tahoe-lafs/src/allmydata/util/observer.py
2010-08-04 00:26:12 -07:00

139 lines
4.6 KiB
Python

# -*- test-case-name: allmydata.test.test_observer -*-
import weakref
from twisted.internet import defer
from foolscap.api import eventually
"""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)
class EventStreamObserver:
"""A simple class to distribute multiple events to a single subscriber.
It accepts arbitrary kwargs, but no posargs."""
def __init__(self):
self._watcher = None
self._undelivered_results = []
self._canceler = None
def set_canceler(self, c, methname):
"""I will call c.METHNAME(self) when somebody cancels me."""
# we use a weakref to avoid creating a cycle between us and the thing
# we're observing: they'll be holding a reference to us to compare
# against the value we pass to their canceler function. However,
# since bound methods are first-class objects (and not kept alive by
# the object they're bound to), we can't just stash a weakref to the
# bound cancel method. Instead, we must hold a weakref to the actual
# object, and obtain its cancel method later.
# http://code.activestate.com/recipes/81253-weakmethod/ has an
# alternative.
self._canceler = (weakref.ref(c), methname)
def subscribe(self, observer, **watcher_kwargs):
self._watcher = (observer, watcher_kwargs)
while self._undelivered_results:
self._notify(self._undelivered_results.pop(0))
def notify(self, **result_kwargs):
if self._watcher:
self._notify(result_kwargs)
else:
self._undelivered_results.append(result_kwargs)
def _notify(self, result_kwargs):
o, watcher_kwargs = self._watcher
kwargs = dict(result_kwargs)
kwargs.update(watcher_kwargs)
eventually(o, **kwargs)
def cancel(self):
wr,methname = self._canceler
o = wr()
if o:
getattr(o,methname)(self)