Delegation is a design pattern in which
one object sends messages to another object—specified as its delegate—to
ask for input or to notify it that an event is occurring. Delegation is
often used as an alternative to class inheritance to extend the
functionality of reusable objects. For example, before a window changes
size, it asks its delegate whether the new size is ok. The delegate
replies to the window, telling it that the suggested size is acceptable
or suggesting a better size. (For more details on window resizing, see
the
windowWillResize:toSize: message.)
Delegate methods are typically grouped
into a protocol. A protocol is basically just a list of methods. The
delegate protocol specifies all the messages an object might send to its
delegate. If a class conforms to (or adopts) a protocol, it
guarantees that it implements the required methods of a protocol.
(Protocols may also include optional methods).In this application, the
application object tells its delegate that the main startup routines
have finished by sending it the
applicationDidFinishLaunching: message. The delegate is then able to perform additional tasks if it wants.
delegate.self means
