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Man Pages

Manual Reference Pages  -  CGI::PROTOTYPE (3)

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CGI::Prototype - Create a CGI application by subclassing



  package My::HelloWorld;
  use base CGI::Prototype;

  sub template { \ <<END_OF_TEMPLATE }
  [% self.CGI.header; %]
  Hello world at [% USE Date; Date.format( | html %]!



The core of every CGI application seems to be roughly the same:
o Analyze the incoming parameters, cookies, and URLs to determine the state of the application (let’s call this dispatch).
o Based on the current state, analyze the incoming parameters to respond to any form submitted (respond).
o From there, decide what response page should be generated, and produce it (render).
CGI::Prototype creates a Class::Prototyped engine for doing all this, with the right amount of callback hooks to customize the process. Because I’m biased toward Template Toolkit for rendering HTML, I’ve also integrated that as my rendering engine of choice. And, being a fan of clean MVC designs, the classes become the controllers, and the templates become the views, with clean separation of responsibilities, and CGI::Prototype a sort of archetypal controller.

You can create the null application by simply activating it:

  use CGI::Prototype;

But this won’t be very interesting. You’ll want to subclass this class in a Class::Prototyped-style manner to override most of its behavior. Slots can be added to add or alter behavior. You can subclass your subclasses when groups of your CGI pages share similar behavior. The possibilities are mind-boggling.

Within the templates, self refers to the current controller. Thus, you can define callbacks trivially. In your template, if you need some data, you can pull it as a request:

  [% my_data = self.get_some_big_data %]

which is supplied by simply adding the same slot (method or data) in the controlling class:

  sub get_some_big_data {
    my $self = shift;
    return $self->some_other_method(size => big);

And since the classes are hierarchical, you can start out with an implementation for one page, then move it to a region or globally quickly.

Although the name CGI::Prototype implies a CGI protocol, I see no reason that this would not work with Apache::Registry in a mod_perl environment, or a direct content handler such as:

  package My::App;
  use base CGI::Prototype;
  sub handler {

Note that the $r request object will have to be created if needed if you use this approach.


These slots provide core functionality. You will probably not need to override these.
activate Invoke the activate slot to activate your application, causing it to process the incoming CGI values, select a page to be respond to the parameters, which in turn selects a page to render, and then responds with that page. For example, your App might consist only of:

  package My::App;
  use base qw(CGI::Prototype);

Again, this will not be interesting, but it shows that the null app is easy to create. Almost always, you will want to override some of the callback slots below.

CGI Invoking $self->CGI gives you access to the object representing the incoming parameters and other values. For example,


generates a self-referencing URL. From a template, this is:

  [% self.CGI.self_url %]

for the same thing.

See initialize_CGI for how this slot gets established.

render The render method uses the results from engine and template to process a selected template through Template Toolkit. If the result does not throw an error, $self->display is called to show the result.
display The display method is called to render the output of the template under normal circumstances, normally dumping the first parameter to STDOUT. Test harnesses may override this method to cause the output to appear into a variable, but normally this method is left alone.
param The param method is a convenience method that maps to $self->CGI->param, because accessing params is a very common thing.
interstitial <B>Please note that this feature is still experimental and subject to change.B>

Use this in your per-page respond methods if you have a lot of heavy processing to perform. For example, suppose you’re deleting something, and it takes 5 seconds to do the first step, and 3 seconds to do the second step, and then you want to go back to normal web interaction. Simulating the heavy lifting with sleep, we get:

  my $p = $self->interstitial
    ({ message => "Your delete is being processed...",
       action => sub { sleep 5 },
     { message => "Just a few seconds more....",
       action => sub { sleep 3 },
  return $p if $p;

interstitial returns either a page that should be returned so that it can be rendered (inside a wrapper that provides the standard top and bottom of your application page), or undef.

The list passed to interstitial should be a series of hashrefs with one or more parameters reflecting the steps:
message What the user should see while the step is computing. (Default: Working....)
action A coderef with the action performed server-side during the message. (Default: no action.)
delay The number of seconds the browser should wait before initiating the next connection, triggering the start of action. (Default: 0 seconds.)

The user sees the first message at the first call to interstitial (via the first returned page), at which time a meta-refresh will immediately repost the same parameters as on the call that got you here. (Thus, it’s important not to have changed the params yet, or you might end up in a different part of your code.) When the call to interstitial is re-executed, the first coderef is then performed. At the end of that coderef, the second interstitial page is returned, and the user sees the second message, which then performs the next meta-refresh, which gets us back to this call to interstitial again (whew). The second coderef is executed while the user is seeing the second message, and then interstitial returns undef, letting us roll through to the final code. Slick.

config_interstitial_param This parameter is used by interstitial to determine the processing step. You should ensure that the name doesn’t conflict with any other param that you might need.

The default value is _interstitial.


engine The engine returns a Template object that will be generating any response. The object is computed lazily (with autoloading) when needed.

The Template object is passed the configuration returned from the engine_config callback.

engine_config Returns a hashref of desired parameters to pass to the Template new method as a configuration. Defaults to an empty hash.
prototype_enter Called when the prototype mechanism is entered, at the very beginning of each hit. Defaults to calling -initialize_CGI>, which see.

Generally, you should not override this method. If you do, be sure to call the SUPER method, in case future versions of this module need additional initialization.

prototype_leave Called when the prototype mechanism is exited, at the very end of each hit. Defaults to no action.

Generally, you should not override this method. If you do, be sure to call the SUPER method, in case future versions of this module need additional teardown.

initialize_CGI Sets up the CGI slot as an autoload, defaulting to creating a new object. Called from prototype_enter.
app_enter Called when the application is entered, at the very beginning of each hit. Defaults to no action.
app_leave Called when the application is left, at the very end of each hit. Defaults to no action.
control_enter Called when a page gains control, either at the beginning for a response, or in the middle when switched for rendering. Defaults to nothing.

This is a great place to hang per-page initialization, because you’ll get this callback at most once per hit.

control_leave Called when a page loses control, either after a response phase because we’re switching to a new page, or render phase after we’ve delivered the new text to the browser.

This is a great place to hang per-page teardown, because you’ll get this callback at most once per hit.

render_enter Called when a page gains control specifically for rendering (delivering text to the browser), just after control_enter if needed.
render_leave Called when a page loses control specifically for rendering (delivering text to the browser), just before control_leave.
respond_enter Called when a page gains control specifically for responding (understanding the incoming parameters, and deciding what page should render the response), just after control_enter.
respond_leave Called when a page loses control specifically for rendering (understanding the incoming parameters, and deciding what page should render the response), just before control_leave (if needed).
template Delivers a template document object (something compatible to the Template process method, such as a Template::Document or a filehandle or a reference to a scalar). The default is a simple this page intentionally left blank template.

When rendered, the <B>onlyB> extra global variable passed into the template is the self variable, representing the controller object. However, as seen earlier, this is sufficient to allow access to anything you need from the template, thanks to Template Toolkit’s ability to call methods on an object and understand the results.

For example, to get at the barney parameter:

  The barney field is [% self.param("barney") | html %].

error Called if an uncaught error is triggered in any of the other steps, passing the error text or object as the first method parameter. The default callback simply displays the output to the browser, which is highly insecure and should be overridden, perhaps with something that logs the error and puts up a generic error message with an incident code for tracking.
dispatch Called to analyze the incoming parameters to define which page object gets control based on the incoming CGI parameters.

This callback <B>must returnB> a page object (the object taking control during the response phase). By default, this callback returns the application itself.

respond Called to determine how to respond specifically to this set of incoming parameters. Probably updates databases and such.

This callback <B>must returnB> a page object (the object taking control during the render phase). By default, this callback returns the same object that had control during the response phase (stay here logic), which works most of the time.


Class::Prototyped, Template::Manual, <>.


Please report any bugs or feature requests to, or through the web interface at I will be notified, and then you’ll automatically be notified of progress on your bug as I make changes.


Randal L. Schwartz, <>

Special thanks to and an unnamed large university for providing funding for the development of this module.


Copyright (C) 2003, 2004, 2005 by Randal L. Schwartz

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.5 or, at your option, any later version of Perl 5 you may have available.

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perl v5.20.3 CGI::PROTOTYPE (3) 2011-07-30

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