There are three steps to using key-value observation:
• Send the addObserver:forKeyPath:options:context: message
to each object that is being observed.
• Override the method observeValueForKeyPath:ofObject:chan
ge:content: in the class definition of the object that is doing the
observing.
• Override the dealloc method and remove the observer reference
in the class definition of the object that is observing.
