Add ObservedPoint class that records changes and passes them on to the owner, and use it for all Point objects returned from Rectangle.

src/basic/Point.js

@@ -159,8 +159,12 @@ var Point = this.Point = Base.extend({
 		if (this.isZero()) {
 			if (this._angle != null) {
 				var a = this._angle;
-				this.x = Math.cos(a) * length;
+				// Use #set() instead of direct assignment, so ObservedPoint
-				this.y = Math.sin(a) * length;
+				// can optimise
+				this.set(
+					Math.cos(a) * length,
+					Math.sin(a) * length
+				);
 			} else {
 				// Assume angle = 0
 				this.x = length;
@@ -173,8 +177,12 @@ var Point = this.Point = Base.extend({
 				// x and y are 0
 				this.getAngle();
 			}
-			this.x *= scale;
+			// Use #set() instead of direct assignment, so ObservedPoint
-			this.y *= scale;
+			// can optimise
+			this.set(
+				this.x * scale,
+				this.y * scale
+			);
 		}
 		return this;
 	},
@@ -229,8 +237,12 @@ var Point = this.Point = Base.extend({
 		angle = this._angle = angle * Math.PI / 180;
 		if (!this.isZero()) {
 			var length = this.getLength();
-			this.x = Math.cos(angle) * length;
+			// Use #set() instead of direct assignment, so ObservedPoint
-			this.y = Math.sin(angle) * length;
+			// can optimise
+			this.set(
+				Math.cos(angle) * length,
+				Math.sin(angle) * length
+			);
 		}
 		return this;
 	},
@@ -560,3 +572,52 @@ var Point = this.Point = Base.extend({
 		}
 	}
 });
+
+var ObservedPoint = Point.extend({
+	beans: true,
+
+	set: function(x, y) {
+		this._x = x;
+		this._y = y;
+		this._observer[this._set](this);
+		return this;
+	},
+
+	getX: function() {
+		return this._x;
+	},
+
+	setX: function(x) {
+		this._x = x;
+		this._observer[this._set](this);
+	},
+
+	getY: function() {
+		return this._y;
+	},
+
+	setY: function(y) {
+		this._y = y;
+		this._observer[this._set](this);
+	},
+
+	statics: {
+		/**
+		 * Provide a faster creator for Points out of two coordinates that
+		 * does not rely on Point#initialize at all. This speeds up all math
+		 * operations a lot.
+		 */
+		create: function(observer, set, x, y) {
+			// Don't use the shorter form as we want absolute maximum
+			// performance here:
+			// return new Point(Point.dont).set(x, y);
+			// TODO: Benchmark and decide
+			var point = new ObservedPoint(ObservedPoint.dont);
+			point._x = x;
+			point._y = y;
+			point._observer = observer;
+			point._set = set;
+			return point;
+		}
+	}
+});

src/basic/Rectangle.js

@@ -71,7 +71,7 @@ var Rectangle = this.Rectangle = Base.extend({
 	},
 
 	getPoint: function() {
-		return Point.create(this.x, this.y);
+		return ObservedPoint.create(this, 'setPoint', this.x, this.y);
 	},
 
 	setPoint: function(point) {
@@ -150,7 +150,8 @@ var Rectangle = this.Rectangle = Base.extend({
 	},
 
 	getCenter: function() {
-		return Point.create(this.getCenterX(), this.getCenterY());
+		return ObservedPoint.create(this, 'setCenter',
+				this.getCenterX(), this.getCenterY());
 	},
 
 	setCenter: function(point) {
@@ -242,10 +243,10 @@ var Rectangle = this.Rectangle = Base.extend({
 			['Right', 'Center'], ['Bottom', 'Center']
 		],
 		function(parts, index) {
-			var key = parts.join('');
+			var part = parts.join('');
 			// find out if the first of the pair is an x or y property,
 			// by checking the first character for [R]ight or [L]eft;
-			var xFirst = /^[RL]/.test(key);
+			var xFirst = /^[RL]/.test(part);
 			// Rename Center to CenterX or CenterY:
 			if (index >= 4)
 				parts[1] += xFirst ? 'Y' : 'X';
@@ -255,10 +256,11 @@ var Rectangle = this.Rectangle = Base.extend({
 				getY = 'get' + y,
 				setX = 'set' + x,
 				setY = 'set' + y;
-			this['get' + key] = function() {
+			this['get' + part] = function() {
-				return Point.create(this[getX](), this[getY]());
+				return ObservedPoint.create(this, 'set' + part,
+						this[getX](), this[getY]());
 			};
-			this['set' + key] = function(point) {
+			this['set' + part] = function(point) {
 				point = Point.read(arguments);
 				return this[setX](point.x)[setY](point.y); // Note: call chaining!
 			};