Event dispatch framework.
All objects that produce events in pyglet implement
providing a consistent interface for registering and manipulating event
handlers. A commonly used event dispatcher is pyglet.window.Window.
For each event dispatcher there is a set of events that it dispatches; these
correspond with the type of event handlers you can attach. Event types are
identified by their name, for example, ‘’on_resize’’. If you are creating a
new class which implements
EventDispatcher, you must call
EventDispatcher.register_event_type for each event type.
Attaching event handlers¶
An event handler is simply a function or method. You can attach an event handler by setting the appropriate function on the instance:
def on_resize(width, height): # ... dispatcher.on_resize = on_resize
There is also a convenience decorator that reduces typing:
@dispatcher.event def on_resize(width, height): # ...
You may prefer to subclass and override the event handlers instead:
class MyDispatcher(DispatcherClass): def on_resize(self, width, height): # ...
Event handler stack¶
When attaching an event handler to a dispatcher using the above methods, it replaces any existing handler (causing the original handler to no longer be called). Each dispatcher maintains a stack of event handlers, allowing you to insert an event handler “above” the existing one rather than replacing it.
There are two main use cases for “pushing” event handlers:
- Temporarily intercepting the events coming from the dispatcher by pushing a custom set of handlers onto the dispatcher, then later “popping” them all off at once.
- Creating “chains” of event handlers, where the event propagates from the top-most (most recently added) handler to the bottom, until a handler takes care of it.
Use EventDispatcher.push_handlers to create a new level in the stack and attach handlers to it. You can push several handlers at once:
If your function handlers have different names to the events they handle, use keyword arguments:
After an event handler has processed an event, it is passed on to the next-lowest event handler, unless the handler returns EVENT_HANDLED, which prevents further propagation.
To remove all handlers on the top stack level, use EventDispatcher.pop_handlers.
Note that any handlers pushed onto the stack have precedence over the
handlers set directly on the instance (for example, using the methods
described in the previous section), regardless of when they were set.
For example, handler
foo is called before handler
bar in the following
dispatcher.push_handlers(on_resize=foo) dispatcher.on_resize = bar
pyglet uses a single-threaded model for all application code. Event handlers are only ever invoked as a result of calling EventDispatcher.dispatch_events`.
It is up to the specific event dispatcher to queue relevant events until they can be dispatched, at which point the handlers are called in the order the events were originally generated.
This implies that your application runs with a main loop that continuously updates the application state and checks for new events:
while True: dispatcher.dispatch_events() # ... additional per-frame processing
Not all event dispatchers require the call to
dispatch_events; check with
the particular class documentation.
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
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)
An exception raised when an event handler could not be attached.
Generic event dispatcher interface.
See the module docstring for usage.
Dispatch a single event to the attached handlers.
The event is propagated to all handlers from from the top of the stack until one returns EVENT_HANDLED. This method should be used only by
EventDispatcherimplementors; applications should call the
Since pyglet 1.2, the method returns EVENT_HANDLED if an event handler returned EVENT_HANDLED or EVENT_UNHANDLED if all events returned EVENT_UNHANDLED. If no matching event handlers are in the stack,
- event_type : str
Name of the event.
- args : sequence
Arguments to pass to the event handler.
bool or None
(Since pyglet 1.2) EVENT_HANDLED if an event handler returned EVENT_HANDLED; EVENT_UNHANDLED if one or more event handlers were invoked but returned only EVENT_UNHANDLED; otherwise
False. In pyglet 1.1 and earlier, the return value is always
Function decorator for an event handler.
win = window.Window() @win.event def on_resize(self, width, height): # ...
@win.event('on_resize') def foo(self, width, height): # ...
Pop the top level of event handlers off the stack.
Push a level onto the top of the handler stack, then attach zero or more event handlers.
If keyword arguments are given, they name the event type to attach. Otherwise, a callable’s __name__ attribute will be used. Any other object may also be specified, in which case it will be searched for callables with event names.
Register an event type with the dispatcher.
Registering event types allows the dispatcher to validate event handler names as they are attached, and to search attached objects for suitable handlers.
- name : str
Name of the event to register.
Remove a single event handler.
The given event handler is removed from the first handler stack frame it appears in. The handler must be the exact same callable as passed to set_handler, set_handlers or
push_handlers(); and the name must match the event type it is bound to.
No error is raised if the event handler is not set.
- name : str
Name of the event type to remove.
- handler : callable
Event handler to remove.
Remove event handlers from the event stack.
push_handlers()for the accepted argument types. All handlers are removed from the first stack frame that contains any of the given handlers. No error is raised if any handler does not appear in that frame, or if no stack frame contains any of the given handlers.
If the stack frame is empty after removing the handlers, it is removed from the stack. Note that this interferes with the expected symmetry of
Attach a single event handler.
- name : str
Name of the event type to attach to.
- handler : callable
Event handler to attach.
Attach one or more event handlers to the top level of the handler stack.
push_handlers()for the accepted argument types.