Ensure Compositor does not make superfluous copies

holoviews/core/options.py

@@ -880,9 +880,10 @@ def map(cls, obj, mode='data', backend=None):
         if overlay_compositors:
             obj = obj.map(lambda obj: cls.collapse_element(obj, mode=mode, backend=backend),
                           [CompositeOverlay])
-        if element_compositors:
+        element_patterns = [c.pattern for c in element_compositors]
+        if element_compositors and obj.traverse(lambda x: x, element_patterns):
             obj = obj.map(lambda obj: cls.collapse_element(obj, mode=mode, backend=backend),
-                          [c.pattern for c in element_compositors])
+                          element_patterns)
         return obj
 
 

tests/testbokehcallbacks.py

@@ -1,12 +1,14 @@
 from unittest import SkipTest
 from nose.plugins.attrib import attr
 
-from holoviews.element.comparison import ComparisonTestCase
+from holoviews.core import NdOverlay
 from holoviews.core.options import Store
-
+from holoviews.element import Curve
+from holoviews.element.comparison import ComparisonTestCase
+from holoviews.streams import Selection1D
 
 try:
-    from holoviews.plotting.bokeh.callbacks import Callback
+    from holoviews.plotting.bokeh.callbacks import Callback, Selection1DCallback
     from holoviews.plotting.bokeh.util import bokeh_version
 
     from bokeh.events import Tap
@@ -52,6 +54,15 @@ def test_customjs_callback_attributes_js_for_cb_data(self):
                 'data["y1"] = cb_data["geometry"]["y1"];\n')
         self.assertEqual(js_code, code)
 
+
+    def test_callback_on_ndoverlay_is_attached(self):
+        ndoverlay = NdOverlay({i: Curve([i]) for i in range(5)})
+        selection = Selection1D(source=ndoverlay)
+        plot = bokeh_renderer.get_plot(ndoverlay)
+        self.assertEqual(len(plot.callbacks), 1)
+        self.assertIsInstance(plot.callbacks[0], Selection1DCallback)
+
+
 @attr(optional=1)
 class TestBokehServerJSCallbacks(ComparisonTestCase):