Provided by: libparams-callbackrequest-perl_1.20-2_all 
NAME
Params::CallbackRequest - Functional and object-oriented callback architecture
SYNOPSIS
Functional parameter-triggered callbacks:
use strict;
use Params::CallbackRequest;
# Create a callback function.
sub calc_time {
my $cb = shift;
my $params = $cb->params;
my $val = $cb->value;
$params->{my_time} = localtime($val || time);
}
# Set up a callback request object.
my $cb_request = Params::CallbackRequest->new(
callbacks => [ { cb_key => 'calc_time',
pkg_key => 'myCallbacker',
cb => \&calc_time } ]
);
# Request callback execution.
my %params = ('myCallbacker|calc_time_cb' => 1);
$cb_request->request(\%params);
# Demonstrate the result.
print "The time is $params{my_time}\n";
Or, in a subclass of Params::Callback:
package MyApp::Callback;
use base qw(Params::Callback);
__PACKAGE__->register_subclass( class_key => 'myCallbacker' );
# Set up a callback method.
sub calc_time : Callback {
my $self = shift;
my $params = $self->request_params;
my $val = $cb->value;
$params->{my_time} = localtime($val || time);
}
And then, in your application:
# Load order is important here!
use MyApp::Callback;
use Params::CallbackRequest;
my $cb_request = Params::Callback->new( cb_classes => [qw(myCallbacker)] );
my %params = ('myCallbacker|calc_time_cb' => 1);
$cb_request->request(\%params);
print "The time is $params{my_time}\n";
DESCRIPTION
Params::CallbackRequest provides functional and object-oriented callbacks to method and
function parameters. Callbacks may be either code references provided to the "new()"
constructor, or methods defined in subclasses of Params::Callback. Callbacks are triggered
either for every call to the Params::CallbackRequest "request()" method, or by specially
named keys in the parameters to "request()".
The idea behind this module is to provide a sort of plugin architecture for Perl
templating systems. Callbacks are triggered by the contents of a request to the Perl
templating server, before the templating system itself executes. This approach allows you
to carry out logical processing of data submitted from a form, to affect the contents of
the request parameters before they're passed to the templating system for processing, and
even to redirect or abort the request before the templating system 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
While some Perl templating systems enforce separation of application logic from
presentation (e.g., TT, HTML::Template), others do not (e.g., HTML::Mason,
Apache::ASP). Even in the former case, application logic is often put into scripts
that are executed alongside the presentation templates, and loaded on-demand under
mod_perl. By moving the application logic into Perl modules and then directing the
templating system to execute that code as callbacks, you obviously benefit from a
cleaner separation of application logic and presentation.
Widgitization
Thanks to their ability to preprocess parameters, callbacks enable developers to
develop easier-to-use, more dynamic widgets that can then be used in any and all
templating systems. 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 parameter) before the
template that responds to the form submission gets the data. See Params::Callback for
an example solution for this very problem.
Shared Memory
If you run your templating system under mod_perl, callbacks are just Perl subroutines
in modules loaded at server startup time. Thus the memory they consume is all in the
Apache parent process, and shared by the child processes. For code that executes
frequently, this can be much less resource-intensive than code in templates, since
templates are loaded separately in each Apache child process on demand.
Performance
Since they're executed before the templating architecture does much processing,
callbacks have the opportunity to short-circuit the template 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
Templating system templates 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 aren't enough reasons, then just consider this: Callbacks are just way cool.
USAGE
Params::CallbackRequest supports two different types of callbacks: those triggered by a
specially named parameter keys, and those executed for every request.
Parameter-Triggered Callbacks
Parameter-triggered callbacks are triggered by specially named parameter 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 specified like this:
my $params = { "world|save_cb" => 'Save World' };
When the parameters hash $params is passed to Params::CallbackRequest's "request()"
method, the "world|save_cb" parameter would trigger the callback associated with the
"save" callback key in the "world" package. If such a callback hasn't been configured,
then Params::CallbackRequest will throw a Params::CallbackRequest::Exceptions::InvalidKey
exception. Here's how to configure a functional callback when constructing your
Params::CallbackRequest object so that that doesn't happen:
my $cb_request = Params::CallbackRequest->new
( callbacks => [ { pkg_key => 'world',
cb_key => 'save',
cb => \&My::World::save } ] );
With this configuration, the "world|save_cb" parameter key will trigger the execution of
the "My::World::save()" subroutine during a callback request:
# Execute parameter-triggered callback.
$cb_request->request($params);
Functional Callback Subroutines
Functional callbacks use a code reference for parameter-triggered callbacks, and
Params::CallbackRequest executes them with a single argument, a Params::Callback object.
Thus, a callback subroutine will generally look something like this:
sub foo {
my $cb = shift;
# Do stuff.
}
The Params::Callback object provides accessors to data relevant to the callback, including
the callback key, the package key, and the parameter hash. It also includes an "abort()"
method. See the Params::Callback documentation for all the goodies.
Note that Params::CallbackRequest installs an exception handler during the execution of
callbacks, so if any of your callback subroutines "die", Params::CallbackRequest will
throw an Params::Callback::Exception::Execution exception. If your callback subroutines
throw their own exception objects, Params::CallbackRequest will simply rethrow them. If
you don't like this configuration, use the "exception_handler" parameter to "new()" to
install your own exception handler.
Object-Oriented Callback Methods
Object-oriented callback methods, which are supported under Perl 5.6 or later, are defined
in subclasses of Params::Callback, and identified by attributes in their declarations.
Unlike functional callbacks, callback methods are not called with a Params::Callback
object, but with an instance of the callback subclass. These classes inherit all the
goodies provided by Params::Callback, so you can essentially use their instances exactly
as you would use the Params::Callback object in functional callback subroutines. But
because they're subclasses, you can add your own methods and attributes. See
Params::Callback for all the gory details on subclassing, along with a few examples.
Generally, callback methods will look like this:
sub foo : Callback {
my $self = shift;
# Do stuff.
}
As with functional callback subroutines, method callbacks are executed with a custom
exception handler. Again, see the "exception_handler" parameter to install your own
exception handler.
Note: Under mod_perl, it's important that you "use" any and all Params::Callback
subclasses before you "use Params::CallbackRequest". This is to get around an issue with
identifying the names of the callback methods in mod_perl. Read the comments in the
Params::Callback source code if you're interested in learning more.
The Package Key
The use of the package key is a convenience so that a system with many functional
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 $cb_request = Params::CallbackRequest->new
( callbacks => [ { cb_key => 'save',
cb => \&My::World::save } ] );
But note that if you don't specify the package key, you'll still need to provide one in
the parameter hash passed to "request()". By default, that key is "DEFAULT". Such a
callback parameter would then look like this:
my $params = { "DEFAULT|save_cb" => 'Save World' };
If you don't like the "DEFAULT" package name, you can set an alternative default using the
"default_pkg_name" parameter to "new()":
my $cb_request = Params::CallbackRequest->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 must then use
the custom default:
my $params = { "MyPkg|save_cb" => 'Save World' };
$cb_request->request($params);
The Class Key
The class key is essentially a synonym for the package key, but applies more directly to
object-oriented callbacks. The difference is mainly that it corresponds to an actual
class, and that all Params::Callback subclasses are required to have a class key; it's not
optional as it is with functional callbacks. The class key may be declared in your
Params::Callback subclass like so:
package MyApp::CallbackHandler;
use base qw(Params::Callback);
__PACKAGE__->register_subclass( class_key => 'MyCBHandler' );
The class key can also be declared by implementing a "CLASS_KEY" subroutine, like so:
package MyApp::CallbackHandler;
use base qw(Params::Callback);
__PACKAGE__->register_subclass;
use constant CLASS_KEY => 'MyCBHandler';
If no class key is explicitly defined, Params::Callback will use the subclass name,
instead. In any event, the "register_callback()" method must be called to register the
subclass with Params::Callback. See the Params::Callback documentation for complete
details.
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 parameters are passed via 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. For functional callbacks, you can do it like this:
my $cb_request = Params::CallbackRequest->new
( callbacks => [ { cb_key => 'setup',
priority => 3,
cb => \&setup },
{ cb_key => 'save',
cb => \&save }
] );
For object-oriented callbacks, you can define the priority right in the callback method
declaration:
sub setup : Callback( priority => 3 ) {
my $self = shift;
# ...
}
sub save : Callback {
my $self = shift;
# ...
}
In these examples, 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". Obviously, this is true regardless of the order of the fields in the
hash:
my $params = { "DEFAULT|save_cb" => 'Save World',
"DEFAULT|setup_cb" => 1 };
In this configuration, 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
functional callbacks and the "default_priority" to the class declaration for object-
oriented callbacks. For example, this functional callback configuration:
my $cb_request = Params::CallbackRequest->new
( callbacks => [ { cb_key => 'setup',
priority => 3,
cb => \&setup },
{ cb_key => 'save',
cb => \&save }
],
default_priority => 2 );
Or this Params::Callback subclass declaration:
package MyApp::CallbackHandler;
use base qw(Params::Callback);
__PACKAGE__->register_subclass( class_key => 'MyCBHandler',
default_priority => 2 );
Will cause the "save" callback to always execute before the "setup" callback, since its
priority level will default to "2".
In addition, the priority level can be overridden via the parameter key itself by
appending a priority level to the end of the key name. Hence, this example:
my $params = { "DEFAULT|save_cb2" => 'Save World',
"DEFAULT|setup_cb" => 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 parameter key that triggers the
"save" callback without a priority override will still execute the "save" callback at its
configured level.
Request Callbacks
Request callbacks come in two flavors: those that execute before the parameter-triggered
callbacks, and those that execute after the parameter-triggered callbacks. Functional
request callbacks may be specified via the "pre_callbacks" and "post_callbacks" parameters
to "new()", respectively:
my $cb_request = Params::CallbackRequest->new
( pre_callbacks => [ \&translate, \&foobarate ],
post_callbacks => [ \&escape, \&negate ] );
Object-oriented request callbacks may be declared via the "PreCallback" and "PostCallback"
method attributes, like so:
sub translate : PreCallback { ... }
sub foobarate : PreCallback { ... }
sub escape : PostCallback { ... }
sub negate : PostCallback { ... }
In these examples, the "translate()" and "foobarate()" subroutines or methods will execute
(in that order) before any parameter-triggered callbacks are executed (none will be in
these examples, since none are specified).
Conversely, the "escape()" and "negate()" subroutines or methods will be executed (in that
order) after all parameter-triggered callbacks have been executed. And regardless of what
parameter-triggered callbacks may be triggered, the request callbacks will always be
executed for every request (unless an exception is thrown by an earlier callback).
Although they may be used for different purposes, the "pre_callbacks" and "post_callbacks"
functional callback code references expect the same argument as parameter-triggered
functional callbacks: a Params::Callback object:
sub foo {
my $cb = shift;
# Do your business here.
}
Similarly, object-oriented request callback methods will be passed an object of the class
defined in the class key portion of the callback trigger -- either an object of the class
in which the callback is defined, or an object of a subclass:
sub foo : PostCallback {
my $self = shift;
# ...
}
Of course, the attributes of the Params::Callback or subclass object will be different
than in parameter-triggered callbacks. For example, the "priority", "pkg_key", and
"cb_key" attributes will naturally be undefined. It will, however, be the same instance of
the object passed to all other functional callbacks -- or to all other class callbacks
with the same class key -- in a single request.
Like the parameter-triggered callbacks, request callbacks run under the nose of a custom
exception handler, so if any of them "die"s, an Params::Callback::Exception::Execution
exception will be thrown. Use the "exception_handler" parameter to "new()" if you don't
like this.
INTERFACE
Parameters To The "new()" Constructor
Params::CallbackRequest supports a number of its own parameters to the "new()" constructor
(though none of them, sadly, trigger callbacks). The parameters to "new()" are as follows:
"callbacks"
Parameter-triggered functional 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 parameter hash 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 parameter hash passed to "request()". Each code
reference should expect a single argument: a Params::Callback object. The same
instance of a Params::Callback object will be used for all functional callbacks in
a single call to "request()".
"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 to "new()".
"priority"
Optional. Indicates the level of priority of a callback. Some callbacks are more
important than others, and should be executed before the others.
Params::CallbackRequest supports priority levels ranging from "0" (highest
priority) to "9" (lowest priority). The default priority for functional callbacks
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 call to "request()" before any parameter-triggered callbacks. They will be
executed in the order in which they're listed in the array reference. Each code
reference should expect a Params::Callback object as its sole argument. The same
instance of a Params::Callback object will be used for all functional callbacks in a
single call to "request()". Use pre-parameter-triggered request callbacks when you
want to do something with the parameters submitted for every call to "request()", such
as convert character sets.
"post_callbacks"
This parameter accepts an array reference of code references that should be executed
for every call to "request()" after all parameter-triggered callbacks have been
called. They will be executed in the order in which they're listed in the array
reference. Each code reference should expect a Params::Callback object as its sole
argument. The same instance of a Params::Callback object will be used for all
functional callbacks in a single call to "request()". Use post-parameter-triggered
request callbacks when you want to do something with the parameters submitted for
every call to "request()", such as encode or escape their values for presentation.
"cb_classes"
An array reference listing the class keys of all of the Params::Callback subclasses
containing callback methods that you want included in your Params::CallbackRequest
object. Alternatively, the "cb_classes" parameter may simply be the word "ALL", in
which case all Params::Callback subclasses will have their callback methods registered
with your Params::CallbackRequest object. See the Params::Callback documentation for
details on creating callback classes and methods.
Note: In a mod_perl environment, be sure to "use Params::CallbackRequest" only after
you've "use"d all of the Params::Callback subclasses you need or else you won't be
able to use their callback methods.
"default_priority"
The priority level at which functional callbacks will be executed. Does not apply to
object-oriented callbacks. 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" (highest priority) to "9" (lowest
priority). If not specified, it defaults to "5".
"default_pkg_key"
The default package key for functional callbacks. Does not apply to object-oriented
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.
"ignore_nulls"
By default, Params::CallbackRequest will execute all callbacks triggered by parameter
hash keys. However, in many situations it may be desirable to skip any callbacks that
have no value for the callback field. One can do this by simply checking "$cbh->value"
in the callback, but if you need to disable the execution of all parameter-triggered
callbacks when the callback parameter value is undefined or the null string (''), pass
the "ignore_null" parameter with a true value. It is set to a false value by default.
"leave_notes"
By default, Params::CallbackRequest will clear out the contents of the hash accessed
via the "notes()" method just before returning from a call to "request()". There may
be some circumstances when it's desirable to allow the notes hash to persist beyond
the duration of a a call to "request()". For example, a templating architecture may
wish to keep the notes around for the duration of the execution of a template request.
In such cases, pass a true value to the "leave_notes" parameter, and use the
"clear_notes()" method to manually clear out the notes hash at the appropriate point.
"exception_handler"
Params::CallbackRequest installs a custom exception handler during the execution of
callbacks. This custom exception handler will simply rethrow any exception objects it
comes across, but will throw a Params::Callback::Exception::Execution exception object
if it is passed only a string value (such as is passed by "die "fool!"").
But if you find that you're throwing your own exceptions in your callbacks, and want
to handle them differently, pass the "exception_handler" parameter a code reference to
do what you need.
Instance Methods
Params::CallbackRequest of course has several instance methods. I cover the most
important, first.
request
$cb_request->request(\%params);
# If you're in a mod_perl environment, pass in an Apache request object
# to be passed to the Callback classes.
$cb_request->request(\%params, apache_req => $r);
# Or pass in argument to be passed to callback class constructors.
$cb_request->request(\%params, @args);
Executes the callbacks specified when the Params::CallbackRequest object was created. It
takes a single required argument, a hash reference of parameters. Any subsequent arguments
are passed to the constructor for each callback class for which callbacks will be
executed. By default, the only extra parameter supported by the Params::Callback base
class is an Apache request object, which can be passed via the "apache_req" parameter.
Returns the Params::CallbackRequest object on success, or the code passed to
Params::Callback's "abort()" method if callback execution was aborted.
A single call to "request()" is referred to as a "callback request" (naturally!). First,
all pre-request callbacks are executed. Then, any parameter-triggered callbacks triggered
by the keys in the parameter hash reference passed as the sole argument are executed. And
finally, all post-request callbacks are executed. "request()" returns the
Params::CallbackRequest object on successful completion of the request.
Any callback that calls "abort()" on its Params::Callback object will prevent any other
callbacks scheduled by the request to run subsequent to its execution from being executed
(including post-request callbacks). Furthermore, any callback that "die"s or throws an
exception will of course also prevent any subsequent callbacks from executing, and in
addition must also be caught by the caller or the whole process will terminate:
eval { $cb_request->request(\%params) };
if (my $err = $@) {
# Handle exception.
}
notes
$cb_request->notes($key => $value);
my $val = $cb_request->notes($key);
my $notes = $cb_request->notes;
The "notes()" method provides a place to store application data, giving developers a way
to share data among multiple callbacks over the course of a call to "request()". Any data
stored here persists for the duration of the request unless the "leave_notes" parameter to
"new()" has been passed a true value. In such cases, use "clear_notes()" to manually clear
the notes.
Conceptually, "notes()" contains a hash of key-value pairs. "notes($key, $value)" stores a
new entry in this hash. "notes($key)" returns a previously stored value. "notes()" without
any arguments returns a reference to the entire hash of key-value pairs.
"notes()" is similar to the mod_perl method "$r->pnotes()". The main differences are that
this "notes()" can be used in a non-mod_perl environment, and that its lifetime is tied to
the lifetime of the call to "request()" unless the "leave_notes" parameter is true.
For the sake of convenience, a shortcut to "notes()" is provide to callback code via the
"notes()" method in Params::Callback.
clear_notes
$cb_request->clear_notes;
Use this method to clear out the notes hash. Most useful when the "leave_notes" parameter
to "new()" has been set to at true value and you need to manage the clearing of notes
yourself. This method is specifically designed for a templating environment, where it may
be advantageous for the templating architecture to allow the notes to persist beyond the
duration of a call to "request()", e.g., to keep them for the duration of a call to the
templating architecture itself. See MasonX::Interp::WithCallbacks for an example of this
strategy.
Accessor Methods
The properties "default_priority" and "default_pkg_key" have standard read-only accessor
methods of the same name. For example:
my $cb_request = Params::CallbackRequest->new;
my $default_priority = $cb_request->default_priority;
my $default_pkg_key = $cb_request->default_pkg_key;
ACKNOWLEDGMENTS
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! This implementation is however completely independent of previous implementations.
SEE ALSO
Params::Callback objects get passed as the sole argument to all functional callbacks, and
offer access to data relevant to the callback. Params::Callback also defines the object-
oriented callback interface, making its documentation a must-read for anyone who wishes to
create callback classes and methods.
MasonX::Interp::WithCallbacks uses this module to provide a callback architecture for
HTML::Mason.
SUPPORT
This module is stored in an open GitHub repository <http://github.com/theory/params-
callbackrequest/>. Feel free to fork and contribute!
Please file bug reports via GitHub Issues <http://github.com/theory/params-
callbackrequest/issues/> or by sending mail to bug-params-callbackrequest@rt.cpan.org
<mailto:bug-params-callbackrequest@rt.cpan.org>.
AUTHOR
David E. Wheeler <david@justatheory.com>
COPYRIGHT AND LICENSE
Copyright 2003-2011 David E. Wheeler. Some Rights Reserved.
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.