Docs + remove buffer hack

I removed the buffer hack. This shouldn't be required anymore since the
removal of json_clean in ipykernel. We should also see if we can improve
performances in jupyter-server.

ipywidgets/_version.py

@@ -9,6 +9,7 @@
   '' if version_info[3]=='final' else _specifier_[version_info[3]]+str(version_info[4]))
 
 __protocol_version__ = '2.0.0'
+__control_protocol_version__ = '1.0.0'
 
 # These are *protocol* versions for each package, *not* npm versions. To check, look at each package's src/version.ts file for the protocol version the package implements.
 __jupyter_widgets_base_version__ = '1.2.0'

ipywidgets/widgets/widget.py

@@ -25,8 +25,9 @@
 
 from base64 import standard_b64encode
 
-from .._version import __protocol_version__, __jupyter_widgets_base_version__
+from .._version import __protocol_version__, __control_protocol_version__, __jupyter_widgets_base_version__
 PROTOCOL_VERSION_MAJOR = __protocol_version__.split('.')[0]
+CONTROL_PROTOCOL_VERSION_MAJOR = __control_protocol_version__.split('.')[0]
 
 def _widget_to_json(x, obj):
     if isinstance(x, dict):
@@ -319,6 +320,10 @@ def _call_widget_constructed(widget):
 
     @classmethod
     def handle_comm_opened_control(cls, comm, msg):
+        version = msg.get('metadata', {}).get('version', '')
+        if version.split('.')[0] != CONTROL_PROTOCOL_VERSION_MAJOR:
+            raise ValueError("Incompatible widget control protocol versions: received version %r, expected version %r"%(version, __control_protocol_version__))
+
         cls.get_manager_state()
         widgets = Widget.widgets.values()
         # build a single dict with the full widget state
@@ -332,10 +337,7 @@ def handle_comm_opened_control(cls, comm, msg):
                 '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)
+        comm.send([full_state, buffer_paths], buffers=buffers)
 
     @staticmethod
     def handle_comm_opened(comm, msg):

packages/base/src/version.ts

@@ -6,4 +6,3 @@ const JUPYTER_WIDGETS_VERSION = '1.2.0';
 
 export
 const PROTOCOL_VERSION = '2.0.0';
-

packages/schema/messages.md

@@ -338,3 +338,14 @@ To display a widget, the kernel sends a Jupyter [iopub `display_data` message](h
   }
 }
 ```
+
+
+
+
+# Control Widget messaging protocol, version 1.0
+
+This is implemented in ipywidgets 7.7.
+
+### The `jupyter.widget.control` comm target
+
+A kernel-side Jupyter widgets library registers a `jupyter.widget.control` comm target that is used for fetching all widgets states through a "one shot" comm message (one for all widget instances). The kernel-side widgets library must answer to the "comm-open" message with a comm message containing the full state of all widget instances.