How to properly subclass a delegate property in Objective-C?

Question

In subclassing a class, I want to also subclass a delegate of the parent class given that the subclass now has additional functionality. What\'s the best way to go about doing

Answer 1

Given this example that produces the warning:

// Class A
@protocol ClassADelegete;

@interface ClassA : NSObject
@property (nonatomic, weak) id<ClassADelegete> delegate;
@end

@protocol ClassADelegete <NSObject>
- (void)classADidSomethingInteresting:(ClassA *)classA;
@end

// Class B
@protocol ClassBDelegete;

@interface ClassB : ClassA
@property (nonatomic, weak) id<ClassBDelegete> delegate; // Warning here
@end

@protocol ClassBDelegete <ClassADelegete>
- (void)classBDidSomethingElse:(ClassB *)classB;
@end

Two solutions that remove the warning are.

1) In the subclass, place the protocol definition before the class definition. This is what UITableViewDelegate in UITableView.h does:

// Class B
@class ClassB;

@protocol ClassBDelegete <ClassADelegete>
- (void)classBDidSomethingElse:(ClassB *)classB;
@end

@interface ClassB : ClassA
@property (nonatomic, weak) id<ClassBDelegete> delegate;
@end

2) In the subclass, add the original protocol alongside the new one:

// Class B
@protocol ClassBDelegete;

@interface ClassB : ClassA
@property (nonatomic, weak) id<ClassADelegete, ClassBDelegete> delegate;
@end

@protocol ClassBDelegete <ClassADelegete>
- (void)classBDidSomethingElse:(ClassB *)classB;
@end

I assume (1) works as Apple do it this way, Option (2) removes the warning but I haven't compiled and run anything setup this way.

Answer 2

Follow the example of NSTableView and NSOutlineView.

NSOutlineView is a subclass of NSTableView, and defines its own protocol for its dataSource and delegate.

NSTableView declares its delegate this way:

- (void)setDelegate:(id <NSTableViewDelegate>)delegate;
- (id <NSTableViewDelegate>)delegate;

and NSOutlineView:

- (void)setDelegate:(id <NSOutlineViewDelegate>)anObject;
- (id <NSOutlineViewDelegate>)delegate;

Apparently the compiler is more lenient with bare method declarations than it is with property declarations.

Unlike NSTable/OutlineView, you might want to make the subclass's protocol inherit from the base class's protocol, e.g.

 @protocol SpecializedProtocol <BaseProtocol>

... it probably depends on the situation.