mouseDown: function(event) {
			this.lastMouseLocation = {
				x: event.pageX,
				y: event.pageY
			};
			return YES;
		},

		mouseDragged: function(event) {
			var deltax = this.lastMouseLocation.x - event.pageX;
			var deltay = this.lastMouseLocation.y - event.pageY;
...
			this.lastMouseLocation = {
				x: event.pageX,
				y: event.pageY
			};
			return YES;
		},

		mouseUp: function(evt) {
			this.lastMouseLocation = undefined;
			return YES;
		},