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139 lines
4.6 KiB
Python
139 lines
4.6 KiB
Python
# -*- test-case-name: allmydata.test.test_observer -*-
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import weakref
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from twisted.internet import defer
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from foolscap.api import eventually
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"""The idiom we use is for the observed object to offer a method named
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'when_something', which returns a deferred. That deferred will be fired when
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something happens. The way this is typically implemented is that the observed
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has an ObserverList whose when_fired method is called in the observed's
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'when_something'."""
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class OneShotObserverList:
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"""A one-shot event distributor."""
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def __init__(self):
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self._fired = False
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self._result = None
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self._watchers = []
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self.__repr__ = self._unfired_repr
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def _unfired_repr(self):
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return "<OneShotObserverList [%s]>" % (self._watchers, )
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def _fired_repr(self):
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return "<OneShotObserverList -> %s>" % (self._result, )
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def _get_result(self):
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return self._result
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def when_fired(self):
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if self._fired:
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return defer.succeed(self._get_result())
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d = defer.Deferred()
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self._watchers.append(d)
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return d
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def fire(self, result):
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assert not self._fired
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self._fired = True
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self._result = result
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self._fire(result)
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def _fire(self, result):
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for w in self._watchers:
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eventually(w.callback, result)
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del self._watchers
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self.__repr__ = self._fired_repr
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def fire_if_not_fired(self, result):
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if not self._fired:
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self.fire(result)
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class LazyOneShotObserverList(OneShotObserverList):
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"""
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a variant of OneShotObserverList which does not retain
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the result it handles, but rather retains a callable()
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through which is retrieves the data if and when needed.
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"""
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def __init__(self):
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OneShotObserverList.__init__(self)
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def _get_result(self):
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return self._result_producer()
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def fire(self, result_producer):
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"""
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@param result_producer: a no-arg callable which
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returns the data which is to be considered the
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'result' for this observer list. note that this
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function may be called multiple times - once
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upon initial firing, and potentially once more
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for each subsequent when_fired() deferred created
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"""
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assert not self._fired
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self._fired = True
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self._result_producer = result_producer
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if self._watchers: # if not, don't call result_producer
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self._fire(self._get_result())
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class ObserverList:
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"""A simple class to distribute events to a number of subscribers."""
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def __init__(self):
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self._watchers = []
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def subscribe(self, observer):
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self._watchers.append(observer)
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def unsubscribe(self, observer):
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self._watchers.remove(observer)
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def notify(self, *args, **kwargs):
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for o in self._watchers:
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eventually(o, *args, **kwargs)
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class EventStreamObserver:
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"""A simple class to distribute multiple events to a single subscriber.
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It accepts arbitrary kwargs, but no posargs."""
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def __init__(self):
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self._watcher = None
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self._undelivered_results = []
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self._canceler = None
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def set_canceler(self, c, methname):
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"""I will call c.METHNAME(self) when somebody cancels me."""
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# we use a weakref to avoid creating a cycle between us and the thing
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# we're observing: they'll be holding a reference to us to compare
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# against the value we pass to their canceler function. However,
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# since bound methods are first-class objects (and not kept alive by
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# the object they're bound to), we can't just stash a weakref to the
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# bound cancel method. Instead, we must hold a weakref to the actual
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# object, and obtain its cancel method later.
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# http://code.activestate.com/recipes/81253-weakmethod/ has an
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# alternative.
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self._canceler = (weakref.ref(c), methname)
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def subscribe(self, observer, **watcher_kwargs):
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self._watcher = (observer, watcher_kwargs)
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while self._undelivered_results:
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self._notify(self._undelivered_results.pop(0))
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def notify(self, **result_kwargs):
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if self._watcher:
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self._notify(result_kwargs)
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else:
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self._undelivered_results.append(result_kwargs)
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def _notify(self, result_kwargs):
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o, watcher_kwargs = self._watcher
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kwargs = dict(result_kwargs)
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kwargs.update(watcher_kwargs)
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eventually(o, **kwargs)
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def cancel(self):
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wr,methname = self._canceler
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o = wr()
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if o:
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getattr(o,methname)(self)
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