13: Protocols in Objective-C

Creator Quick Response Code in Objective-C 13: Protocols

That s it for making Engine copy-capable. We don t have to touch Slant6. Because Slant6 doesn t add any instance variables, it doesn t have to do any extra work when making a copy. Thanks to inheritance, and the technique of using [self class] when creating the object, Slant6 objects can be copied too.

Tires are trickier to copy than Engines. Tire has two instance variables (pressure and treadDepth) that need to be copied into new Tires, and the AllWeatherRadial subclass introduces two additional instance variables (rainHandling and snowHandling) that also must be copied into a new object. First up is Tire. The interface has grown the protocol-adoption syntax:

@interface Tire : NSObject <NSCopying> { float pressure; float treadDepth; } // ... methods @end // Tire

and now the implementation of copyWithZone::

- (id) copyWithZone: (NSZone *) zone { Tire *tireCopy; tireCopy = [[[self class] allocWithZone: zone] initWithPressure: pressure treadDepth: treadDepth]; return (tireCopy); } // copyWithZone

You can see the [[self class] allocWithZone: zone] pattern here, like in Engine. Since we have to call init when we create the object, we can easily use Tire s initWithPressure:treadDepth: to set the pressure and treadDepth of the new tire to be the values of the tire we re copying. This method happens to be Tire s designated initializer, but you don t have to use the designated initializer for copying. If you want, you can use a plain init and use accessor methods to change attributes.

You can access instance variables directly via the C pointer operator, like this:

tireCopy->pressure = pressure; tireCopy->treadDepth = treadDepth;

Generally, we try to use init methods and accessor methods in the unlikely event that setting an attribute involves extra work.

Now, it s time for AllWeatherRadial. The @interface for AllWeatherRadial is unchanged:

@interface AllWeatherRadial : Tire { float rainHandling; float snowHandling; } // ... methods @end // AllWeatherRadial

Wait where s the <NSCopying> You don t need it, and you can probably guess why. When AllWeatherRadial inherits from Tire, it pulls all of Tire s baggage along, including the conformance to the NSCopying protocol. We ll need to implement copyWithZone:, though, because we have to make sure AllWeatherRadial s rain and snow-handling instance variables are copied:

- (id) copyWithZone: (NSZone *) zone { AllWeatherRadial *tireCopy; tireCopy = [super copyWithZone: zone]; [tireCopy setRainHandling: rainHandling]; [tireCopy setSnowHandling: snowHandling]; return (tireCopy); } // copyWithZone

Because AllWeatherRadial is a subclass of a class that can be copied, it doesn t need to do the allocWithZone: and [self class] jazz we used earlier. This class just asks its superclass for a copy and hopes that the superclass does the right thing and uses [self class] when allocating the object. Because Tire s copyWithZone: uses [self class] to determine the kind of object to make, it will create a new AllWeatherRadial, which is just what

we want. That code also handles copying the pressure and treadDepth values for us. Now, isn t that convenient The rest of the work is to set the rain and snow-handling values. The accessor methods are good for doing that.

Now that we can make copies of engines and tires and their subclasses, it s time to make the Car itself copiable. As you d expect, Car needs to adopt the NSCopying protocol:

@interface Car : NSObject <NSCopying> { NSMutableArray *tires; Engine *engine; } // ... methods @end // Car

And to fulfill its promise to NSCopying, Car must implement our old friend copyWithZone:. Here is Car s copyWithZone: method:

- (id) copyWithZone: (NSZone *) zone { Car *carCopy; carCopy = [[[self class] allocWithZone: zone] init]; carCopy.name = self.name; Engine *engineCopy; engineCopy = [[engine copy] autorelease]; carCopy.engine = engineCopy; int i; for (i = 0; i < 4; i++) { Tire *tireCopy; tireCopy = [[self tireAtIndex: i] copy]; [tireCopy autorelease]; [carCopy setTire: tireCopy atIndex: i];