Objective C - How can I create an interface?

Question

I need to be able to create an interface like you create in C# to enforce a group of classes to implement certain methods. Is this possible in objective c?

Answer 1

You can create a protocol. it would look something like this: in MyProtocol.h:

@protocol MyProtocol
-(void)myMethod;
-(void)myMethod2;
@end

in MyClass.h

#import "MyProtocol.h"
@interface MyClass : NSObject<MyProtocol>
@end

If you want to receive objects of a given protocol, you can do so like this:

id<MyProtocol> var;

or

NSObject<MyProtocol> *var;

more info here

Answer 2

You declare a "protocol" (Objective-C's interfaces) using

@protocol MyProtocol <BaseProtocol1,...,BaseProtocolN>
//methods and properties
@end

where <BaseProtocol> is optional and indicates that MyProtocol "inherits" BaseProtocol's interface. The NSObject protocol is useful in this context because it allows you to use

@protocol MyProtocol <NSObject>
//...
@end

to indicate (when appropriate) that conforming instances of MyProtocol have the standard NSObject methods as well (e.g. -retain/-release, etc.).

You then declare that a class "conforms" to a protocol:

@interface MyClass : NSObject <MyProtocol,...,OtherProtocols>
{}

@end

And you can test whether an instance conforms to a protocol:

id myInstance = ...; //some object instance

if([myInstance conformsToProtocol:@protocol(MyProtocol)]) {
  // myInstance conforms to MyProtocol
}

You can further silence compiler warnings by declaring that a variable holds instances that conform to a protocol (note that Objective-C's dynamic nature prevents the compiler from verifying that contract and you can still get runtime errors by assigning a non-conforming instance to the variable):

id<MyProtocol> o;

In this case, the compiler will complain if you send [o retain] without MyProtocol conforming to the NSObject protocol. You can silence these warnings by declaring MyProtocol as conforming to NSObject as described above or by delcaring o as

NSObject<MyProtocol> o;

Since NSObject is not the only root object in Cocoa (i.e. NSProxy does not inherit from NSObject), it's not necessarily true that all instances conforming to MyProtocol also conform to NSObject. If you know that they do, you can declare MyProtocol as conforming to NSObject.