Cast an instance of a class to a @protocol in Objective-C

Question

I have an object (a UIViewController) which may or may not conform to a protocol I've defined.

I know I can determine if the object conforms to the protocol, then safely call the method:

if([self.myViewController conformsToProtocol:@protocol(MyProtocol)]) {
    [self.myViewController protocolMethod]; // <-- warning here
}

However, XCode shows a warning:

warning 'UIViewController' may not respond to '-protocolMethod'

What's the right way to prevent this warning? I can't seem to cast self.myViewController as a MyProtocol class.

Answer 1

The correct way to do this is to do:

if ([self.myViewController conformsToProtocol:@protocol(MyProtocol)])
{
        UIViewController <MyProtocol> *vc = (UIViewController <MyProtocol> *) self.myViewController;
        [vc protocolMethod];
}

The UIViewController <MyProtocol> * type-cast translates to "vc is a UIViewController object that conforms to MyProtocol", whereas using id <MyProtocol> translates to "vc is an object of an unknown class that conforms to MyProtocol".

This way the compiler will give you proper type checking on vc - the compiler will only give you a warning if any method that's not declared on either UIViewController or <MyProtocol> is called. id should only be used in the situation if you don't know the class/type of the object being cast.

Answer 2

You can cast it like this:

if([self.myViewController conformsToProtocol:@protocol(MyProtocol)])
{
    id<MyProtocol> p = (id<MyProtocol>)self.myViewController;
    [p protocolMethod];
}

This threw me for a bit, too. In Objective-C, the protocol isn't the type itself, so you need to specify id (or some other type, such as NSObject) along with the protocol that you want.