Add support for dash client-side callbacks (#149) (#165)

This should close issue #149. This is a purely additive change.

Adding support for client-side callbacks is just a matter of registering a javascript function name. The client-side callback may be registered by calling the `clientside_callback` function on a `DjangoDash` object in the same exact way as it would have been done with a plain Dash app (see: https://community.plot.ly/t/dash-0-41-0-released/22131).

Author: gianlucasalvato

django_plotly_dash/dash_wrapper.py

@@ -100,6 +100,7 @@ def __init__(self, name=None, serve_locally=None,
             self._uid = name
         self.layout = None
         self._callback_sets = []
+        self._clientside_callback_sets = []
 
         self.css = Holder()
         self.scripts = Holder()
@@ -201,6 +202,8 @@ def form_dash_instance(self, replacements=None, ndid=None, base_pathname=None):
 
         for cb, func in self._callback_sets:
             rd.callback(**cb)(func)
+        for cb in self._clientside_callback_sets:
+            rd.clientside_callback(**cb)
         for s in self.css.items:
             rd.css.append_css(s)
         for s in self.scripts.items:
@@ -229,6 +232,16 @@ def expanded_callback(self, output, inputs=[], state=[], events=[]): # pylint: d
         self._expanded_callbacks = True
         return self.callback(output, inputs, state, events)
 
+    def clientside_callback(self, clientside_function, output, inputs=None, state=None):
+        'Form a callback function by wrapping, in the same way as the underlying Dash application would'
+        callback_set = { 'clientside_function': clientside_function,
+                         'output': output,
+                         'inputs': inputs and inputs or dict(),
+                         'state': state and state or dict() }
+
+        self._clientside_callback_sets.append(callback_set)
+
+
     def get_asset_url(self, asset_name):
         '''URL of an asset associated with this component
 
@@ -449,6 +462,21 @@ def callback(self, output, inputs=[], state=[], events=[]): # pylint: disable=da
                                                  [self._fix_callback_item(x) for x in inputs],
                                                  [self._fix_callback_item(x) for x in state])
 
+    def clientside_callback(self, clientside_function, output, inputs=[], state=[]): # pylint: disable=dangerous-default-value
+        'Invoke callback, adjusting variable names as needed'
+
+        if isinstance(output, (list, tuple)):
+            fixed_outputs = [self._fix_callback_item(x) for x in output]
+            # Temporary check; can be removed once the library has been extended
+            raise NotImplementedError("django-plotly-dash cannot handle multiple callback outputs at present")
+        else:
+            fixed_outputs = self._fix_callback_item(output)
+
+        return super(WrappedDash, self).clientside_callback(clientside_function,
+                                                            fixed_outputs,
+                                                            [self._fix_callback_item(x) for x in inputs],
+                                                            [self._fix_callback_item(x) for x in state])
+
     def dispatch(self):
         'Perform dispatch, using request embedded within flask global state'
         import flask