/ 
MasonX-ApacheHandler-WithCallbacks-0.91
/ MasonX::ApacheHandler::WithCallbacks

NAME

MasonX::ApacheHandler::WithCallbacks - Execute code before Mason components

SYNOPSIS

In handler.pl:

use strict;
use MasonX::ApacheHandler::WithCallbacks;

sub calc_time {
    my $cbh = shift;
    my $args = $cbh->request_args;
    my $val = $cbh->value;
    $args->{answer} = localtime($val || time);
}

my $ah = MasonX::ApacheHandler::WithCallbacks->new
  ( callbacks => [ { cb_key => calc_time,
                     cb => \&calc_time } ]
  );

sub handler {
    my $r = shift;
    $ah->handle_request($r);
}

In your component:

% if (exists $ARGS{answer}) {
      <p><b>Answer: <% $ARGS{answer} %></b></p>
% } else {
<form>
<p>Enter an epoch time: <input type="text" name="epoch_time" /><br />
<input type="submit" name="myCallbacker|calc_time_cb" value="Calculate" />
</p>
</form>
% }

DESCRIPTION

MasonX::ApacheHandler::WithCallbacks subclasses HTML::Mason::ApacheHandler in order to provide callbacks. Callbacks are code references provided to the new() constructor, and are triggered either for every request or by specially named keys in the Mason request arguments. The callbacks are executed at the beginning of a request, just before Mason creates and executes the request component stack.

The idea is to configure Mason to execute arbitrary code before executing any components. Doing so allows you to carry out logical processing of data submitted from a form, to affect the contents of the Mason request arguments (and thus the %ARGS hash in components), and even to redirect or abort the request before Mason handles it.

JUSTIFICATION

Why would you want to do this? Well, there are a number of reasons. Some I can think of offhand include:

Stricter separation of logic from presentation

Most application logic handled in Mason components takes place in <%init> blocks, often in the same component as presentation logic. By moving the application logic into subroutines in Perl modules and then directing Mason to execute those subroutines as callbacks, you obviously benefit from a cleaner separation of application logic and presentation.

Wigitization

Thanks to their ability to preprocess arguments, callbacks enable developers to develop easier-to-use, more dynamic widgets that can then be used in any Mason components. For example, a widget that puts many related fields into a form (such as a date selection widget) can have its fields preprocessed by a callback (for example, to properly combine the fields into a unified date field) before the Mason component that responds to the form submission gets the data.

Shared Memory

Callbacks are just Perl subroutines in modules loaded at server startup time. Thus the memory they consume is all in the parent, and then shared by the Apache children. For code that executes frequently, this can be much less resource-intensive than code in Mason components, since components are loaded separately in each Apache child process (unless they're preloaded via the preloads parameter to the HTML::Mason::Interp constructor).

Performance

Since callbacks are executed before Mason creates a component stack and executes the components, they have the opportunity to short-circuit the Mason processing by doing something else. A good example is redirection. Often the application logic in callbacks does its thing and then redirects the user to a different page. Executing the redirection in a callback eliminates a lot of extraneous processing that would otherwise be executed before the redirection, creating a snappier response for the user.

Testing

Mason components are not easy to test via a testing framework such as Test::Harness. Subroutines in modules, on the other hand, are fully testable. This means that you can write tests in your application test suite to test your callback subroutines.

And if those are enough reasons, then just consider this: Callbacks just way cool.

USAGE

MasonX::ApacheHandler::WithCallbacks supports two different types of callbacks: those triggered by a specially named key in the Mason request arguments hash, and those executed for every request.

Argument-Triggered Callbacks

Argument-triggered callbacks are triggered by specially named request argument keys. These keys are constructed as follows: The package name followed by a pipe character ("|"), the callback key with the string "_cb" appended to it, and finally an optional priority number at the end. For example, if you specified a callback with the callback key "save" and the package key "world", a callback field might be added to an HTML form like this:

<input type="button" value="Save World" name="world|save_cb" />

This field, when submitted to the Mason server, would trigger the callback associated with the "save" callback key in the "world" package. If such a callback hasn't been configured, then MasonX::ApacheHandler::WithCallbacks will throw a HTML::Mason::Exception::Callback::InvalidKey exception. Here's how to configure such a callback when constructing your MasonX::ApacheHandler::WithCallbacks object so that that doesn't hapen:

my $cbah = MasonX::ApacheHandler::WithCallbacks->new
  ( callbacks => [ { pkg_key => 'world',
                     cb_key  => 'save',
                     cb      => \&My::World::save } ] );

With this configuration, the request argument created by the above HTML form field will trigger the exectution of the &My::World::save subroutine.

Callback Subroutines

The code references used for argument-triggered callbacks will be executed with a single argument, a MasonX::CallbackHandle object. Thus, a callback subroutine will generally look something like this:

sub foo {
    my $cbh = shift;
    # Do stuff.
}

The MasonX::CallbackHandle object provides accessors to data relevant to the callback, including the callback key, the package key, and the request arguments. It also includes redirect() and abort() methods. See the MasonX::CallbackHandle documentation for all the goodies.

Note that all callbacks are executed in a eval {} block, so if any of your callback subroutines die, MasonX::ApacheHandler::WithCallbacks will throw an HTML::Mason::Exception::Callback::Execution exception.

The Package Key

The use of the package key is a convenience so that a system with many callbacks can use callbacks with the same keys but in different packages. The idea is that the package key will uniquely identify the module in which each callback subroutine is found, but it doesn't necessarily have to be so. Use the package key any way you wish, or not at all:

my $cbah = MasonX::ApacheHandler::WithCallbacks->new
  ( callbacks => [ { cb_key  => 'save',
                     cb      => \&My::World::save } ] );

But note that if you don't use the package key at all, you'll still need to provide one in the field to be submitted to the Mason server. By default, that key is "DEFAULT". Such a callback field in an HTML form would then look like this:

<input type="button" value="Save World" name="DEFAULT|save_cb" />

If you don't like the "DEFAULT" package name, you can set an alternative default using the default_pkg_name parameter to new():

my $cbah = MasonX::ApacheHandler::WithCallbacks->new
  ( callbacks        => [ { cb_key  => 'save',
                            cb      => \&My::World::save } ],
    default_pkg_name => 'MyPkg' );

Then, of course, any callbacks without a specified package key of their own will then use the custom default:

<input type="button" value="Save World" name="MyPkg|save_cb" />

Priority

Sometimes one callback is more important than another. For example, you might rely on the execution of one callback to set up variables needed by another. Since you can't rely on the order in which callbacks are executed (the Mason request arguments are stored in a hash, and the processing of a hash is, of course, unordered), you need a method of ensuring that the setup callback executes first.

In such a case, you can set a higher priority level for the setup callback than for callbacks that depend on it:

my $cbah = MasonX::ApacheHandler::WithCallbacks->new
  ( callbacks        => [ { cb_key   => 'setup',
                            priority => 3,
                            cb       => \&setup },
                          { cb_key   => 'save',
                            cb       => \&save }
                        ] );

In this example, the "setup" callback has been configured with a priority level of "3". This ensures that it will always execute before the "save" callback, which has the default priority of "5". This is true regardless of the order of the fields in the corresponding HTML::Form:

<input type="button" value="Save World" name="DEFAULT|save_cb" />
<input type="hidden" name="DEFAULT|setup_cb" value="1" />

Despite the fact that the "setup" callback field appears after the "save" field (and will generally be submitted by the browser in that order), the "setup" callback will always execute first because of its higher priority.

Although the "save" callback got the default priority of "5", this too can be customized to a different priority level via the default_priority parameter to new(). For example, this configuration:

my $cbah = MasonX::ApacheHandler::WithCallbacks->new
  ( callbacks        => [ { cb_key   => 'setup',
                            priority => 3,
                            cb       => \&setup },
                          { cb_key   => 'save',
                            cb       => \&save }
                        ],
    default_priority => 2 );

Will cause the "save" callback to always execute before the "setup" callback, since it's priority level will default to "2".

Conversely, the priority level can be overridden via the form submission field itself by appending a priority level to the end of the callback field name. Hence, this example:

<input type="button" value="Save World" name="DEFAULT|save_cb2" />
<input type="hidden" name="DEFAULT|setup_cb" value="1" />

causes the "save" callback to execute before the "setup" callback by overriding the "save" callback's priority to level "2". Of course, any other form field that triggers the "save" callback without a priority override will still execute "save" at its configured level.

Request Callbacks

Request callbacks come in two separate flavors: those that execute before the argument-triggered callbacks, and those that execute after the argument-triggered callbacks. These may be specified via the pre_callbacks and post_callbacks parameters to new(), respectively:

my $cbah = MasonX::ApacheHandler::WithCallbacks->new
  ( pre_callbacks  => [ \&translate, \&foobarate ],
    post_callbacks => [ \&escape, \&negate ] );

In this example, the translate() and foobarate() subroutines will execute (in that order) before any argument-triggered callbacks are executed (none will be in this example, since none are specifed). Conversely, the escape() and negate() subroutines will be executed (in that order) after all argument-triggered callbacks have been executed. And regardless of what argument-triggered callbacks may be triggered, the request callbacks will always be executed for every request.

Although they may be used for different purposes, the pre_callbacks and post_callbacks callback code references expect the same argument as argument-triggered callbacks: a MasonX::CallbackHandleObject:

sub foo {
    my $cbh = shift;
    # Do your business here.
}

Of course, the attributes of the MasonX::CallbackHandleObject object will be different than in argument-triggered callbacks. For example, the priority, pkg_key, and cb_key attributes will naturaly be undefined.

Like the argument-triggered callbacks, however, like the argument-triggered callbacks, request callbacks are executed in a eval {} block, so if any of them dies, an HTML::Mason::Exception::Callback::Execution exception will be thrown.

INTERFACE

Parameters To The new() Constructor

In addition to those offered by the HTML::Mason::ApacheHandler base class, this module supports a number of its own parameters to the new() constructor.

callbacks

Argument-triggered callbacks are configured via the callbacks parameter. This parameter is an array reference of hash references, and each hash reference specifies a single callback. The supported keys in the callback specification hashes are:

cb_key

Required. A string that, when found in a properly-formatted Mason request argument key, will trigger the execution of the callback.

cb

Required. A reference to the Perl subroutine that will be executed when the cb_key has been found in a Mason request argument key. Each code reference should a single argument: a MasonX::CallbackHandle object.

pkg_key

Optional. A key to uniquely identify the package in which the callback subroutine is found. This parameter is useful in systems with many callbacks, where developers may wish to use the same cb_key for different subroutines in different packages. The default package key may be set via the default_pkg_key parameter.

priority

Optional. Indicates the level of priority of a callback. Some callbacks are more important than others, and should be executed before others. MasonX::ApacheHandler::WithCallbacks supports priority levels, ranging from "0" (highest priority) to "9" (lowest priority). The default priority may be set via the default_priority parameter.

pre_callbacks

This parameter accepts an array reference of code references that should be executed for every request before any other callbacks. Each code reference should expect a single MasonX::CallbackHandle argument. Use pre-argument-triggered request callbacks when you want to do something with the arguments sumitted for every request, such as convert character sets.

post_callbacks

This parameter accepts an array reference of code references that should be executed for every request after all other callbacks have been called. Each code reference should expect a single MasonX::CallbackHandle argument. Use post-argument-triggered request callbacks when you want to do something with the arguments sumitted for every request, such as HTML-escape their values.

default_priority

The priority level at which callbacks will be executed. This value will be used in each hash reference passed via the callbacks parameter to new() that lacks a priority key. You may specify a default priority level within the range of "0" (highst priority) to "9" (lowest priority). If not specified, it defaults to "5".

default_pkg_key

The default package key for callbacks. This value that will be used in each hash reference passed via the callbacks parameter to new() that lacks a pkg_key key. It can be any string that evaluates to a true value, and defaults to "DEFAULT" if not specified.

Accessor Methods

The properties default_priority and default_pkg_key have standard read-only accessor methods of the same name. For example:

my $cbah = new HTML::Mason::ApacheHandler::WithCallbacks;
my $default_priority = $cbah->default_priority;
my $default_pkg_key = $cbah->default_pkg_key;

ACKNOWLEDGEMENTS

Garth Webb implemented the original callbacks in Bricolage, based on an idea he borrowed from Paul Lindner's work with Apache::ASP. My thanks to them both for planting this great idea!

BUGS

Please report all bugs via the CPAN Request Tracker at http://rt.cpan.org/NoAuth/Bugs.html?Dist=MasonX-ApacheHandler-WithCallbacks.

TODO

  • Create object-oriented interface.

  • Figure out how to use httpd.conf PerlSetVar directives to pass callback specs to new().

  • Add some good real-world examples to the documentation.

SEE ALSO

MasonX::CallbackHandle objects get passed as the sole argument to all callback code references, and offer access to data relevant to the callback.

This module works with HTML::Mason by subclassing HTML::Mason::ApacheHandler. Inspired by the implementation of callbacks in Bricolage (http://bricolage.cc/), it is however a completely new code base with a rather different approach.

AUTHOR

David Wheeler <david@wheeler.net>

COPYRIGHT AND LICENSE

Copyright 2003 by David Wheeler

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.