Objective-C LanguageProtocols

Basic Protocol Definition

Defining a new protocol:

@protocol NewProtocol

- (void)protocolMethod:(id)argument;

- (id)anotherMethod;

@end

Optional and required methods

By default, all the methods declared in a protocol are required. This means that any class that conforms to this protocol must implement those methods.

It is also possible to declare optional methods. These method can be implemented only if needed.

You mark optional methods with the @optional directive.

@protocol NewProtocol
- (void)protocolMethod:(id)argument;
@optional
- (id)anotherMethod;
@end

In this case, only anotherMethod is marked as optional; the methods without the @optional directive are assumed to be required.

The @optional directive applies to methods that follow, until the end of the protocol definition or, until another directive is found.

@protocol NewProtocol
- (void)protocolMethod:(id)argument;
@optional
- (id)anotherMethod;
- (void)andAnotherMethod:(id)argument;
@required
- (void)lastProtocolMethod;
@end

This last example defines a protocol with two optional methods and two required methods.

Conforming to Protocols

The following syntax indicate that a class adopts a protocol, using angle brackets.

@interface NewClass : NSObject <NewProtocol>
...
@end

This means that any instance of NewClass will respond to methods declared in its interface but also it will provide an implementation for all the required methods of NewProtocol.

It is also possible for a class to conform to multiple protocols, by separating them with comma.

@interface NewClass : NSObject <NewProtocol, AnotherProtocol, MyProtocol>
...
@end

Like when conforming to a single protocol, the class must implement each required method of each protocols, and each optional method you choose to implement.

Forward Declarations

It's possible to declare protocol name without methods:

@protocol Person;

use it your code (class definition, etc):

@interface World : NSObject
@property (strong, nonatomic) NSArray<id<some>> *employees;
@end

and later define protocol's method somewhere in your code:

@protocol Person
- (NSString *)gender;
- (NSString *)name;
@end

It's useful when you don't need to know protocols details until you import that file with protocol definition. So, your class header file stays clear and contains details of the class only.

Checking existance of optional method implementations

if ([object respondsToSelector:@selector(someOptionalMethodInProtocol:)])
{
    [object someOptionalMethodInProtocol:argument];
}

Check conforms Protocol

Returns a Boolean indicating if the class conform the protocol:

[MyClass conformsToProtocol:@protocol(MyProtocol)];