Added stream activate and deactivate methods

holoviews/streams.py

@@ -106,6 +106,9 @@ def trigger(cls, streams):
         for subscriber in subscribers:
             subscriber(**dict(union))
 
+        for stream in streams:
+            stream.deactivate()
+
 
     def __init__(self, preprocessors=[], source=None, subscribers=[], **params):
         """
@@ -125,6 +128,21 @@ def __init__(self, preprocessors=[], source=None, subscribers=[], **params):
         if source:
             self.registry[id(source)].append(self)
 
+
+    def activate(self):
+        """
+        Allows defining an action before the stream is updated.
+        """
+        pass
+
+
+    def deactivate(self):
+        """
+        Allows defining an action after the stream has been triggered.
+        """
+        pass
+
+
     @property
     def source(self):
         return self._source
@@ -153,6 +171,7 @@ def update(self, trigger=True, **kwargs):
         If trigger is enabled, the trigger classmethod is invoked on
         this particular Stream instance.
         """
+        self.activate()
         params = self.params().values()
         constants = [p.constant for p in params]
         for param in params: