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# -*- coding: utf-8 -*-
# Copyright 2014, 2015 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from twisted.internet import defer
import logging
logger = logging.getLogger(__name__)
class Distributor(object):
"""A central dispatch point for loosely-connected pieces of code to
register, observe, and fire signals.
Signals are named simply by strings.
TODO(paul): It would be nice to give signals stronger object identities,
so we can attach metadata, docstrings, detect typoes, etc... But this
model will do for today.
"""
def __init__(self, suppress_failures=True):
self.suppress_failures = suppress_failures
self.signals = {}
self.pre_registration = {}
def declare(self, name):
if name in self.signals:
raise KeyError("%r already has a signal named %s" % (self, name))
self.signals[name] = Signal(
name,
suppress_failures=self.suppress_failures,
)
if name in self.pre_registration:
signal = self.signals[name]
for observer in self.pre_registration[name]:
signal.observe(observer)
def observe(self, name, observer):
if name in self.signals:
self.signals[name].observe(observer)
else:
# TODO: Avoid strong ordering dependency by allowing people to
# pre-register observations on signals that don't exist yet.
if name not in self.pre_registration:
self.pre_registration[name] = []
self.pre_registration[name].append(observer)
def fire(self, name, *args, **kwargs):
if name not in self.signals:
raise KeyError("%r does not have a signal named %s" % (self, name))
return self.signals[name].fire(*args, **kwargs)
class Signal(object):
"""A Signal is a dispatch point that stores a list of callables as
observers of it.
Signals can be "fired", meaning that every callable observing it is
invoked. Firing a signal does not change its state; it can be fired again
at any later point. Firing a signal passes any arguments from the fire
method into all of the observers.
"""
def __init__(self, name, suppress_failures):
self.name = name
self.suppress_failures = suppress_failures
self.observers = []
def observe(self, observer):
"""Adds a new callable to the observer list which will be invoked by
the 'fire' method.
Each observer callable may return a Deferred."""
self.observers.append(observer)
def fire(self, *args, **kwargs):
"""Invokes every callable in the observer list, passing in the args and
kwargs. Exceptions thrown by observers are logged but ignored. It is
not an error to fire a signal with no observers.
Returns a Deferred that will complete when all the observers have
completed."""
def eb(failure):
logger.warning(
"%s signal observer %s failed: %r",
self.name, observer, failure,
exc_info=(
failure.type,
failure.value,
failure.getTracebackObject()))
if not self.suppress_failures:
failure.raiseException()
deferreds = [
defer.maybeDeferred(observer, *args, **kwargs)
for observer in self.observers
]
d = defer.gatherResults(deferreds, consumeErrors=True)
d.addErrback(eb)
return d
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