Clarification needed about the rules of properties declared in protocols

Question

I have a few questions about properties declared in protocols.

Return type variance

@protocol IHaveProperties

@required

@property (nonatomic, strong) IAmOfTypeX *propertyOfProtocolType;
@property (nonatomic, strong) NSArray *array;

@end

@interface ClassThatHasProperties : NSObject<IHaveProperties>

@property (nonatomic, strong) ImplementationOfTypeX *propertyOfProtocolType;
@property (nonatomic, strong) NSMutableArray *array;

@end

Okay, so I tried this with a protocol/class combination and to my chagrin, it compiled.

How does that work? Wouldn't this technically not conform to the interface?

Property declaration modifiers

@protocol IHaveProperty

@required

@property (nonatomic, strong, readonly) *example;

@end

@interface HaveProperty : NSObject<IHaveProperty>

@property (nonatomic, strong, readonly) *example;

@end

My Mac is restarting right now so I cannot try this out but I would think this would be okay, because the protocol declaration has nothing backing it. All the modifiers wouldn't be of interest to a caller, only to the class implementing the protocol.

Answer 1

If you look at the <NSObject> Protocol, you can see that it holds methods and properties together:

@protocol NSObject

- (BOOL)isEqual:(id)object; @property (readonly) NSUInteger hash;

@property (readonly) Class superclass;
- (Class)class OBJC_SWIFT_UNAVAILABLE("use 'anObject.dynamicType' instead");
- (instancetype)self;

- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

- (BOOL)isProxy;

- (BOOL)isKindOfClass:(Class)aClass;
- (BOOL)isMemberOfClass:(Class)aClass;
- (BOOL)conformsToProtocol:(Protocol *)aProtocol;

- (BOOL)respondsToSelector:(SEL)aSelector;

- (instancetype)retain OBJC_ARC_UNAVAILABLE;
- (oneway void)release OBJC_ARC_UNAVAILABLE;
- (instancetype)autorelease OBJC_ARC_UNAVAILABLE;
- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;

- (struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;

@property (readonly, copy) NSString *description; @optional @property
(readonly, copy) NSString *debugDescription;

@end

You must have used description or superclass properties. There's no difference in how a property is treated, as compared to a function. The class implementing the protocol would be providing the getter and / or setter of the property of a protocol. And similarly the class which owns the delegate object would call its properties like: self.delegate.propertyName.