Introduction of Streams API

holoviews/core/spaces.py

@@ -436,6 +436,15 @@ class DynamicMap(HoloMap):
         simulation time across the layout).
     """)
 
+    streams = param.List(default=[], doc="""
+       List of Stream instances to associate with the DynamicMap. The
+       set of parameter values across these streams will be supplied as
+       keyword arguments to the callback when the events are received,
+       updating the streams.
+
+       Note that streams may only be used with callable callbacks (i.e
+       not generators).""" )
+
     cache_size = param.Integer(default=500, doc="""
        The number of entries to cache for fast access. This is an LRU
        cache where the least recently used item is overwritten once
@@ -454,6 +463,12 @@ class DynamicMap(HoloMap):
        """)
 
     def __init__(self, callback, initial_items=None, **params):
+
+        # Set source to self if not already specified
+        for stream in params.get('streams',[]):
+            if stream.source is None:
+                stream.source = self
+
         super(DynamicMap, self).__init__(initial_items, callback=callback, **params)
         self.counter = 0
         if self.callback is None:
@@ -541,8 +556,10 @@ def _execute_callback(self, *args):
         if self.call_mode == 'generator':
             retval = next(self.callback)
         else:
-            retval = self.callback(*args)
-
+            # Additional validation needed to ensure kwargs don't clash
+            kwarg_items = [s.value.items() for s in self.streams]
+            flattened = [el for kws in kwarg_items for el in kws]
+            retval = self.callback(*args, **dict(flattened))
         if self.call_mode=='key':
             return self._style(retval)
 
@@ -651,6 +668,8 @@ def __getitem__(self, key):
 
         # Cache lookup
         try:
+            if self.streams:
+                raise KeyError('Using streams disables DynamicMap cache')
             cache = super(DynamicMap,self).__getitem__(key)
             # Return selected cache items in a new DynamicMap
             if isinstance(cache, DynamicMap) and self.mode=='open':

holoviews/streams.py

@@ -0,0 +1,195 @@
+"""
+The streams module defines the streams API that allows visualizations to
+generate and respond to events, originating either in Python on the
+server-side or in Javascript in the Jupyter notebook (client-side).
+"""
+
+import param
+import uuid
+from collections import OrderedDict
+
+
+
+class Preprocessor(param.Parameterized):
+    """
+    A Preprocessor is a callable that takes a dictionary as an argument
+    and returns a dictionary. Where possible, Preprocessors should have
+    valid reprs that can be evaluated.
+
+    Preprocessors are used to set the value of a stream based on the
+    parameter values. They may be used for debugging purposes or to
+    remap or repack parameter values before they are passed onto to the
+    subscribers.
+    """
+
+    def __call__(self, params):
+        return params
+
+
+
+class Rename(Preprocessor):
+    """
+    A preprocesor used to rename parameter values.
+    """
+
+    def __init__(self, **mapping):
+        self.mapping = mapping
+
+    def __call__(self, params):
+        return {self.mapping.get(k,k):v for (k,v) in params.items()}
+
+    def __repr__(self):
+        keywords = ','.join('%s=%r' % (k,v) for (k,v) in sorted(self.mapping.items()))
+        return 'Rename(%s)' % keywords
+
+
+
+class Stream(param.Parameterized):
+    """
+    A Stream is simply a parameterized object with parameters that
+    change over time in response to update events. Parameters are
+    updated via the update method.
+
+    Streams may have one or more subscribers which are callables passed
+    the parameter dictionary when the trigger classmethod is called.
+    """
+
+    # Mapping from uuid to stream instance
+    registry = OrderedDict()
+
+    @classmethod
+    def trigger(cls, streams):
+        """
+        Given a list of streams, collect all the stream parameters into
+        a dictionary and pass it to the union set of subscribers.
+
+        Passing multiple streams at once to trigger can be useful when a
+        subscriber may be set multiple times across streams but only
+        needs to be called once.
+        """
+        # Union of stream values
+        items = [stream.value.items() for stream in streams]
+        union = dict(kv for kvs in items for kv in kvs)
+        # Currently building a simple set of subscribers
+        groups = [stream.subscribers + stream._hidden_subscribers for stream in streams]
+        subscribers = set(s for subscribers in groups for s in subscribers)
+        for subscriber in subscribers:
+            subscriber(union)
+
+
+    @classmethod
+    def find(cls, obj):
+        """
+        Return a set of streams from the registry with a given source.
+        """
+        return set(v for v in cls.registry.values() if v.source is obj)
+
+
+    def __init__(self, preprocessors=[], source=None, subscribers=[], **params):
+        """
+        Mapping allows multiple streams with similar event state to be
+        used by remapping parameter names.
+
+        Source is an optional argument specifying the HoloViews
+        datastructure that the stream receives events from, as supported
+        by the plotting backend.
+        """
+        self.source = source
+        self.subscribers = subscribers
+        self.preprocessors = preprocessors
+        self._hidden_subscribers = []
+
+        self.uuid = uuid.uuid4().hex
+        super(Stream, self).__init__(**params)
+        self.registry[self.uuid] = self
+
+
+    @property
+    def value(self):
+        remapped = {k:v for k,v in self.get_param_values() if k!= 'name' }
+        for preprocessor in self.preprocessors:
+            remapped = preprocessor(remapped)
+        return remapped
+
+
+    def update(self, trigger=True, **kwargs):
+        """
+        The update method updates the stream parameters in response to
+        some event.
+
+        If trigger is enabled, the trigger classmethod is invoked on
+        this particular Stream instance.
+        """
+        params = self.params().values()
+        constants = [p.constant for p in params]
+        for param in params:
+            param.constant = False
+        self.set_param(**kwargs)
+        for (param, const) in zip(params, constants):
+            param.constant = const
+
+        if trigger:
+            self.trigger([self])
+
+
+    def __repr__(self):
+        cls_name = self.__class__.__name__
+        kwargs = ','.join('%s=%r' % (k,v)
+                          for (k,v) in self.get_param_values() if k != 'name')
+        if not self.preprocessors:
+            return '%s(%s)' % (cls_name, kwargs)
+        else:
+            return '%s(%r, %s)' % (cls_name, self.preprocessors, kwargs)
+
+
+    def __str__(self):
+        return repr(self)
+
+
+class PositionX(Stream):
+    """
+    A position along the x-axis in data coordinates.
+
+    With the appropriate plotting backend, this may correspond to the
+    position of the mouse/trackpad cursor.
+    """
+
+    x = param.Number(default=0, doc="""
+           Position along the x-axis in data coordinates""", constant=True)
+
+    def __init__(self, mapping=None, **params):
+        super(PositionX, self).__init__(mapping=mapping, **params)
+
+
+class PositionY(Stream):
+    """
+    A position along the y-axis in data coordinates.
+
+    With the appropriate plotting backend, this may correspond to the
+    position of the mouse/trackpad cursor.
+    """
+
+    y = param.Number(default=0, doc="""
+           Position along the y-axis in data coordinates""", constant=True)
+
+    def __init__(self, mapping=None, **params):
+        super(PositionY, self).__init__(mapping=mapping, **params)
+
+
+class PositionXY(Stream):
+    """
+    A position along the x- and y-axes in data coordinates.
+
+    With the appropriate plotting backend, this may correspond to the
+    position of the mouse/trackpad cursor.
+    """
+
+
+    x = param.Number(default=0, doc="""
+           Position along the x-axis in data coordinates""", constant=True)
+
+    y = param.Number(default=0, doc="""
+           Position along the y-axis in data coordinates""", constant=True)
+
+    def __init__(self, mapping=None, **params):
+        super(PositionXY, self).__init__(mapping=mapping, **params)