One of the saddest aspects of being a Big Nerd Ranch instructor in 2016 is that students these days do not appreciate the Michael Knight is to Delegation, as RoboCop is to Subclassing discussion of yesteryear.
Let’s start with a story: Once upon a time, there was a man with no name. Knight Industries decided that if this man were given guns and wheels and booster rockets, he would be the perfect crime-fighting tool. First they thought, “Let’s subclass him and override everything we need to add the guns and wheels and booster rockets.” The problem was that to subclass Michael Knight, they needed to wire his insides to the guns, wheels, and booster rockets – a time-consuming task requiring lots of specialized knowledge. So instead, Knight Industries created a helper object, the Knight Industries 2000, or “KITT,” a well-equipped car designed to assist Michael Knight in a variety of crime- fighting situations.
While approaching the perimeter of an arms dealer’s compound, Michael Knight would say, “KITT, I need to get to the other side of that wall.” KITT would then blast a big hole in the wall with a small rocket. After destroying the wall, KITT would return control to Michael, who would charge through the rubble and capture the arms dealer. Note how creating a helper object is different from the RoboCop approach. RoboCop was a man subclassed and extended. The RoboCop project involved dozens of surgeons who extended the man into a fighting machine. This is the approach taken by many object-oriented frameworks.
In the Cocoa framework, many objects are extended in the Knight Industries way – by supplying them with helper objects. In this section, you are going to provide the speech synthesizer with a type of helper object called a delegate.
What do you think the new metaphor should be?