
=encoding utf8

=head1 NAME

Mojolicious::Guides::Rendering - Rendering content

=head1 OVERVIEW

This document explains content generation with the L<Mojolicious> renderer.

=head1 CONCEPTS

Essentials every L<Mojolicious> developer should know.

=head2 Renderer

The renderer is a tiny black box turning stash data into actual responses
utilizing multiple template systems and data encoding modules.

  {text => 'Hello.'}                 -> 200 OK, text/html, 'Hello.'
  {json => {x => 3}}                 -> 200 OK, application/json, '{"x":3}'
  {text => 'Oops.', status => '410'} -> 410 Gone, text/html, 'Oops.'

Templates can be automatically detected if enough information is provided by
the developer or routes. Template names are expected to follow the
C<template.format.handler> scheme, with C<template> defaulting to
C<controller/action> or the route name, C<format> defaulting to C<html> and
C<handler> to C<ep>.

  {controller => 'users', action => 'list'} -> 'users/list.html.ep'
  {template => 'foo', format => 'txt'}      -> 'foo.txt.ep'
  {template => 'foo', handler => 'epl'}     -> 'foo.html.epl'

The C<controller> value gets converted from C<CamelCase> to C<snake_case> using
L<Mojo::Util/"decamelize"> and C<-> characters replaced with C</>.

  {controller => 'My::Users', action => 'add'} -> 'my/users/add.html.ep'
  {controller => 'my-users', action => 'show'} -> 'my/users/show.html.ep'

All templates should be in the C<templates> directories of the application,
which can be customized with L<Mojolicious::Renderer/"paths">, or one of the
the C<DATA> sections from L<Mojolicious::Renderer/"classes">.

  __DATA__

  @@ time.html.ep
  % use Time::Piece;
  % my $now = localtime;
  <!DOCTYPE html>
  <html>
    <head><title>Time</title></head>
    <body>The time is <%= $now->hms %>.</body>
  </html>

  @@ hello.txt.ep
  ...

The renderer can be easily extended to support additional template systems with
plugins, but more about that later.

=head2 Embedded Perl

L<Mojolicious> includes a minimalistic but very powerful template system out of
the box called Embedded Perl or C<ep> for short. It is based on
L<Mojo::Template> and allows the embedding of Perl code right into actual
content using a small set of special tags and line start characters. For all
templates L<strict>, L<warnings>, L<utf8> and Perl 5.10 L<features|feature> are
automatically enabled.

  <% Perl code %>
  <%= Perl expression, replaced with XML escaped result %>
  <%== Perl expression, replaced with result %>
  <%# Comment, useful for debugging %>
  <%% Replaced with "<%", useful for generating templates %>
  % Perl code line, treated as "<% line =%>" (explained later)
  %= Perl expression line, treated as "<%= line %>"
  %== Perl expression line, treated as "<%== line %>"
  %# Comment line, useful for debugging
  %% Replaced with "%", useful for generating templates

Tags and lines work pretty much the same, but depending on context one will
usually look a bit better. Semicolons get automatically appended to all
expressions.

  <% my $i = 10; %>
  <ul>
    <% for my $j (1 .. $i) { %>
      <li>
        <%= $j %>
      </li>
    <% } %>
  </ul>

  % my $i = 10;
  <ul>
    % for my $j (1 .. $i) {
      <li>
        %= $j
      </li>
    % }
  </ul>

Aside from differences in whitespace handling, both examples generate similar
Perl code, a naive translation could look like this.

  my $output = '';
  my $i = 10;
  $output .= '<ul>';
  for my $j (1 .. $i) {
    $output .= '<li>';
    $output .= xml_escape scalar + $j;
    $output .= '</li>';
  }
  $output .= '</ul>';
  return $output;

An additional equal sign can be used to disable escaping of the characters
C<E<lt>>, C<E<gt>>, C<&>, C<'> and C<"> in results from Perl expressions, which
is the default to prevent XSS attacks against your application.

  <%= 'I ♥ Mojolicious!' %>
  <%== '<p>I ♥ Mojolicious!</p>' %>

Only L<Mojo::ByteStream> objects are excluded from automatic escaping.

  <%= b('<p>I ♥ Mojolicious!</p>') %>

Whitespace characters around tags can be trimmed by adding an additional equal
sign to the end of a tag.

  <% for (1 .. 3) { %>
    <%= 'Trim all whitespace characters around this expression' =%>
  <% } %>

Newline characters can be escaped with a backslash.

  This is <%= 1 + 1 %> a\
  single line

And a backslash in front of a newline character can be escaped with another
backslash.

  This will <%= 1 + 1 %> result\\
  in multiple\\
  lines

A newline character gets appended automatically to every template, unless the
last character is a backslash. And empty lines at the end of a template are
ignored.

  There is <%= 1 + 1 %> no newline at the end here\

At the beginning of the template, stash values that don't have invalid
characters in their name get automatically initialized as normal variables, and
the controller object as both C<$self> and C<$c>.

  $c->stash(name => 'tester');

  Hello <%= $name %> from <%= $c->tx->remote_address %>.

A prefix like C<myapp.*> is commonly used for stash values that you don't want
to expose in templates.

  $c->stash('myapp.name' => 'tester');

There are also many helper functions available, but more about that later.

  <%= dumper {foo => 'bar'} %>

=head1 BASICS

Most commonly used features every L<Mojolicious> developer should know about.

=head2 Automatic rendering

The renderer can be manually started by calling the method
L<Mojolicious::Controller/"render">, but that's usually not necessary, because
it will get automatically called if nothing has been rendered after the router
finished its work. This also means you can have routes pointing only to
templates without actual actions.

  $c->render;

There is one big difference though, by calling it manually you can make sure
that templates use the current controller object, and not the default
controller specified with the attribute L<Mojolicious/"controller_class">.

  $c->render_later;

You can also disable automatic rendering with the method
L<Mojolicious::Controller/"render_later">, which can be very useful to delay
rendering when a non-blocking operation has to be performed first.

=head2 Rendering templates

The renderer will always try to detect the right template, but you can also use
the C<template> stash value to render a specific one. Everything before the
last slash will be interpreted as the subdirectory path in which to find the
template.

  # foo/bar/baz.*.*
  $c->render(template => 'foo/bar/baz');

Choosing a specific C<format> and C<handler> is just as easy.

  # foo/bar/baz.txt.epl
  $c->render(template => 'foo/bar/baz', format => 'txt', handler => 'epl');

Because rendering a specific template is the most common task it also has a
shortcut.

  $c->render('foo/bar/baz');

If you're not sure in advance if a template actually exists, you can also use
the method L<Mojolicious::Controller/"render_maybe"> to try multiple
alternatives.

  $c->render_maybe('localized/baz') or $c->render('foo/bar/baz');

=head2 Rendering to strings

Sometimes you might want to use the rendered result directly instead of
generating a response, for example, to send emails, this can be done with
L<Mojolicious::Controller/"render_to_string">.

  my $html = $c->render_to_string('mail');

No encoding will be performed, making it easy to reuse the result in other
templates or to generate binary data.

  my $pdf = $c->render_to_string('invoice', format => 'pdf');
  $c->render(data => $pdf, format => 'pdf');

All arguments passed will get localized automatically and are only available
during this render operation.

=head2 Template variants

To make your application look great on many different devices you can also use
the C<variant> stash value to choose between different variants of your
templates.

  # foo/bar/baz.html+phone.ep
  # foo/bar/baz.html.ep
  $c->render('foo/bar/baz', variant => 'phone');

This can be done very liberally since it only applies when a template with the
correct name actually exists and falls back to the generic one otherwise.

=head2 Rendering inline templates

Some renderers such as C<ep> allow templates to be passed C<inline>.

  $c->render(inline => 'The result is <%= 1 + 1 %>.');

Since auto-detection depends on a path you might have to supply a C<handler>
too.

  $c->render(inline => "<%= shift->param('foo') %>", handler => 'epl');

=head2 Rendering text

Characters can be rendered to bytes with the C<text> stash value, the given
content will be automatically encoded with L<Mojolicious::Renderer/"encoding">.

  $c->render(text => 'I ♥ Mojolicious!');

=head2 Rendering data

Bytes can be rendered with the C<data> stash value, no encoding will be
performed.

  $c->render(data => $bytes);

=head2 Rendering JSON

The C<json> stash value allows you to pass Perl data structures to the renderer
which get directly encoded to JSON with L<Mojo::JSON>.

  $c->render(json => {foo => [1, 'test', 3]});

=head2 Status code

Response status codes can be changed with the C<status> stash value.

  $c->render(text => 'Oops.', status => 500);

=head2 Content type

The C<Content-Type> header of the response is actually based on the MIME type
mapping of the C<format> stash value.

  # Content-Type: text/plain
  $c->render(text => 'Hello.', format => 'txt');

  # Content-Type: image/png
  $c->render(data => $bytes, format => 'png');

These mappings can be easily extended or changed with L<Mojolicious/"types">.

  # Add new MIME type
  $app->types->type(md => 'text/markdown');

=head2 Stash data

Any of the native Perl data types can be passed to templates as references
through the L<Mojolicious::Controller/"stash">.

  $c->stash(description => 'web framework');
  $c->stash(frameworks  => ['Catalyst', 'Mojolicious']);
  $c->stash(spinoffs    => {minion => 'job queue'});

  %= $description
  %= $frameworks->[1]
  %= $spinoffs->{minion}

Since everything is just Perl normal control structures just work.

  % for my $framework (@$frameworks) {
    <%= $framework %> is a <%= $description %>.
  % }

  % if (my $description = $spinoffs->{minion}) {
    Minion is a <%= $description %>.
  % }

For templates that might get rendered in different ways and where you're not
sure if a stash value will actually be set, you can just use the helper
L<Mojolicious::Plugin::DefaultHelpers/"stash">.

  % if (my $spinoffs = stash 'spinoffs') {
    Minion is a <%= $spinoffs->{minion} %>.
  % }

=head2 Helpers

Helpers are little functions you can use in templates as well as application
and controller code.

  # Template
  %= dumper [1, 2, 3]

  # Application
  my $serialized = $app->dumper([1, 2, 3]);

  # Controller
  my $serialized = $c->dumper([1, 2, 3]);

We differentiate between default helpers, which are more general purpose like
L<Mojolicious::Plugin::DefaultHelpers/"dumper">, and tag helpers like
L<Mojolicious::Plugin::TagHelpers/"link_to">, which are template specific and
mostly used to generate HTML tags.

  %= link_to Mojolicious => 'http://mojolicious.org'

In controllers you can also use the method L<Mojolicious::Controller/"helpers">
to fully qualify helper calls and ensure that they don't conflict with existing
methods you may already have.

  my $serialized = $c->helpers->dumper([1, 2, 3]);

A list of all built-in helpers can be found in
L<Mojolicious::Plugin::DefaultHelpers> and L<Mojolicious::Plugin::TagHelpers>.

=head2 Content negotiation

For resources with different representations and that require truly RESTful
content negotiation you can also use L<Mojolicious::Controller/"respond_to">
instead of L<Mojolicious::Controller/"render">.

  # /hello (Accept: application/json) -> "json"
  # /hello (Accept: application/xml)  -> "xml"
  # /hello.json                       -> "json"
  # /hello.xml                        -> "xml"
  # /hello?format=json                -> "json"
  # /hello?format=xml                 -> "xml"
  $c->respond_to(
    json => {json => {hello => 'world'}},
    xml  => {text => '<hello>world</hello>'}
  );

The best possible representation will be automatically selected from the
C<Accept> request header, C<format> stash value or C<format> C<GET>/C<POST>
parameter and stored in the C<format> stash value. To change MIME type mappings
for the C<Accept> request header or the C<Content-Type> response header you can
use L<Mojolicious/"types">.

  $c->respond_to(
    json => {json => {hello => 'world'}},
    html => sub {
      $c->content_for(head => '<meta name="author" content="sri">');
      $c->render(template => 'hello', message => 'world')
    }
  );

Callbacks can be used for representations that are too complex to fit into a
single render call.

  # /hello (Accept: application/json) -> "json"
  # /hello (Accept: text/html)        -> "html"
  # /hello (Accept: image/png)        -> "any"
  # /hello.json                       -> "json"
  # /hello.html                       -> "html"
  # /hello.png                        -> "any"
  # /hello?format=json                -> "json"
  # /hello?format=html                -> "html"
  # /hello?format=png                 -> "any"
  $c->respond_to(
    json => {json => {hello => 'world'}},
    html => {template => 'hello', message => 'world'},
    any  => {text => '', status => 204}
  );

And if no viable representation could be found, the C<any> fallback will be
used or an empty C<204> response rendered automatically.

  # /hello                      -> "html"
  # /hello (Accept: text/html)  -> "html"
  # /hello (Accept: text/xml)   -> "xml"
  # /hello (Accept: text/plain) -> undef
  # /hello.html                 -> "html"
  # /hello.xml                  -> "xml"
  # /hello.txt                  -> undef
  # /hello?format=html          -> "html"
  # /hello?format=xml           -> "xml"
  # /hello?format=txt           -> undef
  if (my $format = $c->accepts('html', 'xml')) {
    ...
  }

For even more advanced negotiation logic you can also use the helper
L<Mojolicious::Plugin::DefaultHelpers/"accepts">.

=head2 Rendering C<exception> and C<not_found> pages

By now you've probably already encountered the built-in C<404> (Not Found) and
C<500> (Server Error) pages, that get rendered automatically when you make a
mistake. Those are fallbacks for when your own exception handling fails, which
can be especially helpful during development. You can also render them manually
with the helpers L<Mojolicious::Plugin::DefaultHelpers/"reply-E<gt>exception">
and L<Mojolicious::Plugin::DefaultHelpers/"reply-E<gt>not_found">.

  use Mojolicious::Lite;
  use Scalar::Util 'looks_like_number';

  get '/divide/:dividend/by/:divisor' => sub {
    my $c = shift;

    my $dividend = $c->param('dividend');
    my $divisor  = $c->param('divisor');

    # 404
    return $c->reply->not_found
      unless looks_like_number $dividend && looks_like_number $divisor;

    # 500
    return $c->reply->exception('Division by zero!') if $divisor == 0;

    # 200
    $c->render(text => $dividend / $divisor);
  };

  app->start;

You can also change the templates of those pages, since you most likely want to
show your users something more closely related to your application in
production. The renderer will always try to find C<exception.$mode.$format.*>
or C<not_found.$mode.$format.*> before falling back to the built-in default
templates.

  use Mojolicious::Lite;

  get '/dies' => sub { die 'Intentional error' };

  app->start;
  __DATA__

  @@ exception.production.html.ep
  <!DOCTYPE html>
  <html>
    <head><title>Server error</title></head>
    <body>
      <h1>Exception</h1>
      <p><%= $exception->message %></p>
      <h1>Stash</h1>
      <pre><%= dumper $snapshot %></pre>
    </body>
  </html>

The hook L<Mojolicious/"before_render"> makes even more advanced customizations
possible by allowing you to intercept and modify the arguments passed to the
renderer.

  use Mojolicious::Lite;

  hook before_render => sub {
    my ($c, $args) = @_;

    # Make sure we are rendering the exception template
    return unless my $template = $args->{template};
    return unless $template eq 'exception';

    # Switch to JSON rendering if content negotiation allows it
    $args->{json} = {exception => $args->{exception}} if $c->accepts('json');
  };

  get '/' => sub { die "This sho...ALL GLORY TO THE HYPNOTOAD!\n" };

  app->start;

=head2 Layouts

Most of the time when using C<ep> templates you will want to wrap your
generated content in an HTML skeleton, thanks to layouts that's absolutely
trivial.

  use Mojolicious::Lite;

  get '/' => {template => 'foo/bar'};

  app->start;
  __DATA__

  @@ foo/bar.html.ep
  % layout 'mylayout';
  Hello World!

  @@ layouts/mylayout.html.ep
  <!DOCTYPE html>
  <html>
    <head><title>MyApp</title></head>
    <body><%= content %></body>
  </html>

You just select the right layout template with the helper
L<Mojolicious::Plugin::DefaultHelpers/"layout"> and place the result of the
current template with the helper
L<Mojolicious::Plugin::DefaultHelpers/"content">. You can also pass along
normal stash values to the C<layout> helper.

  use Mojolicious::Lite;

  get '/' => {template => 'foo/bar'};

  app->start;
  __DATA__

  @@ foo/bar.html.ep
  % layout 'mylayout', title => 'Hi there';
  Hello World!

  @@ layouts/mylayout.html.ep
  <!DOCTYPE html>
  <html>
    <head><title><%= $title %></title></head>
    <body><%= content %></body>
  </html>

Instead of the C<layout> helper you could also just use the C<layout> stash
value, or call L<Mojolicious::Controller/"render"> with the C<layout> argument.

  $c->render(template => 'mytemplate', layout => 'mylayout');

To set a C<layout> stash value application-wide you can use
L<Mojolicious/"defaults">.

  $app->defaults(layout => 'mylayout');

Layouts can also be used with L<Mojolicious::Controller/"render_to_string">,
but the C<layout> value needs to be passed as a render argument (not a stash
value).

  my $html = $c->render_to_string('reminder', layout => 'mail');

=head2 Partial templates

You can break up bigger templates into smaller, more manageable chunks. These
partial templates can also be shared with other templates. Just use the helper
L<Mojolicious::Plugin::DefaultHelpers/"include"> to include one template into
another.

  use Mojolicious::Lite;

  get '/' => {template => 'foo/bar'};

  app->start;
  __DATA__

  @@ foo/bar.html.ep
  <!DOCTYPE html>
  <html>
    %= include 'header', title => 'Howdy'
    <body>Bar</body>
  </html>

  @@ header.html.ep
  <head><title><%= $title %></title></head>

=head2 Reusable template blocks

It's never fun to repeat yourself, that's why you can build reusable template
blocks in C<ep> that work very similar to normal Perl functions, with the
C<begin> and C<end> keywords. Just be aware that both keywords are part of the
surrounding tag and not actual Perl code, so there can only be whitespace after
C<begin> and before C<end>.

  use Mojolicious::Lite;

  get '/' => 'welcome';

  app->start;
  __DATA__

  @@ welcome.html.ep
  <% my $block = begin %>
    % my $name = shift;
    Hello <%= $name %>.
  <% end %>
  <%= $block->('Wolfgang') %>
  <%= $block->('Baerbel') %>

A naive translation of the template to Perl code could look like this.

  my $output = '';
  my $block  = sub {
    my $name   = shift;
    my $output = '';
    $output .= 'Hello ';
    $output .= xml_escape scalar + $name;
    $output .= '.';
    return Mojo::ByteStream->new($output);
  };
  $output .= xml_escape scalar + $block->('Wolfgang');
  $output .= xml_escape scalar + $block->('Baerbel');
  return $output;

While template blocks cannot be shared between templates, they are most
commonly used to pass parts of a template to helpers.

=head2 Adding helpers

You should always try to keep your actions small and reuse as much code as
possible. Helpers make this very easy, they get passed the current controller
object as first argument, and you can use them to do pretty much anything an
action could do.

  use Mojolicious::Lite;

  helper debug => sub {
    my ($c, $str) = @_;
    $c->app->log->debug($str);
  };

  get '/' => sub {
    my $c = shift;
    $c->debug('Hello from an action!');
  } => 'index';

  app->start;
  __DATA__

  @@ index.html.ep
  % debug 'Hello from a template!';

Helpers can also accept template blocks as last argument, this for example,
allows very pleasant to use tag helpers and filters. Wrapping the helper result
into a L<Mojo::ByteStream> object can prevent accidental double escaping.

  use Mojolicious::Lite;
  use Mojo::ByteStream;

  helper trim_newline => sub {
    my ($c, $block) = @_;
    my $result = $block->();
    $result =~ s/\n//g;
    return Mojo::ByteStream->new($result);
  };

  get '/' => 'index';

  app->start;
  __DATA__

  @@ index.html.ep
  %= trim_newline begin
    Some text.
    %= 1 + 1
    More text.
  % end

Similar to stash values, you can use a prefix like C<myapp.*> to keep helpers
from getting exposed in templates as functions, and to organize them into
namespaces as your application grows. Every prefix automatically becomes a
helper that returns a proxy object containing the current controller object and
on which you can call the nested helpers.

  use Mojolicious::Lite;

  helper 'cache_control.no_caching' => sub {
    my $c = shift;
    $c->res->headers->cache_control('private, max-age=0, no-cache');
  };

  helper 'cache_control.five_minutes' => sub {
    my $c = shift;
    $c->res->headers->cache_control('public, max-age=300');
  };

  get '/news' => sub {
    my $c = shift;
    $c->cache_control->no_caching;
    $c->render(text => 'Always up to date.');
  };

  get '/some_older_story' => sub {
    my $c = shift;
    $c->cache_control->five_minutes;
    $c->render(text => 'This one can be cached for a bit.');
  };

  app->start;

While helpers can also be redefined, this should only be done very carefully to
avoid conflicts.

=head2 Content blocks

The helper L<Mojolicious::Plugin::DefaultHelpers/"content_for"> allows you to
pass whole blocks of content from one template to another. This can be very
useful when your layout has distinct sections, such as sidebars, where content
should be inserted by the template.

  use Mojolicious::Lite;

  get '/' => 'foo';

  app->start;
  __DATA__

  @@ foo.html.ep
  % layout 'mylayout';
  % content_for header => begin
    <meta http-equiv="Content-Type" content="text/html">
  % end
  <div>Hello World!</div>
  % content_for header => begin
    <meta http-equiv="Pragma" content="no-cache">
  % end

  @@ layouts/mylayout.html.ep
  <!DOCTYPE html>
  <html>
    <head><%= content 'header' %></head>
    <body><%= content %></body>
  </html>

=head2 Forms

To build HTML forms more efficiently you can use tag helpers like
L<Mojolicious::Plugin::TagHelpers/"form_for">, which can automatically select a
request method for you if a route name is provided. And since most browsers
only allow forms to be submitted with C<GET> and C<POST>, but not request
methods like C<PUT> or C<DELETE>, they are spoofed with an C<_method> query
parameter.

  use Mojolicious::Lite;

  get '/' => 'form';

  # PUT  /nothing
  # POST /nothing?_method=PUT
  put '/nothing' => sub {
    my $c = shift;

    # Prevent double form submission with redirect
    my $value = $c->param('whatever');
    $c->flash(confirmation => "We did nothing with your value ($value).");
    $c->redirect_to('form');
  };

  app->start;
  __DATA__

  @@ form.html.ep
  <!DOCTYPE html>
  <html>
    <body>
      % if (my $confirmation = flash 'confirmation') {
        <p><%= $confirmation %></p>
      % }
      %= form_for nothing => begin
        %= text_field whatever => 'I ♥ Mojolicious!'
        %= submit_button
      % end
    </body>
  </html>

The methods L<Mojolicious::Controller/"flash"> and
L<Mojolicious::Controller/"redirect_to"> are often used together to prevent
double form submission, allowing users to receive a confirmation message that
will vanish if they decide to reload the page they've been redirected to.

=head2 Form validation

You can use L<Mojolicious::Controller/"validation"> to validate C<GET> and
C<POST> parameters submitted to your application. All unknown fields will be
ignored by default, so you have to decide which should be required or optional
before you can perform checks on their values. Every check is performed right
away, so you can use the results immediately to build more advanced validation
logic with methods like L<Mojolicious::Validator::Validation/"is_valid">.

  use Mojolicious::Lite;

  get '/' => sub {
    my $c = shift;

    # Check if parameters have been submitted
    my $validation = $c->validation;
    return $c->render unless $validation->has_data;

    # Validate parameters ("pass_again" depends on "pass")
    $validation->required('user')->size(1, 20)->like(qr/^[a-z0-9]+$/);
    $validation->required('pass_again')->equal_to('pass')
      if $validation->optional('pass')->size(7, 500)->is_valid;

    # Render confirmation if validation was successful
    $c->render('thanks') unless $validation->has_error;
  } => 'index';

  app->start;
  __DATA__

  @@ index.html.ep
  <!DOCTYPE html>
  <html>
    <head>
      <style>
        label.field-with-error { color: #dd7e5e }
        input.field-with-error { background-color: #fd9e7e }
      </style>
    </head>
    <body>
      %= form_for index => begin
        %= label_for user => 'Username (required, 1-20 characters, a-z/0-9)'
        <br>
        %= text_field 'user', id => 'user'
        %= submit_button
        <br>
        %= label_for pass => 'Password (optional, 7-500 characters)'
        <br>
        %= password_field 'pass', id => 'pass'
        <br>
        %= label_for pass_again => 'Password again (equal to the value above)'
        <br>
        %= password_field 'pass_again', id => 'pass_again'
      % end
    </body>
  </html>

  @@ thanks.html.ep
  <!DOCTYPE html>
  <html><body>Thank you <%= validation->param('user') %>.</body></html>

Form elements generated with tag helpers from
L<Mojolicious::Plugin::TagHelpers> will automatically remember their previous
values and add the class C<field-with-error> for fields that failed validation
to make styling with CSS easier.

  <label class="field-with-error" for="user">
    Username (required, only characters e-t)
  </label>
  <input class="field-with-error" type="text" name="user" value="sri">

For a full list of available checks see also
L<Mojolicious::Validator/"CHECKS">.

=head2 Adding form validation checks

Validation checks can be registered with L<Mojolicious::Validator/"add_check">
and return a false value if they were successful. A true value may be used to
pass along additional information which can then be retrieved with
L<Mojolicious::Validator::Validation/"error">.

  use Mojolicious::Lite;

  # Add "range" check
  app->validator->add_check(range => sub {
    my ($validation, $name, $value, $min, $max) = @_;
    return $value < $min || $value > $max;
  });

  get '/' => 'form';

  post '/test' => sub {
    my $c = shift;

    # Validate parameters with custom check
    my $validation = $c->validation;
    $validation->required('number')->range(3, 23);

    # Render form again if validation failed
    return $c->render('form') if $validation->has_error;

    # Prevent double form submission with redirect
    $c->flash(number => $validation->param('number'));
    $c->redirect_to('form');
  };

  app->start;
  __DATA__

  @@ form.html.ep
  <!DOCTYPE html>
  <html>
    <body>
      % if (my $number = flash 'number') {
        <p>Thanks, the number <%= $number %> was valid.</p>
      % }
      %= form_for test => begin
        % if (my $err = validation->error('number')) {
          <p>
            %= 'Value is required.' if $err->[0] eq 'required'
            %= 'Value needs to be between 3 and 23.' if $err->[0] eq 'range'
          </p>
        % }
        %= text_field 'number'
        %= submit_button
      % end
    </body>
  </html>

=head2 Cross-site request forgery

CSRF is a very common attack on web applications that trick your logged in
users to submit forms they did not intend to send, with something as mundane as
a link. All you have to do, to protect your users from this, is to add an
additional hidden field to your forms with
L<Mojolicious::Plugin::TagHelpers/"csrf_field">, and validate it with
L<Mojolicious::Validator::Validation/"csrf_protect">.

  use Mojolicious::Lite;

  get '/' => {template => 'target'};

  post '/' => sub {
    my $c = shift;

    # Check CSRF token
    my $validation = $c->validation;
    return $c->render(text => 'Bad CSRF token!', status => 403)
      if $validation->csrf_protect->has_error('csrf_token');

    my $city = $validation->required('city')->param('city');
    $c->render(text => "Low orbit ion cannon pointed at $city!")
      unless $validation->has_error;
  } => 'target';

  app->start;
  __DATA__

  @@ target.html.ep
  <!DOCTYPE html>
  <html>
    <body>
      %= form_for target => begin
        %= csrf_field
        %= label_for city => 'Which city to point low orbit ion cannon at?'
        %= text_field 'city', id => 'city'
        %= submit_button
      %= end
    </body>
  </html>

For Ajax requests and the like, you can also generate a token directly with the
helper L<Mojolicious::Plugin::DefaultHelpers/"csrf_token">, and then pass it
along with the C<X-CSRF-Token> request header.

=head1 ADVANCED

Less commonly used and more powerful features.

=head2 Template inheritance

Inheritance takes the layout concept above one step further, the helpers
L<Mojolicious::Plugin::DefaultHelpers/"content"> and
L<Mojolicious::Plugin::DefaultHelpers/"extends"> allow you to build skeleton
templates with named blocks that child templates can override.

  use Mojolicious::Lite;

  # first > mylayout
  get '/first' => {template => 'first', layout => 'mylayout'};

  # third > second > first > mylayout
  get '/third' => {template => 'third', layout => 'mylayout'};

  app->start;
  __DATA__

  @@ layouts/mylayout.html.ep
  <!DOCTYPE html>
  <html>
    <head><title>Hello</title></head>
    <body><%= content %></body>
  </html>

  @@ first.html.ep
  %= content header => begin
    Default header
  % end
  <div>Hello World!</div>
  %= content footer => begin
    Default footer
  % end

  @@ second.html.ep
  % extends 'first';
  % content header => begin
    New header
  % end

  @@ third.html.ep
  % extends 'second';
  % content footer => begin
    New footer
  % end

This chain could go on and on to allow a very high level of template reuse.

=head2 Serving static files

Static files are automatically served from the C<public> directories of the
application, which can be customized with L<Mojolicious::Static/"paths">, or one
of the C<DATA> sections from L<Mojolicious::Static/"classes">. And if that's not
enough you can also serve them manually with
L<Mojolicious::Plugin::DefaultHelpers/"reply-E<gt>static">.

  use Mojolicious::Lite;

  get '/' => sub {
    my $c = shift;
    $c->reply->static('index.html');
  };

  get '/some_download' => sub {
    my $c = shift;
    $c->res->headers->content_disposition('attachment; filename=bar.png;');
    $c->reply->static('foo/bar.png');
  };

  app->start;

=head2 Custom responses

Most response content, static as well as dynamic, gets served through
L<Mojo::Asset::File> and L<Mojo::Asset::Memory> objects. For somewhat static
content, like cached JSON data or temporary files, you can create your own and
use the helper L<Mojolicious::Plugin::DefaultHelpers/"reply-E<gt>asset"> to
serve them while allowing content negotiation to be performed with C<Range>,
C<If-Modified-Since> and C<If-None-Match> headers.

  use Mojolicious::Lite;

  get '/leak' => sub {
    my $c = shift;
    $c->res->headers->content_type('text/plain');
    $c->reply->asset(Mojo::Asset::File->new(path => '/etc/passwd'));
  };

  app->start;

For even more control you can also just skip the helper and use
L<Mojolicious::Controller/"rendered"> to tell the renderer when you're done
generating a response.

  use Mojolicious::Lite;

  get '/leak' => sub {
    my $c = shift;
    $c->res->headers->content_type('text/plain');
    $c->res->content->asset(Mojo::Asset::File->new(path => '/etc/passwd'));
    $c->rendered(200);
  };

  app->start;

=head2 Helper plugins

Some helpers might be useful enough for you to share them between multiple
applications, plugins make that very simple.

  package Mojolicious::Plugin::DebugHelper;
  use Mojo::Base 'Mojolicious::Plugin';

  sub register {
    my ($self, $app) = @_;
    $app->helper(debug => sub {
      my ($c, $str) = @_;
      $c->app->log->debug($str);
    });
  }

  1;

The C<register> method will be called when you load the plugin. And to add your
helper to the application, you can use L<Mojolicious/"helper">.

  use Mojolicious::Lite;

  plugin 'DebugHelper';

  get '/' => sub {
    my $c = shift;
    $c->debug('It works!');
    $c->render(text => 'Hello!');
  };

  app->start;

A skeleton for a full CPAN compatible plugin distribution can be automatically
generated.

  $ mojo generate plugin DebugHelper

And if you have a C<PAUSE> account (which can be requested at
L<http://pause.perl.org>), you are only a few commands away from releasing it
to CPAN.

  $ perl Makefile.PL
  $ make test
  $ make manifest
  $ make dist
  $ mojo cpanify -u USER -p PASS Mojolicious-Plugin-DebugHelper-0.01.tar.gz

=head2 Bundling assets with plugins

Assets such as templates and static files can be easily bundled with your
plugins, even if you plan to release them to CPAN.

  $ mojo generate plugin AlertAssets
  $ mkdir Mojolicious-Plugin-AlertAssets/lib/Mojolicious/Plugin/AlertAssets
  $ cd Mojolicious-Plugin-AlertAssets/lib/Mojolicious/Plugin/AlertAssets
  $ mkdir public
  $ echo 'alert("Hello World!");' > public/alertassets.js
  $ mkdir templates
  $ echo '%= javascript "/alertassets.js"' > templates/alertassets.html.ep

Just give them reasonably unique names, ideally based on the name of your
plugin, and append their respective directories to the list of search paths
when C<register> is called.

  package Mojolicious::Plugin::AlertAssets;
  use Mojo::Base 'Mojolicious::Plugin';

  use Mojo::File 'path';

  sub register {
    my ($self, $app) = @_;

    # Append "templates" and "public" directories
    my $base = path(__FILE__)->sibling('AlertAssets');
    push @{$app->renderer->paths}, $base->child('templates')->to_string;
    push @{$app->static->paths},   $base->child('public')->to_string;
  }

  1;

Both will work just like normal C<templates> and C<public> directories once
you've installed and loaded the plugin, with slightly lower precedence.

  use Mojolicious::Lite;

  plugin 'AlertAssets';

  get '/alert_me';

  app->start;
  __DATA__

  @@ alert_me.html.ep
  <!DOCTYPE html>
  <html>
    <head>
      <title>Alert me!</title>
      %= include 'alertassets'
    </head>
    <body>You've been alerted.</body>
  </html>

And it works just the same for assets bundled in the C<DATA> section of your
plugin.

  package Mojolicious::Plugin::AlertAssets;
  use Mojo::Base 'Mojolicious::Plugin';

  sub register {
    my ($self, $app) = @_;

    # Append class
    push @{$app->renderer->classes}, __PACKAGE__;
    push @{$app->static->classes},   __PACKAGE__;
  }

  1;
  __DATA__

  @@ alertassets.js
  alert("Hello World!");

  @@ alertassets.html.ep
  %= javascript "/alertassets.js"

=head2 Post-processing dynamic content

While post-processing tasks are generally very easy with the hook
L<Mojolicious/"after_dispatch">, for content generated by the renderer it is a
lot more efficient to use L<Mojolicious/"after_render">.

  use Mojolicious::Lite;
  use IO::Compress::Gzip 'gzip';

  hook after_render => sub {
    my ($c, $output, $format) = @_;

    # Check if "gzip => 1" has been set in the stash
    return unless $c->stash->{gzip};

    # Check if user agent accepts gzip compression
    return unless ($c->req->headers->accept_encoding // '') =~ /gzip/i;
    $c->res->headers->append(Vary => 'Accept-Encoding');

    # Compress content with gzip
    $c->res->headers->content_encoding('gzip');
    gzip $output, \my $compressed;
    $$output = $compressed;
  };

  get '/' => {template => 'hello', title => 'Hello', gzip => 1};

  app->start;
  __DATA__

  @@ hello.html.ep
  <!DOCTYPE html>
  <html>
    <head><title><%= title %></title></head>
    <body>Compressed content.</body>
  </html>

=head2 Streaming

You don't have to render all content at once, the method
L<Mojolicious::Controller/"write"> can also be used to stream a series of
smaller chunks.

  use Mojolicious::Lite;

  get '/' => sub {
    my $c = shift;

    # Prepare body
    my $body = 'Hello World!';
    $c->res->headers->content_length(length $body);

    # Start writing directly with a drain callback
    my $drain;
    $drain = sub {
      my $c = shift;
      my $chunk = substr $body, 0, 1, '';
      $drain = undef unless length $body;
      $c->write($chunk, $drain);
    };
    $c->$drain;
  };

  app->start;

The drain callback will be executed whenever the entire previous chunk of data
has actually been written.

  HTTP/1.1 200 OK
  Date: Sat, 13 Sep 2014 16:48:29 GMT
  Content-Length: 12
  Server: Mojolicious (Perl)

  Hello World!

=head2 Chunked transfer encoding

For very dynamic content you might not know the response content length in
advance, that's where the chunked transfer encoding and
L<Mojolicious::Controller/"write_chunk"> come in handy. A common use would be
to send the C<head> section of an HTML document to the browser in advance and
speed up preloading of referenced images and stylesheets.

  use Mojolicious::Lite;

  get '/' => sub {
    my $c = shift;
    $c->write_chunk('<html><head><title>Example</title></head>' => sub {
      my $c = shift;
      $c->finish('<body>Example</body></html>');
    });
  };

  app->start;

The optional drain callback ensures that all previous chunks have been written
before processing continues. To end the stream you can call
L<Mojolicious::Controller/"finish"> or write an empty chunk of data.

  HTTP/1.1 200 OK
  Date: Sat, 13 Sep 2014 16:48:29 GMT
  Transfer-Encoding: chunked
  Server: Mojolicious (Perl)

  29
  <html><head><title>Example</title></head>
  1b
  <body>Example</body></html>
  0

Especially in combination with long inactivity timeouts this can be very useful
for Comet (long polling). Due to limitations in some web servers this might not
work perfectly in all deployment environments.

=head2 Encoding

Templates stored in files are expected to be C<UTF-8> by default, but that can
be easily changed with L<Mojolicious::Renderer/"encoding">.

  $app->renderer->encoding('koi8-r');

All templates from the C<DATA> section are bound to the encoding of the Perl
script.

  use Mojolicious::Lite;

  get '/heart';

  app->start;
  __DATA__

  @@ heart.html.ep
  I ♥ Mojolicious!

=head2 Base64 encoded DATA files

Base64 encoded static files such as images can be easily stored in the C<DATA>
section of your application, similar to templates.

  use Mojolicious::Lite;

  get '/' => {text => 'I ♥ Mojolicious!'};

  app->start;
  __DATA__

  @@ favicon.ico (base64)
  ...base64 encoded image...

=head2 Inflating DATA templates

Templates stored in files get preferred over files from the C<DATA> section,
this allows you to include a default set of templates in your application that
the user can later customize. The command L<Mojolicious::Command::inflate> will
write all templates and static files from the C<DATA> section into actual files
in the C<templates> and C<public> directories.

  $ ./myapp.pl inflate

=head2 Customizing the template syntax

You can easily change the whole template syntax by loading
L<Mojolicious::Plugin::EPRenderer> with a custom configuration.

  use Mojolicious::Lite;

  plugin EPRenderer => {
    name     => 'mustache',
    template => {
      tag_start => '{{',
      tag_end   => '}}'
    }
  };

  get '/:name' => {name => 'Anonymous'} => 'index';

  app->start;
  __DATA__

  @@ index.html.mustache
  Hello {{= $name }}.

L<Mojo::Template> contains the whole list of available options.

=head2 Adding your favorite template system

Maybe you would prefer a different template system than C<ep>, which is provided
by L<Mojolicious::Plugin::EPRenderer>, and there is not already a plugin on CPAN
for your favorite one. All you have to do, is to add a new C<handler> with
L<Mojolicious::Renderer/"add_handler"> when C<register> is called.

  package Mojolicious::Plugin::MyRenderer;
  use Mojo::Base 'Mojolicious::Plugin';

  sub register {
    my ($self, $app) = @_;

    # Add "mine" handler
    $app->renderer->add_handler(mine => sub {
      my ($renderer, $c, $output, $options) = @_;

      # Check for one-time use inline template
      my $inline_template = $options->{inline};

      # Check for appropriate template in "templates" directories
      my $template_path = $renderer->template_path($options);

      # Check for appropriate template in DATA sections
      my $data_template = $renderer->get_data_template($options);

      # This part is up to you and your template system :)
      ...

      # Pass the rendered result back to the renderer
      $$output = 'Hello World!';

      # Or just die if an error occurs
      die 'Something went wrong with the template';
    });
  }

  1;

An C<inline> template, if provided by the user, will be passed along with the
options. You can use L<Mojolicious::Renderer/"template_path"> to search the
C<templates> directories of the application, and
L<Mojolicious::Renderer/"get_data_template"> to search the C<DATA> sections.

  use Mojolicious::Lite;

  plugin 'MyRenderer';

  # Render an inline template
  get '/inline' => {inline => '...', handler => 'mine'};

  # Render a template from the DATA section
  get '/data' => {template => 'test'};

  app->start;
  __DATA__

  @@ test.html.mine
  ...

=head2 Adding a handler to generate binary data

By default the renderer assumes that every C<handler> generates characters that
need to be automatically encoded, but this can be easily disabled if you're
generating bytes instead.

  use Mojolicious::Lite;
  use Storable 'nfreeze';

  # Add "storable" handler
  app->renderer->add_handler(storable => sub {
    my ($renderer, $c, $output, $options) = @_;

    # Disable automatic encoding
    delete $options->{encoding};

    # Encode data from stash value
    $$output = nfreeze delete $c->stash->{storable};
  });

  # Set "handler" value automatically if "storable" value is set already
  app->hook(before_render => sub {
    my ($c, $args) = @_;
    $args->{handler} = 'storable'
      if exists $args->{storable} || exists $c->stash->{storable};
  });

  get '/' => {storable => {i => '♥ mojolicious'}};

  app->start;

The hook L<Mojolicious/"before_render"> can be used to make stash values like
C<storable> special, so that they no longer require a C<handler> value to be set
explicitly.

  # Explicit "handler" value
  $c->render(storable => {i => '♥ mojolicious'}, handler => 'storable');

  # Implicit "handler" value (with "before_render" hook)
  $c->render(storable => {i => '♥ mojolicious'});

=head1 MORE

You can continue with L<Mojolicious::Guides> now or take a look at the
L<Mojolicious wiki|http://github.com/kraih/mojo/wiki>, which contains a lot
more documentation and examples by many different authors.

=head1 SUPPORT

If you have any questions the documentation might not yet answer, don't
hesitate to ask on the
L<mailing list|http://groups.google.com/group/mojolicious> or the official IRC
channel C<#mojo> on C<irc.perl.org>
(L<chat now!|https://chat.mibbit.com/?channel=%23mojo&server=irc.perl.org>).

=cut
