set plack_middlewares => [
[ 'SomeMiddleware' => [ qw(some options for somemiddleware) ]],
];
For instance, if you want to enable Plack::Middleware::Debug in your
Dancer application, all you have to do is to set "plack_middlewares"
like that:
set plack_middlewares => [
[ 'Debug' => [ 'panels' => qw(DBITrace Memory Timer) ]],
];
Of course, you can also put this configuration into your config.yml
file, or even in your environment configuration files:
# environments/development.yml
...
plack_middlewares:
-
- Debug # first element of the array is the name of the middleware
- panels # following elements are the configuration ofthe middleware
-
- DBITrace
- Memory
- Timer
Path-based middlewares
If you want to setup a middleware for a specific path, you can do that
using "plack_middlewares_map". You'll need Plack::App::URLMap to do
that.
plack_middlewares_map:
'/': ['Debug']
'/timer': ['Timer'],
AUTHORS
Dancer contributors - see AUTHORS file.
perl v5.14.1 2011-07-26 Dancer::Cookbook(3)
