NAME

       App::Info::Handler - App::Info event handler base class

SYNOPSIS

         use App::Info::Category::FooApp;
         use App::Info::Handler;

         my $app = App::Info::Category::FooApp->new( on_info => ['default'] );

DESCRIPTION

       This class defines the interface for subclasses that wish to handle events triggered by App::Info
       concrete subclasses. The different types of events triggered by App::Info can all be handled by
       App::Info::Handler (indeed, by default they're all handled by a single App::Info::Handler object), and
       App::Info::Handler subclasses may be designed to handle whatever events they wish.

       If you're interested in using an App::Info event handler, this is probably not the class you should look
       at, since all it does is define a simple handler that does nothing with an event. Look to the
       App::Info::Handler subclasses included in this distribution to do more interesting things with App::Info
       events.

       If, on the other hand, you're interested in implementing your own event handlers, read on!

INTERFACE

       This section documents the public interface of App::Info::Handler.

   Class Method
       register_handler

         App::Info::Handler->register_handler( $key => $code_ref );

       This class method may be used by App::Info::Handler subclasses to register themselves with
       App::Info::Handler. Multiple registrations are supported. The idea is that a subclass can define
       different functionality by specifying different strings that represent different modes of constructing an
       App::Info::Handler subclass object. The keys are case-sensitive, and should be unique across
       App::Info::Handler subclasses so that many subclasses can be loaded and used separately. If the $key is
       already registered, "register_handler()" will throw an exception. The values are code references that,
       when executed, return the appropriate App::Info::Handler subclass object.

   Constructor
       new

         my $handler = App::Info::Handler->new;
         $handler =  App::Info::Handler->new( key => $key);

       Constructs an App::Info::Handler object and returns it. If the key parameter is provided and has been
       registered by an App::Info::Handler subclass via the "register_handler()" class method, then the relevant
       code reference will be executed and the resulting App::Info::Handler subclass object returned. This
       approach provides a handy shortcut for having "new()" behave as an abstract factory method, returning an
       object of the subclass appropriate to the key parameter.

   Instance Method
       handler

         $handler->handler($req);

       App::Info::Handler defines a single instance method that must be defined by its subclasses, "handler()".
       This is the method that will be executed by an event triggered by an App::Info concrete subclass. It
       takes as its single argument an App::Info::Request object, and returns a true value if it has handled the
       event request. Returning a false value declines the request, and App::Info will then move on to the next
       handler in the chain.

       The "handler()" method implemented in App::Info::Handler itself does nothing more than return a true
       value. It thus acts as a very simple default event handler. See the App::Info::Handler subclasses for
       more interesting handling of events, or create your own!

SUBCLASSING

       I hatched the idea of the App::Info event model with its subclassable handlers as a way of separating the
       aggregation of application meta data from writing a user interface for handling certain conditions. I
       felt it a better idea to allow people to create their own user interfaces, and instead to provide only a
       few examples. The App::Info::Handler class defines the API interface for handling these conditions, which
       App::Info refers to as "events".

       There are various types of events defined by App::Info ("info", "error", "unknown", and "confirm"), but
       the App::Info::Handler interface is designed to be flexible enough to handle any and all of them. If
       you're interested in creating your own App::Info event handler, this is the place to learn how.

   The Interface
       To create an App::Info event handler, all one need do is subclass App::Info::Handler and then implement
       the "new()" constructor and the "handler()" method. The "new()" constructor can do anything you like, and
       take any arguments you like. However, I do recommend that the first thing you do in your implementation
       is to call the super constructor:

         sub new {
             my $pkg = shift;
             my $self = $pkg->SUPER::new(@_);
             # ... other stuff.
             return $self;
         }

       Although the default "new()" constructor currently doesn't do much, that may change in the future, so
       this call will keep you covered. What it does do is take the parameterized arguments and assign them to
       the App::Info::Handler object. Thus if you've specified a "mode" argument, where clients can construct
       objects of you class like this:

         my $handler = FooHandler->new( mode => 'foo' );

       You can access the mode parameter directly from the object, like so:

         sub new {
             my $pkg = shift;
             my $self = $pkg->SUPER::new(@_);
             if ($self->{mode} eq 'foo') {
                 # ...
             }
             return $self;
         }

       Just be sure not to use a parameter key name required by App::Info::Handler itself. At the moment, the
       only parameter accepted by App::Info::Handler is "key", so in general you'll be pretty safe.

       Next, I recommend that you take advantage of the "register_handler()" method to create some shortcuts for
       creating handlers of your class. For example, say we're creating a handler subclass FooHandler. It has
       two modes, a default "foo" mode and an advanced "bar" mode. To allow both to be constructed by
       stringified shortcuts, the FooHandler class implementation might start like this:

         package FooHandler;

         use strict;
         use App::Info::Handler;
         use vars qw(@ISA);
         @ISA = qw(App::Info::Handler);

         foreach my $c (qw(foo bar)) {
             App::Info::Handler->register_handler
               ( $c => sub { __PACKAGE__->new( mode => $c) } );
         }

       The strings "foo" and "bar" can then be used by clients as shortcuts to have App::Info objects
       automatically create and use handlers for certain events.  For example, if a client wanted to use a "bar"
       event handler for its info events, it might do this:

         use App::Info::Category::FooApp;
         use FooHandler;

         my $app = App::Info::Category::FooApp->new(on_info => ['bar']);

       Take a look at App::Info::Handler::Print and App::Info::Handler::Carp to see concrete examples of
       "register_handler()" usage.

       The final step in creating a new App::Info event handler is to implement the "handler()" method itself.
       This method takes a single argument, an App::Info::Request object, and is expected to return true if it
       handled the request, and false if it did not. The App::Info::Request object contains all the meta data
       relevant to a request, including the type of event that triggered it; see App::Info::Request for its
       documentation.

       Use the App::Info::Request object however you like to handle the request however you like. You are,
       however, expected to abide by a a few guidelines:

       •   For error and info events, you are expected (but not required) to somehow display the info or error
           message for the user. How your handler chooses to do so is up to you and the handler.

       •   For unknown and confirm events, you are expected to prompt the user for a value. If it's a confirm
           event, offer the known value (found in "$req->value") as a default.

       •   For unknown and confirm events, you are expected to call "$req->callback" and pass in the new value.
           If "$req->callback" returns a false value, you are expected to display the error message in
           "$req->error" and prompt the user again. Note that "$req->value" calls "$req->callback" internally,
           and thus assigns the value and returns true if "$req->callback" returns true, and does not assign the
           value and returns false if "$req->callback" returns false.

       •   For unknown and confirm events, if you've collected a new value and "$req->callback" returns true for
           that value, you are expected to assign the value by passing it to "$req->value". This allows
           App::Info to give the value back to the calling App::Info concrete subclass.

       Probably the easiest way to get started creating new App::Info event handlers is to check out the simple
       handlers provided with the distribution and follow their logical examples. Consult the App::Info
       documentation of the event methods for details on how App::Info constructs the App::Info::Request object
       for each event type.

SUPPORT

       This module is stored in an open GitHub repository <http://github.com/theory/app-info/>. Feel free to
       fork and contribute!

       Please file bug reports via GitHub Issues <http://github.com/theory/app-info/issues/> or by sending mail
       to bug-App-Info@rt.cpan.org <mailto:bug-App-Info@rt.cpan.org>.

AUTHOR

       David E. Wheeler <david@justatheory.com>

SEE ALSO

       App::Info thoroughly documents the client interface for setting event handlers, as well as the event
       triggering interface for App::Info concrete subclasses.

       App::Info::Request documents the interface for the request objects passed to App::Info::Handler
       "handler()" methods.

       The following App::Info::Handler subclasses offer examples for event handler authors, and, of course,
       provide actual event handling functionality for App::Info clients.

       App::Info::Handler::Carp
       App::Info::Handler::Print
       App::Info::Handler::Prompt

COPYRIGHT AND LICENSE

       Copyright (c) 2002-2011, David E. Wheeler. Some Rights Reserved.

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