Why does cocoa use delegates rather than inheritance?

Question

Why does cocoa use delegates rather than inheritance?

Answer 1

With delegates, you can have one object be the delegate of many other objects. For example, you can have your MyController instance be the delegate of an NSTableView, an NSTextField, an NSWindow, and any other objects that compose your interface. This gives a compact place to put all of your user interface code related to one section of your UI.

If you'd done that with subclassing, you'd have to create one subclass every object you wanted callbacks from.

Also, this is a classic inheritance vs composition question

Answer 2

In general creating a subclass can be a time consuming process, requiring a lot of groundwork, and overriding various template methods.

Meanwhile, using a delegate allows you to create a simple object that answers a few specific question or reacts in various ways.

Now when you combine this with the dynamism that you can achieve by swapping out delegates on the fly it can create a very flexible robust system that promotes more code reuse.

There is some general discussions regarding these things here and here. You can also find some older SO questions here and here.