How to create a protocol with methods that are optional?

Question

I noticed methods marked optional in several protocols defined in the iPhone SDK, such as the UIActionSheetDelegate protocol for example.

How can I define a protocol of my own, and set a few of the methods as optional?

Answer 1

Use the @optional keyword before your method declaration to make it optional. Simple as that!

// myProtocol.h
@protocol myProtocol
- (void)myMandatoryMethod:(id)someArgument;
@optional
- (void)myOptionalMethod:(id)someArgument;
@end

// myClass.m
@interface myClass : someSuperClass <myProtocol>
    //...
@end

Answer 2

From the Apple page on "Formal Protocols":

Optional Protocol Methods Protocol methods can be marked as optional using the @optional keyword. Corresponding to the @optional modal keyword, there is a @required keyword to formally denote the semantics of the default behavior. You can use @optional and @required to partition your protocol into sections as you see fit. If you do not specify any keyword, the default is @required.

@protocol MyProtocol

- (void)requiredMethod;

@optional
- (void)anOptionalMethod;
- (void)anotherOptionalMethod;

@required
- (void)anotherRequiredMethod;

@end

Answer 3

Keep in mind that the @optional keyword is new in Objective-C 2.0. Prior to Leopard, you would have typically declared an informal protocol as a category of NSObject.