ObjectViewers offers the same kind of interception of messages (via doesnotUnderstand:) as ObjectTracers, but instead of just being wrappers, they actually replace the object being viewed. This makes them a lot more dangerous to use, but one can do amazing things. For instance, the example below actually intercepts the InputSensor object, and prints the mouse coordinates asynchronously, every time they change:
Sensor evaluate: [Sensor cursorPoint printString displayAt: 0@0]
wheneverChangeIn: [Sensor cursorPoint].
To exit from this example, execute:
Sensor xxxUnTrace

	very few messages	top
	doesNotUnderstand: Check for change after sending aMessage
	xxxViewedObject:evaluate:wheneverChangeIn: This message name must not clash with any other (natch).

	instance creation	top
	on:evaluate:wheneverChangeIn: