如何比较声明为ID的实例 <protocol> 在Objective-C中

Question

I have a class that has a property declared as type id that adheres to a protocol and I'd like to ensure equality for it.

How do check for value equality on an property declared as type id ?

@interface MyClass : NSObject
    @property (nonatomic, copy) NSString *name;
    @property (nonatomic, strong) id<SomeProtocol> attribute;
@end

@implementation MyClass 

- (BOOL)isEqual:(id)object {
    if (self == object) {
        return YES;
    }

    if (![object isKindOfClass:[MyClass class]]) {
        return NO;
    }

    return [self isEqualToMyClass:(MyClass *) object];
}

- (BOOL)isEqualToMyClass:(MyClass *)rhsValue {
    if (rhsValue == nil) {
        return NO;
    }

    return 
        ([self.name isEqualToString:rhsValue.name] && 
        // Compiler produces error: Error:(90, 36) no known instance method for selector 'isEqual:'
        [self.attribute isEqual:rhs.attribute]); 
}

@end

SomeProtocol is defined as:

@protocol SomeProtocol <NSObject>

@end

Class that extends SomeProtocol :

@interface MyAttributeClass : NSObject <SomeProtocol>

@end

MyAttributeClass implements the protocol SomeProtocol and it has its own isEqual and when an instance of it is stored by MyClass in attribute, I'd like to be able to check that the values are equivalent.

It is MyAttributeClass that gets assigned into MyClass:

MyClass *myClass = [[MyClass alloc] init];
myClass.name = "HAL";
myClass.attribute = [[MyAttributeClass alloc] init];

Answer 1

Probably you forgot to import the header containing the protocol definition. (Instead of only having a forward declaration.) Add that in front of the implementation.