Question

The following code is an alternative to the usual style (using decorators) pyglet makes use of.

Can anyone explain how the on_draw() method is called here?

import pyglet

class HelloWorldWindow(pyglet.window.Window): 
    def __init__(self):
        super(HelloWorldWindow, self).__init__() 
        self.label = pyglet.text.Label('Hello, world!') 

    def on_draw(self):
        self.clear() 
        self.label.draw() 

if __name__ == '__main__':
    window = HelloWorldWindow() 
    pyglet.app.run()

The code written using decorators can be found here.

Answer 1

You can just dig through the source to find the answer.

The EventLoop class (you use it by pyglet.app.run()) dispatches the on_draw event regulary.

From the source:

Calling run begins the application event loop, which processes
operating system events, calls pyglet.clock.tick to call scheduled
functions and calls pyglet.window.Window.on_draw and
pyglet.window.Window.flip to update window contents.

The Window class subscripes to this event:

BaseWindow.register_event_type('on_draw')

So by subclassing Window, you ensure your on_draw method gets called.

Look at the programming guide for an example of how the event system of pyglet works.