Chapter 30 Properties

There is no property lifetime specifier called mutableCopy. If you wish for your setter to set the property to be a mutable copy of an object, you must implement the setter yourself so that it calls the mutableCopy method on the incoming object. For example, in OwnedAppliance, you might create a setOwnerNames: method:

- (void)setOwnerNames:(NSSet *)newNames

{

ownerNames = [newNames mutableCopy];

}

More about copying

Most Objective-C classes have no copyWithZone: method at all. Objective-C programmers make fewer copies than you might think.

Curiously, the copy and mutableCopy methods are defined in NSObject like this:

-(id)copy

{

return [self copyWithZone:NULL];

}

-(id)mutableCopy

{

return [self mutableCopyWithZone:NULL];

}

Thus, if you have some code like this:

Appliance *b = [[Appliance alloc] init];

Appliance *c = [b copy];

You will get an error like this:

-[Appliance copyWithZone:]: unrecognized selector sent to instance 0x100110130

Advice on atomic vs. nonatomic

This is an introductory book on programming, and the atomic/nonatomic option relates to a relatively advanced topic known as multithreading. Here is what you need to know: the nonatomic option will make your setter method run a tiny bit faster. If you look at the headers for Apple’s UIKit, every property is marked as nonatomic. You should make your properties nonatomic, too.

(I give this advice to everyone. In every group, however, there is someone who knows just enough to be a pain. That person says, “But when I make my app multithreaded, I’ll need the protection that atomic setter methods get me.” And I should say, “I don’t think you will write multithreaded code anytime soon. And when you do, I don’t think atomic setter methods are going to help.” But what I really say is “OK, then you should leave your setters atomic.” Because you can’t tell someone something they aren’t ready to hear.)

In Appliance.h, make your accessors non-atomic:

@property (copy, nonatomic) NSString *productName; @property (nonatomic) int voltage;

Sadly, at this time, the default for properties is atomic, so you do have to make this change.

220