Add apply_when operation (#4289)

holoviews/operation/element.py

@@ -140,6 +140,48 @@ def _process(self, element, key=None):
         return fn(element, *self.p.args, **self.p.kwargs)
 
 
+class apply_when(param.ParameterizedFunction):
+    """
+    Applies a selection depending on the current zoom range. If the
+    supplied predicate function returns a True it will apply the
+    operation otherwise it will return the raw element after the
+    selection. For example the following will apply datashading if
+    the number of points in the current viewport exceed 1000 otherwise
+    just returning the selected points element:
+
+       apply_when(points, operation=datashade, predicate=lambda x: x > 1000)
+    """
+
+    operation = param.Callable(default=lambda x: x)
+
+    predicate = param.Callable(default=None)
+
+    def _apply(self, element, x_range, y_range, invert=False):
+        selected = element
+        if x_range is not None and y_range is not None:
+            selected = element[x_range, y_range]
+        condition = self.predicate(selected)
+        if (not invert and condition) or (invert and not condition):
+            return selected
+        elif selected.interface.gridded:
+            return selected.clone([])
+        else:
+            return selected.iloc[:0]
+
+    def __call__(self, obj, **params):
+        self.set_param(**params)
+        streams = [RangeXY()]
+        if not self.predicate:
+            raise ValueError(
+                'Must provide a predicate function to determine when '
+                'to apply the operation and when to return the selected '
+                'data.'
+            )
+        applied = self.operation(obj.apply(self._apply, streams=streams))
+        raw = obj.apply(self._apply, streams=streams, invert=True)
+        return applied * raw
+
+
 class chain(Operation):
     """
     Defining an Operation chain is an easy way to define a new