David E. Wheeler


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


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

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


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!


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

Class Method


  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.



  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



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!


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)) {
        ( $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.


This module is stored in an open GitHub repository. Feel free to fork and contribute!

Please file bug reports via GitHub Issues or by sending mail to bug-App-Info@rt.cpan.org.


David E. Wheeler <david@justatheory.com>


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.



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.