How does an Obj-C protocol differ from a Swift protocol?

Middle PRO

A protocol in Objective-C

A list of selectors a class promises to implement. @required (default) and @optional sections; the presence of an optional method is checked at runtime with respondsToSelector: before every call — otherwise unrecognized selector. Protocols can inherit from each other (@protocol Foo <NSObject>) and type variables as id<Foo>, but:

• they apply only to classes (and id);
• they cannot contain implementation — no default implementations;
• no associated types or generic constraints;
• conformance is checked formally at compile time, but at runtime you can conformsToProtocol: and even add a protocol to a class (class_addProtocol).

#import <Foundation/Foundation.h>

@protocol Downloadable <NSObject>
- (NSURL *)downloadURL;
@optional
- (NSInteger)priority;
@end

static NSInteger priorityOf(id<Downloadable> item) {
    if ([item respondsToSelector:@selector(priority)]) {
        return [item priority];
    }
    return 0;
}