observer.py

# -*- coding: utf-8; fill-column: 77 -*-
# -*- indent-tabs-mode: nil -*-
# -*- 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 = []

    def __repr__(self):
        if self._fired:
            return "<OneShotObserverList -> %s>" % (self._result, )
        else:
            return "<OneShotObserverList [%s]>" % (self._watchers, )

    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

    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)