Make client ready event optional

This is for xcanvas to not fail at construction. There seem to be an
issue in xwidgets for when we send a custom comm message too early.

Author: martinRenou

ipycanvas/canvas.py

@@ -254,6 +254,8 @@ class Canvas(_CanvasBase):
     _model_name = Unicode('CanvasModel').tag(sync=True)
     _view_name = Unicode('CanvasView').tag(sync=True)
 
+    _send_client_ready_event = Bool(True).tag(sync=True)
+
     #: (valid HTML color or Gradient or Pattern) The color for filling rectangles and paths. Default to ``'black'``.
     fill_style = Union((Color(), Instance(_CanvasGradient), Instance(Pattern)), default_value='black')
 

src/widget.ts

@@ -244,6 +244,7 @@ class CanvasModel extends DOMWidgetModel {
       height: 500,
       sync_image_data: false,
       image_data: null,
+      _send_client_ready_event: true,
     };
   }
 
@@ -275,7 +276,9 @@ class CanvasModel extends DOMWidgetModel {
     this.on('change:sync_image_data', this.syncImageData.bind(this));
     this.on('msg:custom', this.onCommand.bind(this));
 
-    this.send({ event: 'client_ready' }, {});
+    if (this.get('_send_client_ready_event')) {
+      this.send({ event: 'client_ready' }, {});
+    }
   }
 
   private async drawImageData() {