On 19 May '08, at 9:22 AM, john darnell wrote:
As far as I can tell, it is kind of like a virtual function (virtual because I, the programmer, am expected to flesh it out) that resembles an event attached to a given class.
You're basically correct, but I think you're overcomplicating the concept.
A delegate is basically just the OOP equivalent of a group of callbacks. Many C APIs will let the client set a number of callback function pointers that will be invoked in particular situations. (CFReadStreamSetClient is a good example in CFNetwork.) The callback lets the library tell your code when something happens, or ask it about something.
In object-oriented programming, this is often done with a delegate. Instead of callbacks, the library class defines one or more methods that the delegate should implement. The client then sets the delegate object as a property, and the object in the library can call the delegate methods on it as necessary. (Compare the CFStream example with NSStream, which uses a delegate interface for basically the same purpose.)
Cocoa is more delegate-happy than most frameworks I've seen. A more common design pattern is to have the library class define methods on _itself_ that will be called for this purpose. The user of the class then has to subclass it and override those methods. It's a similar idea but it makes the object its own delegate. There are some advantages to this, but it has the disadvantage of requiring lots and lots of subclassing in application code, whereas in Cocoa you don't have to create subclasses nearly as often; this helps keep your code organized better.
—Jens
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