Objective-C Protocols

I came across the following bit of code in a file called TileOverlay.h开发者_开发技巧:

@protocol TileOverlay <MKOverlay>

I'm trying to understand explicitly what this is doing. Several other overlays then import this .h file. Does this file essentially just create a modified version of the MKOverlay class?

If not, can you please clarify what it does?

MKOverlay is a protocol, and TileOverlay is a protocol that extends MKOverlay.

Any class that conforms to the TileOverlay protocol also conforms to the MKOverlay protocol.

Refer to Protocols Within Protocols in the documentation for a detailed explanation.

You can think of protocols like interfaces in Java or C#, basically they declare a contract that the implementing classes must follow. The difference is that in Objective C you can make some of the declared methods optional.

It's called protocol inheritance.

MKOverlay is a protocol, defining a set of methods for objects to implement.

TileOverlay inherits for MKOverlay, meaning that an object conforming to TileOverlay, should implement methods from MKOverlay and methods from TileOverlay

Here is a link that talks a little bit more about protocol inheritance

A protocol in Objective-C is what you in Java or C# would call an interface. It is a contract that any other class can fulfill, in Obj-C parlance conform to.

This:

@protocol TileOverlay <MKOverlay>

Defines a protocol named TileOverlay that in itself extends MKOverlay. That is by conforming to TileOverlay you must also conform to MKOverlay.

Protocols in Obj-C, just as interfaces in Jave or C#, is not related to implementation. It does not do anything. It's just a marker at compile time, and at run-time if you want to, to check if some functionally exist.