Make the model.views dict a dict of promises for views

Otherwise, there is a race condition where the model doesn't know about views until they are actually created.

IPython/html/static/widgets/js/manager.js

@@ -1,239 +1,240
 // Copyright (c) IPython Development Team.
 // Distributed under the terms of the Modified BSD License.
 
 define([
     "underscore",
     "backbone",
     "jquery",
     "base/js/utils",
     "base/js/namespace",
 ], function (_, Backbone, $, utils, IPython) {
     "use strict";
     //--------------------------------------------------------------------
     // WidgetManager class
     //--------------------------------------------------------------------
     var WidgetManager = function (comm_manager, notebook) {
         // Public constructor
         WidgetManager._managers.push(this);
 
         // Attach a comm manager to the 
         this.keyboard_manager = notebook.keyboard_manager;
         this.notebook = notebook;
         this.comm_manager = comm_manager;
         this._models = {}; /* Dictionary of model ids and model instances */
 
         // Register with the comm manager.
         this.comm_manager.register_target('ipython.widget', $.proxy(this._handle_comm_open, this));
     };
 
     //--------------------------------------------------------------------
     // Class level
     //--------------------------------------------------------------------
     WidgetManager._model_types = {}; /* Dictionary of model type names (target_name) and model types. */
     WidgetManager._view_types = {}; /* Dictionary of view names and view types. */
     WidgetManager._managers = []; /* List of widget managers */
 
     WidgetManager.register_widget_model = function (model_name, model_type) {
         // Registers a widget model by name.
         WidgetManager._model_types[model_name] = model_type;
     };
 
     WidgetManager.register_widget_view = function (view_name, view_type) {
         // Registers a widget view by name.
         WidgetManager._view_types[view_name] = view_type;
     };
 
     //--------------------------------------------------------------------
     // Instance level
     //--------------------------------------------------------------------
     WidgetManager.prototype.display_view = function(msg, model) {
         // Displays a view for a particular model.
         var that = this;
         var cell = this.get_msg_cell(msg.parent_header.msg_id);
         if (cell === null) {
             return Promise.reject(new Error("Could not determine where the display" +
                                             " message was from.  Widget will not be displayed"));
         } else if (cell.widget_subarea) {
             var dummy = $('<div />');
             cell.widget_subarea.append(dummy);
             return this.create_view(model, {cell: cell}).then(
                 function(view) {
                     that._handle_display_view(view);
                     dummy.replaceWith(view.$el);
                     view.trigger('displayed');
                     return view;
                 }).catch(utils.reject('Could not display view', true));
         }
     };
 
     WidgetManager.prototype._handle_display_view = function (view) {
         // Have the IPython keyboard manager disable its event
         // handling so the widget can capture keyboard input.
         // Note, this is only done on the outer most widgets.
         if (this.keyboard_manager) {
             this.keyboard_manager.register_events(view.$el);
         
             if (view.additional_elements) {
                 for (var i = 0; i < view.additional_elements.length; i++) {
                     this.keyboard_manager.register_events(view.additional_elements[i]);
                 }
             } 
         }
     };
     
     WidgetManager.prototype.create_view = function(model, options) {
         // Creates a promise for a view of a given model
 
         // Make sure the view creation is not out of order with 
         // any state updates.
         model.state_change = model.state_change.then(function() {
             
             return utils.load_class(model.get('_view_name'), model.get('_view_module'),
             WidgetManager._view_types).then(function(ViewType) {
 
                 // If a view is passed into the method, use that view's cell as
                 // the cell for the view that is created.
                 options = options || {};
                 if (options.parent !== undefined) {
                     options.cell = options.parent.options.cell;
                 }
                 // Create and render the view...
                 var parameters = {model: model, options: options};
                 var view = new ViewType(parameters);
                 view.listenTo(model, 'destroy', view.remove);
                 return Promise.resolve(view.render()).then(function() {return view;});
             }).catch(utils.reject("Couldn't create a view for model id '" + String(model.id) + "'", true));
         });
+        model.views[utils.uuid()] = model.state_change;
         return model.state_change;
     };
 
     WidgetManager.prototype.get_msg_cell = function (msg_id) {
         var cell = null;
         // First, check to see if the msg was triggered by cell execution.
         if (this.notebook) {
             cell = this.notebook.get_msg_cell(msg_id);
         }
         if (cell !== null) {
             return cell;
         }
         // Second, check to see if a get_cell callback was defined
         // for the message.  get_cell callbacks are registered for
         // widget messages, so this block is actually checking to see if the
         // message was triggered by a widget.
         var kernel = this.comm_manager.kernel;
         if (kernel) {
             var callbacks = kernel.get_callbacks_for_msg(msg_id);
             if (callbacks && callbacks.iopub &&
                 callbacks.iopub.get_cell !== undefined) {
                 return callbacks.iopub.get_cell();
             }
         }
         
         // Not triggered by a cell or widget (no get_cell callback 
         // exists).
         return null;
     };
 
     WidgetManager.prototype.callbacks = function (view) {
         // callback handlers specific a view
         var callbacks = {};
         if (view && view.options.cell) {
 
             // Try to get output handlers
             var cell = view.options.cell;
             var handle_output = null;
             var handle_clear_output = null;
             if (cell.output_area) {
                 handle_output = $.proxy(cell.output_area.handle_output, cell.output_area);
                 handle_clear_output = $.proxy(cell.output_area.handle_clear_output, cell.output_area);
             }
 
             // Create callback dictionary using what is known
             var that = this;
             callbacks = {
                 iopub : {
                     output : handle_output,
                     clear_output : handle_clear_output,
 
                     // Special function only registered by widget messages.
                     // Allows us to get the cell for a message so we know
                     // where to add widgets if the code requires it.
                     get_cell : function () {
                         return cell;
                     },
                 },
             };
         }
         return callbacks;
     };
 
     WidgetManager.prototype.get_model = function (model_id) {
         // Get a promise for a model by model id.
         return this._models[model_id];
     };
 
     WidgetManager.prototype._handle_comm_open = function (comm, msg) {
         // Handle when a comm is opened.
         return this.create_model({
             model_name: msg.content.data.model_name, 
             model_module: msg.content.data.model_module, 
             comm: comm}).catch(utils.reject("Couldn't create a model.", true));
     };
 
     WidgetManager.prototype.create_model = function (options) {
         // Create and return a promise for a new widget model
         //
         // Minimally, one must provide the model_name and widget_class
         // parameters to create a model from Javascript.
         //
         // Example
         // --------
         // JS:
         // IPython.notebook.kernel.widget_manager.create_model({
         //      model_name: 'WidgetModel', 
         //      widget_class: 'IPython.html.widgets.widget_int.IntSlider'})
         //      .then(function(model) { console.log('Create success!', model); },
         //      $.proxy(console.error, console));
         //
         // Parameters
         // ----------
         // options: dictionary
         //  Dictionary of options with the following contents:
         //      model_name: string
         //          Target name of the widget model to create.
         //      model_module: (optional) string
         //          Module name of the widget model to create.
         //      widget_class: (optional) string
         //          Target name of the widget in the back-end.
         //      comm: (optional) Comm
         
         // Create a comm if it wasn't provided.
         var comm = options.comm;
         if (!comm) {
             comm = this.comm_manager.new_comm('ipython.widget', {'widget_class': options.widget_class});
         }
 
         var that = this;
         var model_id = comm.comm_id;
         var model_promise =  utils.load_class(options.model_name, options.model_module, WidgetManager._model_types)
             .then(function(ModelType) {
                 var widget_model = new ModelType(that, model_id, comm);
                 widget_model.once('comm:close', function () {
                     delete that._models[model_id];
                 });
                 return widget_model;
 
             }, function(error) {
                 delete that._models[model_id];
                 var wrapped_error = new utils.WrappedError("Couldn't create model", error);
                 return Promise.reject(wrapped_error);
             });
         this._models[model_id] = model_promise;
         return model_promise;
     };
 
     // Backwards compatibility.
     IPython.WidgetManager = WidgetManager;
 
     return {'WidgetManager': WidgetManager};
 });

IPython/html/static/widgets/js/widget.js

@@ -1,617 +1,616
 // Copyright (c) IPython Development Team.
 // Distributed under the terms of the Modified BSD License.
 
 define(["widgets/js/manager",
         "underscore",
         "backbone",
         "jquery",
         "base/js/utils",
         "base/js/namespace",
 ], function(widgetmanager, _, Backbone, $, utils, IPython){
 
     var WidgetModel = Backbone.Model.extend({
         constructor: function (widget_manager, model_id, comm) {
             // Constructor
             //
             // Creates a WidgetModel instance.
             //
             // Parameters
             // ----------
             // widget_manager : WidgetManager instance
             // model_id : string
             //      An ID unique to this model.
             // comm : Comm instance (optional)
             this.widget_manager = widget_manager;
             this.state_change = Promise.resolve();
             this._buffered_state_diff = {};
             this.pending_msgs = 0;
             this.msg_buffer = null;
             this.state_lock = null;
             this.id = model_id;
             this.views = {};
 
             if (comm !== undefined) {
                 // Remember comm associated with the model.
                 this.comm = comm;
                 comm.model = this;
 
                 // Hook comm messages up to model.
                 comm.on_close($.proxy(this._handle_comm_closed, this));
                 comm.on_msg($.proxy(this._handle_comm_msg, this));
             }
             return Backbone.Model.apply(this);
         },
 
         send: function (content, callbacks) {
             // Send a custom msg over the comm.
             if (this.comm !== undefined) {
                 var data = {method: 'custom', content: content};
                 this.comm.send(data, callbacks);
                 this.pending_msgs++;
             }
         },
 
         _handle_comm_closed: function (msg) {
             // Handle when a widget is closed.
             this.trigger('comm:close');
             this.stopListening();
             this.trigger('destroy', this);
             delete this.comm.model; // Delete ref so GC will collect widget model.
             delete this.comm;
             delete this.model_id; // Delete id from model so widget manager cleans up.
-            for (var id in this.views) {
-                if (this.views.hasOwnProperty(id)) {
-                    this.views[id].remove();
-                }
-            }
+            _.each(this.views, function(v, id, views) {
+                v.then(function(view) {
+                    view.remove();
+                    delete views[id];
+                });
+            });
         },
 
         _handle_comm_msg: function (msg) {
             // Handle incoming comm msg.
             var method = msg.content.data.method;
             var that = this;
             switch (method) {
                 case 'update':
                     this.state_change = this.state_change.then(function() {
                         return that.set_state(msg.content.data.state);
                     }).catch(utils.reject("Couldn't process update msg for model id '" + String(that.id) + "'", true));
                     break;
                 case 'custom':
                     this.trigger('msg:custom', msg.content.data.content);
                     break;
                 case 'display':
                     this.widget_manager.display_view(msg, this);
                     break;
             }
         },
 
         set_state: function (state) {
             var that = this;
             // Handle when a widget is updated via the python side.
             return this._unpack_models(state).then(function(state) {
                 that.state_lock = state;
                 try {
                     WidgetModel.__super__.set.call(that, state);
                 } finally {
                     that.state_lock = null;
                 }
             }).catch(utils.reject("Couldn't set model state", true));
         },
 
         _handle_status: function (msg, callbacks) {
             // Handle status msgs.
 
             // execution_state : ('busy', 'idle', 'starting')
             if (this.comm !== undefined) {
                 if (msg.content.execution_state ==='idle') {
                     // Send buffer if this message caused another message to be
                     // throttled.
                     if (this.msg_buffer !== null &&
                         (this.get('msg_throttle') || 3) === this.pending_msgs) {
                         var data = {method: 'backbone', sync_method: 'update', sync_data: this.msg_buffer};
                         this.comm.send(data, callbacks);
                         this.msg_buffer = null;
                     } else {
                         --this.pending_msgs;
                     }
                 }
             }
         },
 
         callbacks: function(view) {
             // Create msg callbacks for a comm msg.
             var callbacks = this.widget_manager.callbacks(view);
 
             if (callbacks.iopub === undefined) {
                 callbacks.iopub = {};
             }
 
             var that = this;
             callbacks.iopub.status = function (msg) {
                 that._handle_status(msg, callbacks);
             };
             return callbacks;
         },
 
         set: function(key, val, options) {
             // Set a value.
             var return_value = WidgetModel.__super__.set.apply(this, arguments);
 
             // Backbone only remembers the diff of the most recent set()
             // operation.  Calling set multiple times in a row results in a 
             // loss of diff information.  Here we keep our own running diff.
             this._buffered_state_diff = $.extend(this._buffered_state_diff, this.changedAttributes() || {});
             return return_value;
         },
 
         sync: function (method, model, options) {
             // Handle sync to the back-end.  Called when a model.save() is called.
 
             // Make sure a comm exists.
             var error = options.error || function() {
                 console.error('Backbone sync error:', arguments);
             };
             if (this.comm === undefined) {
                 error();
                 return false;
             }
 
             // Delete any key value pairs that the back-end already knows about.
             var attrs = (method === 'patch') ? options.attrs : model.toJSON(options);
             if (this.state_lock !== null) {
                 var keys = Object.keys(this.state_lock);
                 for (var i=0; i<keys.length; i++) {
                     var key = keys[i];
                     if (attrs[key] === this.state_lock[key]) {
                         delete attrs[key];
                     }
                 }
             }
 
             // Only sync if there are attributes to send to the back-end.
             attrs = this._pack_models(attrs);
             if (_.size(attrs) > 0) {
 
                 // If this message was sent via backbone itself, it will not
                 // have any callbacks.  It's important that we create callbacks
                 // so we can listen for status messages, etc...
                 var callbacks = options.callbacks || this.callbacks();
 
                 // Check throttle.
                 if (this.pending_msgs >= (this.get('msg_throttle') || 3)) {
                     // The throttle has been exceeded, buffer the current msg so
                     // it can be sent once the kernel has finished processing 
                     // some of the existing messages.
                     
                     // Combine updates if it is a 'patch' sync, otherwise replace updates
                     switch (method) {
                         case 'patch':
                             this.msg_buffer = $.extend(this.msg_buffer || {}, attrs);
                             break;
                         case 'update':
                         case 'create':
                             this.msg_buffer = attrs;
                             break;
                         default:
                             error();
                             return false;
                     }
                     this.msg_buffer_callbacks = callbacks;
 
                 } else {
                     // We haven't exceeded the throttle, send the message like 
                     // normal.
                     var data = {method: 'backbone', sync_data: attrs};
                     this.comm.send(data, callbacks);
                     this.pending_msgs++;
                 }
             }
             // Since the comm is a one-way communication, assume the message 
             // arrived.  Don't call success since we don't have a model back from the server
             // this means we miss out on the 'sync' event.
             this._buffered_state_diff = {};
         },
 
         save_changes: function(callbacks) {
             // Push this model's state to the back-end
             //
             // This invokes a Backbone.Sync.
             this.save(this._buffered_state_diff, {patch: true, callbacks: callbacks});
         },
 
         _pack_models: function(value) {
             // Replace models with model ids recursively.
             var that = this;
             var packed;
             if (value instanceof Backbone.Model) {
                 return "IPY_MODEL_" + value.id;
 
             } else if ($.isArray(value)) {
                 packed = [];
                 _.each(value, function(sub_value, key) {
                     packed.push(that._pack_models(sub_value));
                 });
                 return packed;
             } else if (value instanceof Date || value instanceof String) {
                 return value;
             } else if (value instanceof Object) {
                 packed = {};
                 _.each(value, function(sub_value, key) {
                     packed[key] = that._pack_models(sub_value);
                 });
                 return packed;
 
             } else {
                 return value;
             }
         },
 
         _unpack_models: function(value) {
             // Replace model ids with models recursively.
             var that = this;
             var unpacked;
             if ($.isArray(value)) {
                 unpacked = [];
                 _.each(value, function(sub_value, key) {
                     unpacked.push(that._unpack_models(sub_value));
                 });
                 return Promise.all(unpacked);
             } else if (value instanceof Object) {
                 unpacked = {};
                 _.each(value, function(sub_value, key) {
                     unpacked[key] = that._unpack_models(sub_value);
                 });
                 return utils.resolve_promises_dict(unpacked);
             } else if (typeof value === 'string' && value.slice(0,10) === "IPY_MODEL_") {
                 // get_model returns a promise already
                 return this.widget_manager.get_model(value.slice(10, value.length));
             } else {
                 return Promise.resolve(value);
             }
         },
 
         on_some_change: function(keys, callback, context) {
             // on_some_change(["key1", "key2"], foo, context) differs from
             // on("change:key1 change:key2", foo, context).
             // If the widget attributes key1 and key2 are both modified, 
             // the second form will result in foo being called twice
             // while the first will call foo only once.
             this.on('change', function() {
                 if (keys.some(this.hasChanged, this)) {
                     callback.apply(context);
                 }
             }, this);
 
        },
     });
     widgetmanager.WidgetManager.register_widget_model('WidgetModel', WidgetModel);
 
 
     var WidgetView = Backbone.View.extend({
         initialize: function(parameters) {
             // Public constructor.
             this.model.on('change',this.update,this);
             this.options = parameters.options;
-            this.id = this.id || utils.uuid();
-            this.model.views[this.id] = this;
             this.on('displayed', function() { 
                 this.is_displayed = true; 
             }, this);
         },
 
         update: function(){
             // Triggered on model change.
             //
             // Update view to be consistent with this.model
         },
 
         create_child_view: function(child_model, options) {
             // Create and promise that resolves to a child view of a given model
             var that = this;
             options = $.extend({ parent: this }, options || {});
             return this.model.widget_manager.create_view(child_model, options).catch(utils.reject("Couldn't create child view"), true);
         },
 
         callbacks: function(){
             // Create msg callbacks for a comm msg.
             return this.model.callbacks(this);
         },
 
         render: function(){
             // Render the view.
             //
             // By default, this is only called the first time the view is created
         },
 
         show: function(){
             // Show the widget-area
             if (this.options && this.options.cell &&
                 this.options.cell.widget_area !== undefined) {
                 this.options.cell.widget_area.show();
             }
         },
 
         send: function (content) {
             // Send a custom msg associated with this view.
             this.model.send(content, this.callbacks());
         },
 
         touch: function () {
             this.model.save_changes(this.callbacks());
         },
 
         after_displayed: function (callback, context) {
             // Calls the callback right away is the view is already displayed
             // otherwise, register the callback to the 'displayed' event.
             if (this.is_displayed) {
                 callback.apply(context);
             } else {
                 this.on('displayed', callback, context);
             }
         },
     });
 
 
     var DOMWidgetView = WidgetView.extend({
         initialize: function (parameters) {
             // Public constructor
             DOMWidgetView.__super__.initialize.apply(this, [parameters]);
             this.on('displayed', this.show, this);
             this.model.on('change:visible', this.update_visible, this);
             this.model.on('change:_css', this.update_css, this);
 
             this.model.on('change:_dom_classes', function(model, new_classes) {
                 var old_classes = model.previous('_dom_classes');
                 this.update_classes(old_classes, new_classes);
             }, this);
 
             this.model.on('change:color', function (model, value) { 
                 this.update_attr('color', value); }, this);
 
             this.model.on('change:background_color', function (model, value) { 
                 this.update_attr('background', value); }, this);
 
             this.model.on('change:width', function (model, value) { 
                 this.update_attr('width', value); }, this);
 
             this.model.on('change:height', function (model, value) { 
                 this.update_attr('height', value); }, this);
 
             this.model.on('change:border_color', function (model, value) { 
                 this.update_attr('border-color', value); }, this);
 
             this.model.on('change:border_width', function (model, value) { 
                 this.update_attr('border-width', value); }, this);
 
             this.model.on('change:border_style', function (model, value) { 
                 this.update_attr('border-style', value); }, this);
 
             this.model.on('change:font_style', function (model, value) { 
                 this.update_attr('font-style', value); }, this);
 
             this.model.on('change:font_weight', function (model, value) { 
                 this.update_attr('font-weight', value); }, this);
 
             this.model.on('change:font_size', function (model, value) { 
                 this.update_attr('font-size', this._default_px(value)); }, this);
 
             this.model.on('change:font_family', function (model, value) { 
                 this.update_attr('font-family', value); }, this);
 
             this.model.on('change:padding', function (model, value) { 
                 this.update_attr('padding', value); }, this);
 
             this.model.on('change:margin', function (model, value) { 
                 this.update_attr('margin', this._default_px(value)); }, this);
 
             this.model.on('change:border_radius', function (model, value) { 
                 this.update_attr('border-radius', this._default_px(value)); }, this);
 
             this.after_displayed(function() {
                 this.update_visible(this.model, this.model.get("visible"));
                 this.update_classes([], this.model.get('_dom_classes'));
                 
                 this.update_attr('color', this.model.get('color'));
                 this.update_attr('background', this.model.get('background_color'));
                 this.update_attr('width', this.model.get('width'));
                 this.update_attr('height', this.model.get('height'));
                 this.update_attr('border-color', this.model.get('border_color'));
                 this.update_attr('border-width', this.model.get('border_width'));
                 this.update_attr('border-style', this.model.get('border_style'));
                 this.update_attr('font-style', this.model.get('font_style'));
                 this.update_attr('font-weight', this.model.get('font_weight'));
                 this.update_attr('font-size', this.model.get('font_size'));
                 this.update_attr('font-family', this.model.get('font_family'));
                 this.update_attr('padding', this.model.get('padding'));
                 this.update_attr('margin', this.model.get('margin'));
                 this.update_attr('border-radius', this.model.get('border_radius'));
 
                 this.update_css(this.model, this.model.get("_css"));
             }, this);
         },
 
         _default_px: function(value) {
             // Makes browser interpret a numerical string as a pixel value.
             if (/^\d+\.?(\d+)?$/.test(value.trim())) {
                 return value.trim() + 'px';
             }
             return value;
         },
 
         update_attr: function(name, value) {
             // Set a css attr of the widget view.
             this.$el.css(name, value);
         },
 
         update_visible: function(model, value) {
             // Update visibility
             this.$el.toggle(value);
          },
 
         update_css: function (model, css) {
             // Update the css styling of this view.
             var e = this.$el;
             if (css === undefined) {return;}
             for (var i = 0; i < css.length; i++) {
                 // Apply the css traits to all elements that match the selector.
                 var selector = css[i][0];
                 var elements = this._get_selector_element(selector);
                 if (elements.length > 0) {
                     var trait_key = css[i][1];
                     var trait_value = css[i][2];
                     elements.css(trait_key ,trait_value);
                 }
             }
         },
 
         update_classes: function (old_classes, new_classes, $el) {
             // Update the DOM classes applied to an element, default to this.$el.
             if ($el===undefined) {
                 $el = this.$el;
             }
             _.difference(old_classes, new_classes).map(function(c) {$el.removeClass(c);})
             _.difference(new_classes, old_classes).map(function(c) {$el.addClass(c);})
         },
 
         update_mapped_classes: function(class_map, trait_name, previous_trait_value, $el) {
             // Update the DOM classes applied to the widget based on a single
             // trait's value.
             //
             // Given a trait value classes map, this function automatically
             // handles applying the appropriate classes to the widget element
             // and removing classes that are no longer valid.
             //
             // Parameters
             // ----------
             // class_map: dictionary
             //  Dictionary of trait values to class lists.
             //  Example:
             //      {
             //          success: ['alert', 'alert-success'],
             //          info: ['alert', 'alert-info'],
             //          warning: ['alert', 'alert-warning'],
             //          danger: ['alert', 'alert-danger']
             //      };
             // trait_name: string
             //  Name of the trait to check the value of.
             // previous_trait_value: optional string, default ''
             //  Last trait value
             // $el: optional jQuery element handle, defaults to this.$el
             //  Element that the classes are applied to.
             var key = previous_trait_value;
             if (key === undefined) {
                 key = this.model.previous(trait_name);
             }
             var old_classes = class_map[key] ? class_map[key] : [];
             key = this.model.get(trait_name);
             var new_classes = class_map[key] ? class_map[key] : [];
 
             this.update_classes(old_classes, new_classes, $el || this.$el);
         },
         
         _get_selector_element: function (selector) {
             // Get the elements via the css selector.
             var elements;
             if (!selector) {
                 elements = this.$el;
             } else {
                 elements = this.$el.find(selector).addBack(selector);
             }
             return elements;
         },
     });
 
 
     var ViewList = function(create_view, remove_view, context) {
         // * create_view and remove_view are default functions called when adding or removing views
         // * create_view takes a model and returns a view or a promise for a view for that model
         // * remove_view takes a view and destroys it (including calling `view.remove()`)
         // * each time the update() function is called with a new list, the create and remove
         //   callbacks will be called in an order so that if you append the views created in the
         //   create callback and remove the views in the remove callback, you will duplicate 
         //   the order of the list.
         // * the remove callback defaults to just removing the view (e.g., pass in null for the second parameter)
         // * the context defaults to the created ViewList.  If you pass another context, the create and remove
         //   will be called in that context.
 
         this.initialize.apply(this, arguments);
     };
 
     _.extend(ViewList.prototype, {
         initialize: function(create_view, remove_view, context) {
             this.state_change = Promise.resolve();
             this._handler_context = context || this;
             this._models = [];
             this.views = [];
             this._create_view = create_view;
             this._remove_view = remove_view || function(view) {view.remove();};
         },
 
         update: function(new_models, create_view, remove_view, context) {
             // the create_view, remove_view, and context arguments override the defaults
             // specified when the list is created.
             // returns a promise that resolves after this update is done
             var remove = remove_view || this._remove_view;
             var create = create_view || this._create_view;
             if (create === undefined || remove === undefined){
                 console.error("Must define a create a remove function");
             }
             var context = context || this._handler_context;
             var added_views = [];
             var that = this;
             this.state_change = this.state_change.then(function() {
                 var i;
                  // first, skip past the beginning of the lists if they are identical
                 for (i = 0; i < new_models.length; i++) {
                     if (i >= that._models.length || new_models[i] !== that._models[i]) {
                         break;
                     }
                 }
                 var first_removed = i;
                 // Remove the non-matching items from the old list.
                 for (var j = first_removed; j < that._models.length; j++) {
                     remove.call(context, that.views[j]);
                 }
 
                 // Add the rest of the new list items.
                 for (; i < new_models.length; i++) {
                     added_views.push(create.call(context, new_models[i]));
                 }
                 // make a copy of the input array
                 that._models = new_models.slice();
                 return Promise.all(added_views).then(function(added) {
                     Array.prototype.splice.apply(that.views, [first_removed, that.views.length].concat(added));
                     return that.views;
                 });
             });
             return this.state_change;
         },
 
         remove: function() {
             // removes every view in the list; convenience function for `.update([])`
             // that should be faster
             // returns a promise that resolves after this removal is done
             var that = this;
             this.state_change = this.state_change.then(function() {
                 for (var i = 0; i < that.views.length; i++) {
                     that._remove_view.call(that._handler_context, that.views[i]);
                 }
                 that._models = [];
                 that.views = [];
             });
             return this.state_change;
         },
     });
 
     var widget = {
         'WidgetModel': WidgetModel,
         'WidgetView': WidgetView,
         'DOMWidgetView': DOMWidgetView,
         'ViewList': ViewList,
     };
 
     // For backwards compatability.
     $.extend(IPython, widget);
 
     return widget;
 });