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# -*- test-case-name: allmydata.test.test_observer -*-
from twisted.internet import defer
try:
from foolscap.eventual import eventually
eventually # http://divmod.org/trac/ticket/1499
except ImportError:
from twisted.internet import reactor
def eventually(f, *args, **kwargs):
return reactor.callLater(0, f, *args, **kwargs)
"""The idiom we use is for the observed object to offer a method named
'when_something', which returns a deferred. That deferred will be fired when
something happens. The way this is typically implemented is that the observed
has an ObserverList whose when_fired method is called in the observed's
'when_something'."""
class OneShotObserverList:
"""A one-shot event distributor."""
def __init__(self):
self._fired = False
self._result = None
self._watchers = []
self.__repr__ = self._unfired_repr
def _unfired_repr(self):
return "<OneShotObserverList [%s]>" % (self._watchers, )
def _fired_repr(self):
return "<OneShotObserverList -> %s>" % (self._result, )
def _get_result(self):
return self._result
def when_fired(self):
if self._fired:
return defer.succeed(self._get_result())
d = defer.Deferred()
self._watchers.append(d)
return d
def fire(self, result):
assert not self._fired
self._fired = True
self._result = result
self._fire(result)
def _fire(self, result):
for w in self._watchers:
eventually(w.callback, result)
del self._watchers
self.__repr__ = self._fired_repr
def fire_if_not_fired(self, result):
if not self._fired:
self.fire(result)
class LazyOneShotObserverList(OneShotObserverList):
"""
a variant of OneShotObserverList which does not retain
the result it handles, but rather retains a callable()
through which is retrieves the data if and when needed.
"""
def __init__(self):
OneShotObserverList.__init__(self)
def _get_result(self):
return self._result_producer()
def fire(self, result_producer):
"""
@param result_producer: a no-arg callable which
returns the data which is to be considered the
'result' for this observer list. note that this
function may be called multiple times - once
upon initial firing, and potentially once more
for each subsequent when_fired() deferred created
"""
assert not self._fired
self._fired = True
self._result_producer = result_producer
if self._watchers: # if not, don't call result_producer
self._fire(self._get_result())
class ObserverList:
"""A simple class to distribute events to a number of subscribers."""
def __init__(self):
self._watchers = []
def subscribe(self, observer):
self._watchers.append(observer)
def unsubscribe(self, observer):
self._watchers.remove(observer)
def notify(self, *args, **kwargs):
for o in self._watchers:
eventually(o, *args, **kwargs)
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