An object that restricts the messages that can be sent to another object (referred to as the checker’s delegate).
- macOS 10.0+
NSProtocol object can be particularly useful when an object with many methods, only a few of which ought to be remotely accessible, is made available using the distributed objects system.
A protocol checker acts as a kind of proxy; when it receives a message that is in its designated protocol, it forwards the message to its target and consequently appears to be the target object itself. However, when it receives a message not in its protocol, it raises an
NSInvalid to indicate that the message isn’t allowed, whether or not the target object implements the method.
Typically, an object that is to be distributed (yet must restrict messages) creates an
NSProtocol for itself and returns the checker rather than returning itself in response to any messages. The object might also register the checker as the root object of an NSConnection.
The object should be careful about vending references to
self—the protocol checker will convert a return value of
self to indicate the checker rather than the object for any messages forwarded by the checker, but direct references to the object (bypassing the checker) could be passed around by other objects.