WIP refactor: fetch the full widget state via a control Comm

ipywidgets/__init__.py

@@ -38,6 +38,7 @@ def register_comm_target(kernel=None):
     if kernel is None:
         kernel = get_ipython().kernel
     kernel.comm_manager.register_target('jupyter.widget', Widget.handle_comm_opened)
+    kernel.comm_manager.register_target('jupyter.widget.control', Widget.handle_comm_opened_control)
 
 # deprecated alias
 handle_kernel = register_comm_target

ipywidgets/widgets/tests/test_send_state.py

@@ -3,7 +3,7 @@
 
 from traitlets import Bool, Tuple, List
 
-from .utils import setup, teardown
+from .utils import setup, teardown, DummyComm
 
 from ..widget import Widget
 
@@ -23,3 +23,11 @@ def test_empty_hold_sync():
     with w.hold_sync():
         pass
     assert w.comm.messages == []
+
+
+def test_control():
+    comm = DummyComm()
+    Widget.close_all()
+    w = SimpleWidget()
+    Widget.handle_comm_opened_control(comm, {})
+    assert comm.messages

ipywidgets/widgets/widget.py

@@ -317,6 +317,26 @@ def _call_widget_constructed(widget):
         if Widget._widget_construction_callback is not None and callable(Widget._widget_construction_callback):
             Widget._widget_construction_callback(widget)
 
+    @classmethod
+    def handle_comm_opened_control(cls, comm, msg):
+        cls.get_manager_state()
+        widgets = Widget.widgets.values()
+        # build a single dict with the full widget state
+        full_state = {}
+        drop_defaults = False
+        for widget in widgets:
+            full_state[widget.model_id] = {
+                'model_name': widget._model_name,
+                'model_module': widget._model_module,
+                'model_module_version': widget._model_module_version,
+                'state': widget.get_state(drop_defaults=drop_defaults),
+            }
+        full_state, buffer_paths, buffers = _remove_buffers(full_state)
+        # the message is also send as buffer, so it does not get handled by jupyter_server
+        msg = jsondumps([full_state, buffer_paths]).encode('utf8')
+        buffers.insert(0, msg)
+        comm.send(buffers=buffers)
+
     @staticmethod
     def handle_comm_opened(comm, msg):
         """Static method, called when a widget is constructed."""