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