Dispatching Events¶

The pyglet.window, pyglet.media, pyglet.app, pyglet.text, 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 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 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 Window class subclasses 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 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 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 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 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. 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 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 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 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 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 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 Window, 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:

Subclass EventDispatcher
Call the register_event_type() class method on your subclass for each event your subclass will recognise.
Call 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 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 EventDispatcher suitable for use even on light-weight objects that may not always have handlers. For example, Player is an 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.

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.