Document tween update event as Tween#onUpdate.

src/item/Item.js

@@ -4694,8 +4694,8 @@ new function() { // Injection scope for hit-test functions shared with project
      * var path = new Path.Circle({
      *     radius: view.bounds.height * 0.4,
      *     center: view.center
-     * })
-     * path.tween({ fillColor: 'blue' }, { fillColor: 'red' }, 3000)
+     * });
+     * path.tween({ fillColor: 'blue' }, { fillColor: 'red' }, 3000);
      */
     /**
      * Tween item to a state.
@@ -4713,14 +4713,14 @@ new function() { // Injection scope for hit-test functions shared with project
      *     size: [100, 100],
      *     position: view.center,
      *     fillColor: 'red',
-     * })
+     * });
      *
-     * var delta = { x: path.bounds.width / 2, y: 0 }
+     * var delta = { x: path.bounds.width / 2, y: 0 };
      *
      * path.tween({
      *     'segments[1].point': ['+=', delta],
      *     'segments[2].point.x': '-= 50'
-     * }, 3000)
+     * }, 3000);
      *
      * @see Item#tween(from, to, options)
      */
@@ -4741,16 +4741,16 @@ new function() { // Injection scope for hit-test functions shared with project
      *     fillColor: 'blue',
      *     radius: view.bounds.height * 0.4,
      *     center: view.center,
-     * })
+     * });
      * var pathFrom = path.clone({ insert: false })
      * var pathTo = new Path.Rectangle({
      *     position: view.center,
      *     rectangle: path.bounds,
      *     insert: false
-     * })
-     * path.tween(2000).on('update', function(event) {
+     * });
+     * path.tween(2000).onUpdate = function(event) {
      *     path.interpolate(pathFrom, pathTo, event.factor)
-     * })
+     * };
      */
     tween: function(from, to, options) {
         if (!options) {
@@ -4814,8 +4814,8 @@ new function() { // Injection scope for hit-test functions shared with project
      *     fillColor: 'blue',
      *     radius: view.bounds.height * 0.4,
      *     center: view.center
-     * })
-     * path.tweenFrom({ fillColor: 'red' }, { duration: 1000 })
+     * });
+     * path.tweenFrom({ fillColor: 'red' }, { duration: 1000 });
      */
     tweenFrom: function(state, options) {
         return this.tween(state, null, options);

src/item/Tween.js

@@ -257,19 +257,15 @@ var Tween = Base.extend(Emitter, /** @lends Tween# */{
     },
 
     /**
-     * {@grouptitle Event Handling}
+     * {@grouptitle Event Handlers}
      *
-     * Attaches an event handler to the tween.
-     *
-     * @name Tween#on
-     * @function
-     * @param {String} type the type of event (currently only 'update')
-     * @param {Function} function the function to be called when the event
-     *     occurs, receiving an object as its
-     *     sole argument, containing the current progress of the
-     *     tweening and the factor calculated by the easing function
-     * @return {Tween} this tween itself, so calls can be chained
+     * The function to be called when the tween is updated. It receives an
+     * object as its sole argument, containing the current progress of the
+     * tweening and the factor calculated by the easing function.
      *
+     * @name Tween#onUpdate
+     * @property
+     * @type Function
      *
      * @example {@paperscript}
      * // Display tween progression values:
@@ -284,11 +280,14 @@ var Tween = Base.extend(Emitter, /** @lends Tween# */{
      * );
      * var progressText = new PointText(view.center + [60, -10]);
      * var factorText = new PointText(view.center + [60, 10]);
-     * tween.on('update', function(event) {
+     * tween.onUpdate = function(event) {
      *     progressText.content = 'progress: ' + event.progress.toFixed(2);
      *     factorText.content = 'factor: ' + event.factor.toFixed(2);
-     * });
+     * };
      */
+    _events: {
+        onUpdate: {}
+    },
 
     _getState: function(state) {
         var keys = this._keys,