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Declaring Interfaces for Anonymous Objects

A protocol can be used to declare the methods of an anonymous object, an object of unknown class. An anonymous object may represent a service or handle a limited set of functions, especially where only one object of its kind is needed. (Objects that play a fundamental role in defining an application’s architecture and objects that you must initialize before using are not good candidates for anonymity.)

Objects are not anonymous to their developers, of course, but they are anonymous when the developer supplies them to someone else. For example, consider the following situations:

• Someone who supplies a framework or a suite of objects for others to use can include objects that are not identified by a class name or an interface file. Lacking the name and class interface, users have no way of creating instances of the class. Instead, the supplier must provide a ready-made instance. Typically, a method in another class returns a usable object:

  id formatter = [receiver formattingService];

  The object returned by the method is an object without a class identity, at least not one the supplier is willing to reveal. For it to be of any use at all, the supplier must be willing to identify at least some of the messages that it can respond to. This is done by associating the object with a list of methods declared in a protocol.

• You can send Objective-C messages to remote objects—objects in other applications. (“Remote Messaging,” discusses this possibility in more detail.)

  Each application has its own structure, classes, and internal logic. But you don’t need to know how another application works or what its components are to communicate with it. As an outsider, all you need to know is what messages you can send (the protocol) and where to send them (the receiver).

  An application that publishes one of its objects as a potential receiver of remote messages must also publish a protocol declaring the methods the object will use to respond to those messages. It doesn’t have to disclose anything else about the object. The sending application doesn’t need to know the class of the object or use the class in its own design. All it needs is the protocol.

Protocols make anonymous objects possible. Without a protocol, there would be no way to declare an interface to an object without identifying its class.

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