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Protocols and Data Types
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You can specify protocol names in the data types you use for instance variables and method arguments. By doing this, you give the Objective-C compiler a little more information so it can help error-check your code. Recall that the id type represents a pointer to any kind of object; it s the generic object type. You can assign any object to an id variable, and you can assign an id variable to any kind of object pointer. If you follow id with a protocol name, complete with angle brackets, you re telling the compiler (and any humans reading the code) that you are expecting any kind of object, as long as it conforms to that protocol.
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CHAPTER 13: Protocols
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For example, NSControl has a method called setObjectValue:, which requires an object that conforms to NSCopying:
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- (void) setObjectValue: (id<NSCopying>) obj;
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When you compile this, the compiler checks the type of the argument and gives you a warning, like class 'Triangle' does not implement the 'NSCopying' protocol. Handy!
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Apple never leaves well enough alone. Objective-C 2.0 adds two new modifiers for protocols: @optional and @required. Wait a minute. Did we just say that if you conform to a protocol, you re required to implement all of the protocol s methods Yes, that s true, for older versions of Objective-C. If you have the luxury of Objective-C 2.0, you can do groovy stuff like this:
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@protocol BaseballPlayer - (void)drawHugeSalary; @optional - (void)slideHome; - (void)catchBall; - (void)throwBall; @required - (void)swingBat; @end // BaseballPlayer
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So, a class that adopts the BaseballPlayer protocol is required to implement -drawHugeSalary and -swingBat but has the option of sliding home, catching the ball, or throwing the ball. Why would Apple do this, when informal protocols seem to work OK It s one more tool in our arsenal to explicitly express our intent in class declarations and our method declarations. Say you saw this in a header file:
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@interface CalRipken : Person <BaseballPlayer>
You know immediately that we re dealing with someone who gets paid a lot and can swing a bat and who might slide home or catch or throw the ball. With an informal protocol, there s no way to say this. Likewise, you can decorate arguments to methods with a protocol:
-(void)draft:(Person<BaseballPlayer>);
CHAPTER 13: Protocols
This code makes it obvious what kind of person can get drafted to play baseball. And if you do any iPhone development, you ll notice the things that are informal protocols in Cocoa become formal protocols with a lot of @optional methods.
Summary
In this chapter, we introduced the concept of a formal protocol. You define a formal protocol by listing a set of methods inside a @protocol block. Objects adopt this formal protocol by listing the protocol name in angle brackets after the class name in an @interface statement. When an object adopts a formal protocol, it promises to implement every required method that s listed in the protocol. The compiler helps you keep your promise by giving you a warning if you don t implement all the protocol s methods. Along the way, we explored some of the nuances that occur with object-oriented programming, particularly the issues that crop up when making copies of objects that live in a hierarchy of classes. And now, congratulations! You ve covered a great majority of the Objective-C language and have delved deeply into a number of topics that come up often in OOP. You have a good foundation for moving on to Cocoa programming or jumping into your own projects. In the next chapter of this book, you ll get a quick taste of writing a graphical Cocoa application using Interface Builder and the AppKit. Interface Builder and AppKit are the soul of Cocoa programming and are the central topic of most Cocoa books and projects and they re also a lot of fun. After that, we ll delve more into some of Cocoa s lower-level features.
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