[feature] HookableService

ad-hoc register/trigger mechanism used for service composition.
to be used in bitmask.core and bitmask.bonafide in the first place.

2 files changed, 76 insertions, 0 deletions

diff --git a/changes/next-changelog.rst b/changes/next-changelog.rst
index a1af28c..ba897df 100644
--- a/changes/next-changelog.rst
+++ b/changes/next-changelog.rst
@@ -10,6 +10,7 @@ I've added a new category `Misc` so we can track doc/style/packaging stuff.
 
 Features
 ~~~~~~~~
+- Add HookableService, allowing inter-service notification for hooks.
 - `#1234 <https://leap.se/code/issues/1234>`_: Description of the new feature corresponding with issue #1234.
 - New feature without related issue number.
 
diff --git a/src/leap/common/service_hooks.py b/src/leap/common/service_hooks.py
new file mode 100644
index 0000000..96e95cc
--- /dev/null
+++ b/src/leap/common/service_hooks.py
@@ -0,0 +1,75 @@
+# -*- coding: utf-8 -*-
+# service_hooks.py
+# Copyright (C) 2016 LEAP
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+"""
+Hooks for service composition.
+"""
+from collections import defaultdict
+
+from twisted.application.service import IService, Service
+from twisted.python import log
+
+from zope.interface import implementer
+
+
+@implementer(IService)
+class HookableService(Service):
+
+    """
+    This service allows for other services in a Twisted Service tree to be
+    notified whenever a certain kind of hook is triggered.
+
+    During the service composition, one is expected to register
+    a hook name with the name of the service that wants to react to the
+    triggering of the hook. All the services, both hooked and listeners, should
+    be registered against the same parent service.
+
+    Upon the hook being triggered, the method "hook_<name>" will be called with
+    the passed data in the listener service.
+    """
+
+    def register_hook(self, name, listener):
+        if not hasattr(self, 'event_listeners'):
+            self.event_listeners = defaultdict(list)
+        log.msg("Registering hook %s->%s" % (name, listener))
+        self.event_listeners[name].append(listener)
+
+    def trigger_hook(self, name, **data):
+
+        def react_to_hook(listener, name, **kw):
+            try:
+                getattr(listener, 'hook_' + name)(**kw)
+            except AttributeError:
+                raise RuntimeError(
+                    "Tried to notify a hook, but the listener service class %s"
+                    "has not defined the proper method" % listener.__class__)
+
+        if not hasattr(self, 'event_listeners'):
+            self.event_listeners = defaultdict(list)
+        listeners = self._get_listener_services(name)
+
+        for listener in listeners:
+            react_to_hook(listener, name, **data)
+
+    def _get_sibling_service(self, name):
+        return self.parent.getServiceNamed(name)
+
+    def _get_listener_services(self, hook):
+        if hook in self.event_listeners:
+            service_names = self.event_listeners[hook]
+            services = [
+                self._get_sibling_service(name) for name in service_names]
+            return services