Question

In Objective-C / Cocoa how do I discover the messages I can implement in a delegate class?

From what I have read in the documentation a class will only allow certain chosen messages to be handled via a delegate but so far have found difficulty finding a list of messages for a class.

To ask the question another way, if I created a delegate for NSApplication, which messages are available for me to handle?

The documentation for NSApplication states a delegate

responds to certain messages on behalf of NSApp.

The documentation then goes onto say

NSApplication defines over twenty delegate methods that offer opportunities for modifying specific aspects of application behavior.

but as far as I can see it fails to list these methods / messages so how do I know which ones will be called from my delegate?

Answer 1

The delegate is usually supposed to conform to a protocol declared for that purpose, e.g. NSApplicationDelegate. If it’s not clearly spelled out already, you can often find the name of the appropriate protocol by looking at the type of the delegate property (in this case, id<NSApplicationDelegate>). You can check the documentation for details on the methods, or the @protocol declaration in the appropriate header file for specifics on which methods are available and which are required or optional.

There are some cases where the object does not define a protocol for its delegate, for example NSURLConnection. In this case, you just have to follow the documentation of the class with respect to what it expects of its delegate.

In either case, it is completely up to the class when it sends a message to the delegate and what the semantics of any message are.