NAME

Dancer::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.

RECIPES

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); you can point your browser at http://localhost:3000/hello/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
    + [D] mywebapp 
    + [F] mywebapp/config.yml
    + [D] mywebapp/views
    + [D] mywebapp/views/layouts
    + [F] mywebapp/views/layouts/main.tt
    + [F] mywebapp/views/index.tt
    + [D] mywebapp/environments
    + [F] mywebapp/environments/production.yml
    + [F] mywebapp/environments/development.yml
    + [F] mywebapp/mywebapp.pm
    + [F] mywebapp/mywebapp.pl
    + [F] mywebapp/app.psgi

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, and an app.psgi file - this will only be of interest if you're running your web app via Plack/PSGI - more on that later.

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.

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 - in your config file, add:

    session: "YAML"

Or, to enable it 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}

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
    };

    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';
        }
    };

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 is used Dancer::Template::Simple 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 and route params are automatically accessible in the view, named request, session and params) - for example:

    get '/hello/:name' => sub {
        my $name = params->{name};
        my $browser = request->{user_agent};
        template 'hello.tt', { name => $name, $browser => $browser };
    };

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 %>

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:

    layout 'main';

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'
    access_log: 1
    show_errors: 1

And in a production one:

    # appdir/environments/production.yml
    log: 'warning'
    access_log: 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 :)

Logging

Configuring logging

It's possible to log messages sent by the application. In the current version, only logging to a simple file is available, but later versions may provide additional logging options.

In order to enable the logging system for your application, you first have to start the logger engine in your config.yml

    log: 'file'

Then you can choose which kind of messages you want to actually log:

    log: 'debug'     # will log debug, warning and errors
    log: 'warning'   # will log warning and errors
    log: 'error'     # will log only errors

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.";

AUTHORS

Dancer contributors - see AUTHORS file.

cpanm Dancer

perl -MCPAN -e shell
install Dancer