Dispatching Events
==================

The :py:mod:`pyglet.window`, :py:mod:`pyglet.media`, :py:mod:`pyglet.app`,
:py:mod:`pyglet.text`, :py:mod:`pyglet.input` and other modules make use
of a consistent event pattern.  This provides several ways to attach event
handlers to objects.  You can also reuse this pattern in your own
classes easily, by subclassing :py:class:`~pyglet.event.EventDispatcher`.

Throughout this documentation, an "event dispatcher" is an object that has
events it needs to notify other objects about, and an "event handler" is some
code that can be attached to a dispatcher.

Setting event handlers
----------------------

An event handler is simply a function with a formal parameter list
corresponding to the event type. For example, the
:py:meth:`pyglet.window.Window.on_resize` event has the parameters
``(width, height)``, so an event handler for this event could be written as::

    def on_resize(width, height):
        pass

The :py:class:`~pyglet.window.Window` class subclasses
:py:class:`~pyglet.event.EventDispatcher`, which enables it to dispatch
its own events.  There are a few different ways in which event handlers
can be attached to recieve them. The simplest way is to directly attach the
event handler to the corresponding attribute on the object.  This will
completely replace the default event handler::

    window = pyglet.window.Window()

    def on_resize(width, height):
        pass
    window.on_resize = on_resize

If you don't want to replace the default event handler, but instead want to
add an additional one, pyglet provides a shortcut using the
:py:class:`~pyglet.event.EventDispatcher.event` decorator.
Your custom event handler will run, followed by the default event handler::

    window = window.Window()

    @window.event
    def on_resize(width, height):
        pass

or if your handler has a different name::

    @window.event('on_resize')
    def my_resize_handler(width, height):
        pass

In some cases, replacing the default event handler may be desired.
For example, the default :py:meth:`pyglet.window.Window.on_resize` event
sets up a 2D orthographic OpenGL projection. If you wish to use another
OpenGL projection, such as for a 3D scene, then you will likely want
to replace this with your own custom event handler.

In most simple cases, the :py:class:`~pyglet.event.EventDispatcher.event`
decorator is most convienent.  One limitation of using the decorator,
however, is that you can only add one additional event handler.
If you want to add multiple additional event handlers, the next section
describes how to accomplish that.

As a quick note, as shown in :ref:`guide_subclassing-window`,
you can also replace default event handlers by subclassing the event
dispatcher and adding the event handler as a method::

    class MyWindow(pyglet.window.Window):
        def on_resize(self, width, height):
            pass

Stacking event handlers
-----------------------

It is often convenient to attach more than one event handler for an event.
:py:class:`~pyglet.event.EventDispatcher` allows you to stack event handlers
upon one another, rather than replacing them outright. The event will
propagate from the top of the stack to the bottom, but can be stopped
by any handler along the way.

To push an event handler onto the stack,
use the :py:meth:`~pyglet.event.EventDispatcher.push_handlers` method::

    def on_key_press(symbol, modifiers):
        if symbol == key.SPACE:
            fire_laser()

    window.push_handlers(on_key_press)

One use for pushing handlers instead of setting them is to handle different
parameterisations of events in different functions.  In the above example, if
the spacebar is pressed, the laser will be fired.  After the event handler
returns control is passed to the next handler on the stack, which on a
:py:class:`~pyglet.window.Window` is a function that checks for the ESC key
and sets the ``has_exit`` attribute if it is pressed.  By pushing the event
handler instead of setting it, the application keeps the default behaviour
while adding additional functionality.

You can prevent the remaining event handlers in the stack from receiving the
event by returning a true value.  The following event handler, when pushed
onto the window, will prevent the escape key from exiting the program::

    def on_key_press(symbol, modifiers):
        if symbol == key.ESCAPE:
            return True

    window.push_handlers(on_key_press)

You can push more than one event handler at a time, which is especially useful
when coupled with the :py:meth:`~pyglet.event.EventDispatcher.pop_handlers`
function. In the following example, when the game starts some additional
event handlers are pushed onto the stack. When the game ends (perhaps
returning to some menu screen) the handlers are popped off in one go::

    def start_game():
        def on_key_press(symbol, modifiers):
            print('Key pressed in game')
            return True

        def on_mouse_press(x, y, button, modifiers):
            print('Mouse button pressed in game')
            return True

        window.push_handlers(on_key_press, on_mouse_press)

    def end_game():
        window.pop_handlers()

Note that you do not specify which handlers to pop off the stack -- the entire
top "level" (consisting of all handlers specified in a single call to
:py:meth:`~pyglet.event.EventDispatcher.push_handlers`) is popped.

You can apply the same pattern in an object-oriented fashion by grouping
related event handlers in a single class.  In the following example, a
``GameEventHandler`` class is defined.  An instance of that class can be
pushed on and popped off of a window::

    class GameEventHandler:
        def on_key_press(self, symbol, modifiers):
            print('Key pressed in game')
            return True

        def on_mouse_press(self, x, y, button, modifiers):
            print('Mouse button pressed in game')
            return True

    game_handlers = GameEventHandler()

    def start_game()
        window.push_handlers(game_handlers)

    def stop_game()
        window.pop_handlers()

.. note::

    In order to prevent issues with garbage collection, the
    :py:class:`~pyglet.event.EventDispatcher` class only holds weak
    references to pushed event handlers. That means the following example
    will not work, because the pushed object will fall out of scope and be
    collected::

        dispatcher.push_handlers(MyHandlerClass())

    Instead, you must make sure to keep a reference to the object before pushing
    it. For example::

        my_handler_instance = MyHandlerClass()
        dispatcher.push_handlers(my_handler_instance)

Creating your own event dispatcher
----------------------------------

pyglet provides the :py:class:`~pyglet.window.Window`,
:py:class:`~pyglet.media.player.Player`, and other event dispatchers,
but exposes a public interface for creating and dispatching your own events.

The steps for creating an event dispatcher are:

1. Subclass :py:class:`~pyglet.event.EventDispatcher`
2. Call the :py:meth:`~pyglet.event.EventDispatcher. register_event_type`
   class method on your subclass for each event your subclass will recognise.
3. Call :py:meth:`~pyglet.event.EventDispatcher. dispatch_event` to create and
   dispatch an event as needed.

In the following example, a hypothetical GUI widget provides several events::

    class ClankingWidget(pyglet.event.EventDispatcher):
        def clank(self):
            self.dispatch_event('on_clank')

        def click(self, clicks):
            self.dispatch_event('on_clicked', clicks)

        def on_clank(self):
            print('Default clank handler.')

    ClankingWidget.register_event_type('on_clank')
    ClankingWidget.register_event_type('on_clicked')

Event handlers can then be attached as described in the preceding sections::

    widget = ClankingWidget()

    @widget.event
    def on_clank():
        pass

    @widget.event
    def on_clicked(clicks):
        pass

    def override_on_clicked(clicks):
        pass

    widget.push_handlers(on_clicked=override_on_clicked)

The :py:class:`~pyglet.event.EventDispatcher` takes care of propagating the
event to all attached handlers or ignoring it if there are no handlers for
that event.

There is zero instance overhead on objects that have no event handlers
attached (the event stack is created only when required).  This makes
:py:class:`~pyglet.event.EventDispatcher` suitable for use even on light-weight
objects that may not always have handlers.  For example,
:py:class:`~pyglet.media.player.Player` is an
:py:class:`~pyglet.event.EventDispatcher` even though potentially hundreds
of these objects may be created and destroyed each second, and most will
not need an event handler.

Implementing the Observer pattern
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The `Observer design pattern`_, also known as Publisher/Subscriber, is a
simple way to decouple software components.  It is used extensively in many
large software projects; for example, Java's AWT and Swing GUI toolkits and the
Python ``logging`` module; and is fundamental to any Model-View-Controller
architecture.

:py:class:`~pyglet.event.EventDispatcher` can be used to easily add
observerable components to your application.  The following example recreates
the `ClockTimer` example from `Design Patterns` (pages 300-301), though
without needing the bulky ``Attach``, ``Detach`` and ``Notify`` methods::

    # The subject
    class ClockTimer(pyglet.event.EventDispatcher):
        def tick(self):
            self.dispatch_event('on_update')
    ClockTimer.register_event_type('on_update')

    # Abstract observer class
    class Observer:
        def __init__(self, subject):
            subject.push_handlers(self)

    # Concrete observer
    class DigitalClock(Observer):
        def on_update(self):
            pass

    # Concrete observer
    class AnalogClock(Observer):
        def on_update(self):
            pass

    timer = ClockTimer()
    digital_clock = DigitalClock(timer)
    analog_clock = AnalogClock(timer)

The two clock objects will be notified whenever the timer is "ticked", though
neither the timer nor the clocks needed prior knowledge of the other.  During
object construction any relationships between subjects and observers can be
created.

.. _Observer design pattern: Gamma, et al., `Design Patterns` Addison-Wesley 1994
