Provided by: libparams-callbackrequest-perl_1.20-2_all bug


       Params::Callback - Parameter callback base class


       Functional callback interface:

         sub my_callback {
             # Sole argument is a Params::Callback object.
             my $cb = shift;
             my $params = $cb->params;
             my $value = $cb->value;
             # Do stuff with above data.

       Object-oriented callback interface:

         package MyApp::Callback;
         use base qw(Params::Callback);
         use constant CLASS_KEY => 'MyHandler';
         use strict;

         sub my_callback : Callback {
             my $self = shift;
             my $params = $self->params;
             my $value = $self->value;
             # Do stuff with above data.


       Params::Callback provides the interface for callbacks to access parameter hashes
       Params::CallbackRequest object, and callback metadata, as well as for executing common
       request actions, such as aborting a callback execution request. There are two ways to use
       Params::Callback: via functional-style callback subroutines and via object-oriented
       callback methods.

       For functional callbacks, a Params::Callback object is constructed by
       Params::CallbackRequest for each call to its "request()" method, and passed as the sole
       argument for every execution of a callback function. See Params::CallbackRequest for
       details on how to create a Params::CallbackRequest object to execute your callback code.

       In the object-oriented callback interface, Params::Callback is the parent class from which
       all callback classes inherit. Callback methods are declared in such subclasses via
       "Callback", "PreCallback", and "PostCallback" attributes to each method declaration.
       Methods and subroutines declared without one of these callback attributes are not callback
       methods, but normal methods or subroutines of the subclass. Read subclassing for details
       on subclassing Params::Callback.


       Params::Callback provides the parameter hash accessors and utility methods that will help
       manage a callback request (where a "callback request" is considered a single call to the
       "request()" method on a Params::CallbackRequest object).  Functional callbacks always get
       a Params::Callback object passed as their first argument; the same Params::Callback object
       will be used for all callbacks in a single request. For object-oriented callback methods,
       the first argument will of course always be an object of the class corresponding to the
       class key used in the callback key (or, for request callback methods, an instance of the
       class for which the request callback method was loaded), and the same object will be
       reused for all subsequent callbacks to the same class in a single request.

   Accessor Methods
       All of the Params::Callback accessor methods are read-only. Feel free to add other
       attributes in your Params::Callback subclasses if you're using the object-oriented
       callback interface.


         my $cb_request = $cb->cb_request;

       Returns a reference to the Params::CallbackRequest object that executed the callback.


         my $params = $cb->params;

       Returns a reference to the request parameters hash. Any changes you make to this hash will
       propagate beyond the lifetime of the request.


         my $r = $cb->apache_req;

       Returns the Apache request object for the current request, provided you've passed one to
       "Params::CallbackRequest->request". This will be most useful in a mod_perl environment, of
       course. Use Apache:FakeRequest in tests to emmulate the behavior of an Apache request


         my $r = $cb->requester;

       Returns the object that executed the callback by calling "request()" on a
       Params::CallbackRequest object. Only available if the "requester" parameter is passed to
       "Params::CallbackRequest->request". This can be useful for callbacks to get access to the
       object that executed the callbacks.


         my $priority = $cb->priority;

       Returns the priority level at which the callback was executed. Possible values range from
       "0" to "9", and may be set by a default priority setting, by the callback configuration or
       method declaration, or by the parameter callback trigger key. See Params::CallbackRequest
       for details.


         my $cb_key = $cb->cb_key;

       Returns the callback key that triggered the execution of the callback. For example, this
       callback-triggering parameter hash:

         my $params = { "DEFAULT|save_cb" => 'Save' };

       Will cause the "cb_key()" method in the relevant callback to return "save".


         my $pkg_key = $cb->pkg_key;

       Returns the package key used in the callback trigger parameter key. For example, this
       callback-triggering parameter hash:

         my $params = { "MyCBs|save_cb" => 'Save' };

       Will cause the "pkg_key()" method in the relevant callback to return "MyCBs".


         my $class_key = $cb->class_key;

       An alias for "pkg_key", only perhaps a bit more appealing for use in object-oriented
       callback methods.


         my $trigger_key = $cb->trigger_key;

       Returns the complete parameter key that triggered the callback. For example, if the
       parameter key that triggered the callback looks like this:

         my $params = { "MyCBs|save_cb6" => 'Save' };

       Then the value returned by "trigger_key()" method will be "MyCBs|save_cb6".

       Note: Most browsers will submit "image" input fields with two arguments, one with ".x"
       appended to its name, and the other with ".y" appended to its name. Because
       Params::CallbackRequest is designed to be used with Web form fields populating a parameter
       hash, it will ignore these fields and either use the field that's named without the ".x"
       or ".y", or create a field with that name and give it a value of "1". The reasoning behind
       this approach is that the names of the callback-triggering fields should be the same as
       the names that appear in the HTML form fields. If you want the actual x and y image click
       coordinates, access them directly from the request parameters:

         my $params = $cb->params;
         my $trigger_key = $cb->trigger_key;
         my $x = $params->{"$trigger_key.x"};
         my $y = $params->{"$trigger_key.y"};


         my $value = $cb->value;

       Returns the value of the parameter that triggered the callback. This value can be anything
       that can be stored in a hash value -- that is, any scalar value. Thus, in this example:

         my $params = { "DEFAULT|save_cb" => 'Save',
                        "DEFAULT|open_cb" => [qw(one two)] };

       "value()" will return the string "Save" in the save callback, but the array reference
       "['one', 'two']" in the open callback.

       Although you may often be able to retrieve the value directly from the hash reference
       returned by "params()", if multiple callback keys point to the same subroutine or if the
       parameter that triggered the callback overrode the priority, you may not be able to
       determine which value was submitted for a particular callback execution. So
       Params::Callback kindly provides the value for you. The exception to this rule is values
       submitted under keys named for HTML "image" input fields. See the note about this under
       the documentation for the "trigger_key()" method.


         $cb->redirect($url) unless $cb->redirected;

       If the request has been redirected, this method returns the redirection URL. Otherwise, it
       returns false. This method is useful for conditions in which one callback has called
       "$cb->redirect" with the optional $wait argument set to a true value, thus allowing
       subsequent callbacks to continue to execute. If any of those subsequent callbacks want to
       call "$cb->redirect" themselves, they can check the value of "$cb->redirected" to make
       sure it hasn't been done already.

   Other Methods
       Params::Callback offers has a few other publicly accessible methods.


         $cb->notes($key => $value);
         my $val = $cb->notes($key);
         my $notes = $cb->notes;

       Shortcut for "$cb->cb_request->notes". It provides a place to store application data,
       giving developers a way to share data among multiple callbacks. See "notes()" for more


         $cb->redirect($url, $wait);
         $cb->redirect($url, $wait, $status);

       This method can be used to redirect a request in a mod_perl environment, provided that an
       Apache request object has been passed to "Params::CallbackRequest->new".  Outide of a
       mod_perl environment or without an Apache request object, "redirect()" will still set the
       proper value for the the "redirected()" method to return, and will still abort the
       callback request.

       Given a URL, this method generates a proper HTTP redirect for that URL. By default, the
       status code used is "302", but this can be overridden via the $status argument. If the
       optional $wait argument is true, any callbacks scheduled to be executed after the call to
       "redirect" will continue to be executed. In that case, "$cb->abort" will not be called;
       rather, Params::CallbackRequest will finish executing all remaining callbacks and then
       return the abort status. If the $wait argument is unspecified or false, then the request
       will be immediately terminated without executing subsequent callbacks or. This approach
       relies on the execution of "$cb->abort".

       Since "$cb->redirect" calls "$cb->abort", it will be trapped by an "eval {}" block. If you
       are using an "eval {}" block in your code to trap exceptions, you need to make sure to
       rethrow these exceptions, like this:

         eval {

         die $@ if $cb->aborted;

         # handle other exceptions



       Aborts the current request without executing any more callbacks. The $status argument
       specifies a request status code to be returned to by "Params::CallbackRequest->request()".

       "abort()" is implemented by throwing a Params::Callback::Exception::Abort object and can
       thus be caught by "eval{}". The "aborted()" method is a shortcut for determining whether
       an exception was generated by "abort()".


         die $err if $cb->aborted;
         die $err if $cb->aborted($err);

       Returns true or "undef" to indicate whether the specified $err was generated by "abort()".
       If no $err argument is passed, "aborted()" examines $@, instead.

       In this code, we catch and process fatal errors while letting "abort()" exceptions pass

         eval { code_that_may_die_or_abort() };
         if (my $err = $@) {
             die $err if $cb->aborted($err);

             # handle fatal errors...

       $@ can lose its value quickly, so if you're planning to call "$cb->aborted" more than a
       few lines after the "eval", you should save $@ to a temporary variable and explicitly pass
       it to "aborted()" as in the above example.


       Under Perl 5.6.0 and later, Params::Callback offers an object-oriented callback interface.
       The object-oriented approach is to subclass Params::Callback, add the callback methods you
       need, and specify a class key that uniquely identifies your subclass across all
       Params::Callback subclasses in your application. The key is to use Perl method attributes
       to identify methods as callback methods, so that Params::Callback can find them and
       execute them when the time comes. Here's an example:

         package MyApp::CallbackHandler;
         use base qw(Params::Callback);
         use strict;

         __PACKAGE__->register_subclass( class_key => 'MyHandler' );

         sub build_utc_date : Callback( priority => 2 ) {
             my $self = shift;
             my $params = $self->params;
             $params->{date} = sprintf "%04d-%02d-%02dT%02d:%02d:%02d",
               delete @{$params}{qw(year month day hour minute second)};

       This parameter-triggered callback can then be executed via a parameter hash such as this:

         my $params = { "MyHandler|build_utc_date_cb" => 1 };

       Think of the part of the name preceding the pipe (the package key) as the class name, and
       the part of the name after the pipe (the callback key) as the method to call (plus '_cb').
       If multiple parameters use the "MyHandler" class key in a single request, then a single
       MyApp::CallbackHandler object instance will be used to execute each of those callback
       methods for that request.

       To configure your Params::CallbackRequest object to use this callback, use its
       "cb_classes" constructor parameter:

         my $cb_request = Params::CallbackRequest->new
           ( cb_classes => [qw(MyHandler)] );

       Now, there are a few of things to note in the above callback class example.  The first is
       the call to "__PACKAGE__->register_subclass". This step is required in all callback
       subclasses in order that Params::Callback will know about them, and thus they can be
       loaded into an instance of a Params::CallbackRequest object via its "cb_classes"
       constructor parameter.

       Second, a callback class key must be declared for the class. This can be done either by
       implementing the "CLASS_KEY()" class method or constant in your subclass, or by passing
       the "class_key" parameter to "__PACKAGE__->register_subclass", which will then create the
       "CLASS_KEY()" method for you. If no callback key is declared, then Params::Callback will
       throw an exception when you try to load your subclass' callback methods into a
       Params::CallbackRequest object.

       One other, optional parameter, "default_priority", may also be passed to
       "register_subclass()". The value of this parameter (an integer between 0 and 9) will be
       used to create a "DEFAULT_PRIORITY()" class method in the subclass. You can also
       explicitly implement the "DEFAULT_PRIORITY()" class method or constant in the subclass, if
       you'd rather. All parameter-triggered callback methods in that class will have their
       priorities set to the value returned by "DEFAULT_PRIORITY()", unless they override it via
       their "Callback" attributes.

       And finally, notice the "Callback" attribute on the "build_utc_date" method declaration in
       the example above. This attribute is what identifies "build_utc_date" as a parameter-
       triggered callback. Without the "Callback" attribute, any subroutine declaration in your
       subclass will just be a subroutine or a method; it won't be a callback, and it will never
       be executed by Params::CallbackRequest. One parameter, "priority", can be passed via the
       "Callback" attribute. In the above example, we pass "priority => 2", which sets the
       priority for the callback. Without the "priority" parameter, the callback's priority will
       be set to the value returned by the "DEFAULT_PRIORITY()" class method. Of course, the
       priority can still be overridden by adding it to the callback trigger key. For example,
       here we force the callback priority for the execution of the "build_utc_date" callback
       method for this one field to be the highest priority, "0":

         my $params = { "MyHandler|build_utc_date_cb0" => 1 };

       Other parameters to the "Callback" attribute may be added in future versions of

       Request callbacks can also be implemented as callback methods using the "PreCallback" and
       "PostCallback" attributes, which currently support no parameters.

   Subclassing Examples
       At this point, you may be wondering what advantage the object-oriented callback interface
       offer over functional callbacks. There are a number of advantages. First, it allows you to
       make use of callbacks provided by other users without having to reinvent the wheel for
       yourself. Say someone has implemented the above class with its exceptionally complex
       "build_utc_date()" callback method. You need to have the same functionality, only with
       fractions of a second added to the date format so that you can insert them into your
       database without an error. (This is admittedly a contrived example, but you get the idea.)
       To make it happen, you merely have to subclass the above class and override the
       "build_utc_date()" method to do what you need:

         package MyApp::Callback::Subclass;
         use base qw(MyApp::CallbackHandler);
         use strict;


         # Implement CLASS_KEY ourselves.
         use constant CLASS_KEY => 'SubHandler';

         sub build_utc_date : Callback( priority => 1 ) {
             my $self = shift;
             my $params = $self->params;
             $params->{date} .= '.000000';

       This callback can then be triggered by a parameter hash such as this:

         my $params = { "SubHandler|build_utc_date_cb" => 1 };

       Note that we've used the "SubHandler" class key. If we used the "MyHandler" class key,
       then the "build_utc_date()" method would be called on an instance of the
       MyApp::CallbackHandler class, instead.

       Request Callback Methods

       I'll admit that the case for request callback methods is a bit more tenuous. Granted, a
       given application may have 100s or even 1000s of parameter-triggered callbacks, but only
       one or two request callbacks, if any. But the advantage of request callback methods is
       that they encourage code sharing, in that Params::Callback creates a kind of plug-in
       architecture Perl templating architectures.

       For example, say someone has kindly created a Params::Callback subclass,
       Params::Callback::Unicodify, with the request callback method "unicodify()", which
       translates character sets, allowing you to always store data in the database in Unicode.
       That's all well and good, as far as it goes, but let's say that you want to make sure that
       your Unicode strings are actually encoded using the Perl "\x{..}" notation. Again, just

         package Params::Callback::Unicodify::PerlEncode;
         use base qw(Params::Callback::Unicodify);
         use strict;

         __PACKAGE__->register_subclass( class_key => 'PerlEncode' );

         sub unicodify : PreCallback {
             my $self = shift;
             my $params = $self->params;
             encode_unicode($params); # Hand waving.

       Now you can just tell Params::CallbackRequest to use your subclassed callback handler:

         my $cb_request = Params::CallbackRequest->new
           ( cb_classes => [qw(PerlEncode)] );

       Yeah, okay, you could just create a second pre-callback request callback to encode the
       Unicode characters using the Perl "\x{..}" notation. But you get the idea. Better examples

       Overriding the Constructor

       Another advantage to using callback classes is that you can override the Params::Callback
       "new()" constructor. Since every callback for a single class will be executed on the same
       instance object in a single request, you can set up object properties in the constructor
       that subsequent callback methods in the same request can then access.

       For example, say you had a series of pages that all do different things to manage objects
       in your application. Each of those pages might have a number of parameters in common to
       assist in constructing an object:

         my $params = { class  => "MyApp::Spring",
                        obj_id => 10,
                        # ...

       Then the remaining parameters created for each of these pages have different key/value
       pairs for doing different things with the object, perhaps with numerous parameter-
       triggered callbacks. Here's where subclassing comes in handy: you can override the
       constructor to construct the object when the callback object is constructed, so that each
       of your callback methods doesn't have to:

         package MyApp::Callback;
         use base qw(Params::Callback);
         use strict;
         __PACKAGE__->register_subclass( class_key => 'MyCBHandler' );

         sub new {
             my $class = shift;
             my $self = $class->SUPER::new(@_);
             my $params = $self->params;
             $self->object($params->{class}->lookup( id => $params->{obj_id} ));

         sub object {
             my $self = shift;
             if (@_) {
                 $self->{object} = shift;
             return $self->{object};

         sub save : Callback {
             my $self = shift;


       Much of the interface for subclassing Params::Callback is evident in the above examples.
       Here is a reference to the complete callback subclassing API.

   Callback Class Declaration
       Callback classes always subclass Params::Callback, so of course they must always declare
       such. In addition, callback classes must always call "__PACKAGE__->register_subclass" so
       that Params::Callback is aware of them and can tell Params::CallbackRequest about them.

       Second, callback classes must have a class key. The class key can be created either by
       implementing a "CLASS_KEY()" class method or constant that returns the class key, or by
       passing the "class_key" parameter to "register_subclass()" method. If no "class_key"
       parameter is passed to "register_subclass()" and no "CLASS_KEY()" method exists,
       "register_subclass()" will create the "CLASS_KEY()" class method to return the actual
       class name. So here are a few example callback class declarations:

         package MyApp::Callback;
         use base qw(Params::Callback);
         __PACKAGE__->register_subclass( class_key => 'MyCBHandler' );

       In this declaration "register_subclass()" will create a "CLASS_KEY()" class method
       returning "MyCBHandler" in the MyApp::CallbackHandler class.

         package MyApp::AnotherCallback;
         use base qw(MyApp::Callback);
         use constant CLASS_KEY => 'AnotherCallback';

       In this declaration, we've created an explicit "CLASS_KEY()" class method (using the handy
       "use constant" syntax, so that "register_subclass()" doesn't have to.

         package MyApp::Callback::Foo;
         use base qw(Params::Callback);

       And in this callback class declaration, we've specified neither a "class_key" parameter to
       "register_subclass()", nor created a "CLASS_KEY()" class method. This causes
       "register_subclass()" to create the "CLASS_KEY()" class method returning the name of the
       class itself, i.e., "MyApp::FooHandler". Thus any parameter-triggered callbacks in this
       class can be triggered by using the class name in the trigger key:

         my $params = { "MyApp::Callback::Foo|take_action_cb" => 1 };

       A second, optional parameter, "default_priority", may also be passed to
       "register_subclass()" in order to set a default priority for all of the methods in the
       class (and for all the methods in subclasses that don't declare their own

         package MyApp::Callback;
         use base qw(Params::Callback);
         __PACKAGE__->register_subclass( class_key => 'MyCB',
                                         default_priority => 7 );

       As with the "class_key" parameter, the "default_priority" parameter creates a class
       method, "DEFAULT_PRIORITY()". If you'd rather, you can create this class method yourself;
       just be sure that its value is a valid priority -- that is, an integer between "0" and

         package MyApp::Callback;
         use base qw(Params::Callback);
         use constant DEFAULT_PRIORITY => 7;
         __PACKAGE__->register_subclass( class_key => 'MyCB' );

       Any callback class that does not specify a default priority via the "default_priority" or
       by implementing a <DEFAULT_PRIORITY()> class method will simply inherit the priority
       returned by "Params::Callback->DEFAULT_PRIORITY", which is "5".

       Note: In a mod_perl environment, 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 source code if you're interested in learning more.

   Method Attributes
       These method attributes are required to create callback methods in Params::Callback


         sub take_action : Callback {
             my $self = shift;
             # Do stuff.

       This attribute identifies a parameter-triggered callback method. The callback key is the
       same as the method name ("take_action" in this example). The priority for the callback may
       be set via an optional "priority" parameter to the "Callback" attribute, like so:

         sub take_action : Callback( priority => 5 ) {
             my $self = shift;
             # Do stuff.

       Otherwise, the priority will be that returned by "$self->DEFAULT_PRIORITY".

       Note: The priority set via the "priority" parameter to the "Callback" attribute is not
       inherited by any subclasses that override the callback method. This may change in the


         sub early_action : PreCallback {
             my $self = shift;
             # Do stuff.

       This attribute identifies a method as a request callback that gets executed for every
       request before any parameter-triggered callbacks are executed .  No parameters to
       "PreCallback" are currently supported.


         sub late_action : PostCallback {
             my $self = shift;
             # Do stuff.

       This attribute identifies a method as a request callback that gets executed for every
       request after any parameter-triggered callbacks are executed . No parameters to
       "PostCallback" are currently supported.


       ยท   Allow methods that override parent methods to inherit the parent method's priority?


       Params::CallbackRequest constructs Params::Callback objects and executes the appropriate
       callback functions and/or methods. It's worth a read.


       This module is stored in an open repository at the following address:


       Patches against Params::CallbackRequest are welcome. Please send bug reports to


       David E. Wheeler <>


       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.