"Example usage:
mock := MockObject new.
mock on: #selector do: [ aBlockToReturn ]."

Object subclass: #MockObject
	instanceVariableNames: 'callbacks'
	classVariableNames: ''
	poolDictionaries: ''
	category: 'MockObjects'
	
class>>new
	^super basicNew initialize

initialize
	callbacks := Dictionary new.

doesNotUnderstand: aMessage
	^(callbacks includesKey: aMessage selector)
		ifTrue: [ self performCallback: aMessage ]
		ifFalse: [ super doesNotUnderstand: aMessage ].

performCallback: aMessage
	^(callbacks at: aMessage selector)

		valueWithArguments: aMessage arguments

on: aSymbol do: aBlock
	callbacks at: aSymbol put: aBlock