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synapse/util/distributor.py

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+# -*- coding: utf-8 -*-
+# Copyright 2014 matrix.org
+#
+# 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):
+        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)
+
+        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):
+        self.name = name
+        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."""
+        deferreds = []
+        for observer in self.observers:
+            d = defer.maybeDeferred(observer, *args, **kwargs)
+
+            def eb(failure):
+                logger.warning(
+                    "%s signal observer %s failed: %r",
+                    self.name, observer, failure,
+                    exc_info=(
+                        failure.type,
+                        failure.value,
+                        failure.getTracebackObject()))
+            deferreds.append(d.addErrback(eb))
+
+        return defer.DeferredList(deferreds)