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POPython duck-typing for MVC event handling in pygame
 

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    <p>A friend and I have been playing around with pygame some and came across <a href="http://ezide.com/games/writing-games.html" rel="nofollow">this tutorial for building games</a> using pygame. We really liked how it broke out the game into a model-view-controller system with events as a go-between, but the code makes <em>heavy</em> use of <code>isinstance</code> checks for the event system.</p> <p>Example:</p> <pre><code>class CPUSpinnerController: ... def Notify(self, event): if isinstance( event, QuitEvent ): self.keepGoing = 0 </code></pre> <p>This results in some extremely unpythonic code. Does anyone have any suggestions on how this could be improved? Or an alternative methodology for implementing MVC?</p> <hr> <p>This is a bit of code I wrote based on @Mark-Hildreth answer (how do I link users?) Does anyone else have any good suggestions? I'm going to leave this open for another day or so before picking a solution.</p> <pre><code>class EventManager: def __init__(self): from weakref import WeakKeyDictionary self.listeners = WeakKeyDictionary() def add(self, listener): self.listeners[ listener ] = 1 def remove(self, listener): del self.listeners[ listener ] def post(self, event): print "post event %s" % event.name for listener in self.listeners.keys(): listener.notify(event) class Listener: def __init__(self, event_mgr=None): if event_mgr is not None: event_mgr.add(self) def notify(self, event): event(self) class Event: def __init__(self, name="Generic Event"): self.name = name def __call__(self, controller): pass class QuitEvent(Event): def __init__(self): Event.__init__(self, "Quit") def __call__(self, listener): listener.exit(self) class RunController(Listener): def __init__(self, event_mgr): Listener.__init__(self, event_mgr) self.running = True self.event_mgr = event_mgr def exit(self, event): print "exit called" self.running = False def run(self): print "run called" while self.running: event = QuitEvent() self.event_mgr.post(event) em = EventManager() run = RunController(em) run.run() </code></pre> <p>This is another build using the examples from @Paul - impressively simple!</p> <pre><code>class WeakBoundMethod: def __init__(self, meth): import weakref self._self = weakref.ref(meth.__self__) self._func = meth.__func__ def __call__(self, *args, **kwargs): self._func(self._self(), *args, **kwargs) class EventManager: def __init__(self): # does this actually do anything? self._listeners = { None : [ None ] } def add(self, eventClass, listener): print "add %s" % eventClass.__name__ key = eventClass.__name__ if (hasattr(listener, '__self__') and hasattr(listener, '__func__')): listener = WeakBoundMethod(listener) try: self._listeners[key].append(listener) except KeyError: # why did you not need this in your code? self._listeners[key] = [listener] print "add count %s" % len(self._listeners[key]) def remove(self, eventClass, listener): key = eventClass.__name__ self._listeners[key].remove(listener) def post(self, event): eventClass = event.__class__ key = eventClass.__name__ print "post event %s (keys %s)" % ( key, len(self._listeners[key])) for listener in self._listeners[key]: listener(event) class Event: pass class QuitEvent(Event): pass class RunController: def __init__(self, event_mgr): event_mgr.add(QuitEvent, self.exit) self.running = True self.event_mgr = event_mgr def exit(self, event): print "exit called" self.running = False def run(self): print "run called" while self.running: event = QuitEvent() self.event_mgr.post(event) em = EventManager() run = RunController(em) run.run() </code></pre>
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    1. COBtw, your `name` parameter in `Event.__init__` is unnecessary. The name of the class is already stored by Python. Print `QuitEvent.__name__` to see. :) Also, if you have an object instance, you can do `obj.__class__.__name__` to get its class' name string.
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    2. COAll that seems alright. But don't forget to remove the listener from the event manager before (or when) your RunController object gets destroyed! Other than that, I don't see any issues. I still think you should create the WeakBoundMethod inside RunController__init__ instead of inside Eventmanager.add. The EventManager should be agnostic to the kind of listener it receives.
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    3. CORe: *'does this actually do anything?'* -- No, but I like to clearly state in my `__init__` functions what attributes the class has. That line makes it clear that self._listeners is a dict that has objects as keys and lists as values.
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