Merge pull request #1232 from maartenbreddels/_view_count

added _view_count trait to Widget, to count the number of views

Author: jasongrout

ipywidgets/widgets/widget.py

@@ -288,6 +288,8 @@ def get_view_spec(self):
         help="Name of the view object.").tag(sync=True)
     _view_module_version = Unicode('*',
         help="A semver requirement for the view module.").tag(sync=True)
+    _view_count = Int(read_only=True,
+        help="EXPERIMENTAL: The number of views of the model displayed in the frontend. This attribute is experimental and may change or be removed in the future.").tag(sync=True)
     comm = Instance('ipykernel.comm.Comm', allow_none=True)
 
     msg_throttle = Int(1, help="""Maximum number of msgs the front-end can send before receiving an idle msg from the back-end.""").tag(sync=True)

jupyter-js-widgets/src/widget.ts

@@ -60,6 +60,7 @@ class WidgetModel extends Backbone.Model {
             _view_module: "jupyter-js-widgets",
             _view_name: null,
             _view_module_version: "*",
+            _view_count: 0,
             msg_throttle: 1,
         };
     }
@@ -100,7 +101,6 @@ class WidgetModel extends Backbone.Model {
         this.state_lock = null;
         this._buffered_state_diff = {};
 
-
         this.views = {};
 
         if (comm) {
@@ -623,6 +623,17 @@ abstract class WidgetView extends NativeView<WidgetModel> {
         this.listenTo(this.model, 'change', this.update);
 
         this.options = parameters.options;
+
+        this.once('remove', () => {
+            this.model.set('_view_count', this.model.get('_view_count') - 1);
+            this.model.save_changes()
+        });
+
+        this.once('displayed', () => {
+            this.model.set('_view_count', this.model.get('_view_count') + 1);
+            this.model.save_changes()
+        });
+
         this.displayed = new Promise((resolve, reject) => {
             this.once('displayed', resolve);
         });