如何正确实现mutableCopyWithZone和copyWithZone

Question

我读了很多关于它的话题,但我还是迷路了.

我想创建两种对象,一种是只有"只读"属性的不可变对象,另一种是只有"readwrite"属性的可变对象.

让我们称他们为EXCar和EXMutableCar.

EXCar是NSObject的子类,EXMutableCar是EXCar的子类.

ExCar将拥有其界面

@property (nonatomic, strong, readonly) NSString *name;

EXMutableCar将在其界面中具有

@property (nonatomic, strong) NSString *name;

所以当我使用它的子类EXMutableCar时,我"打开"EXCar的属性.然后它是可变的.问题是在它们之间正确复制.

我在EXCar中实现了mutableCopyWithZone:

- (id)mutableCopyWithZone:(NSZone *)zone {
    EXMutableCar *mutableCopy = [[EXMutableCar allocWithZone:zone] init];
    mutableCopy.name = _name;

    return mutableCopy;
}

第一个问题,是这样做的好方法吗？(我想要燕子副本)

问题出在copyWithZone上.由于EXCar的属性是readonly,我既不能在EXCar中创建,也不能在EXMutableCar中创建EXCar的新实例,并填充其属性,如下所示:

- (id)copyWithZone:(NSZone *)zone {
    EXCar *copy = [[EXCar allocWithZone:zone] init];
    copy.name = _name; // This can't work...

    return copy;
}

我真的不想做一个"init"方法,传入15个属性(当然,EXCar就是一个例子,真正的类充满了许多属性).通常它们是从服务器的JSON消息启动的,因此它们不需要复杂的init方法.

第二个问题是这样的,如何做一个使我的类不可变的copyWithZone？

谢谢你的帮助 :)

Answer 1

码:

// EXCar.h
#import <Foundation/Foundation.h>

@interface EXCar : NSObject <NSCopying, NSMutableCopying>

@property (nonatomic, strong, readonly) NSString* name;

@end

// EXCar.m
#import "EXCar.h"
#import "EXMutableCar.h"

@implementation EXCar

- (id)copyWithZone:(NSZone *)zone {
  EXCar* car = [[[self class] allocWithZone:zone] init];
  car->_name = [_name copyWithZone:zone];
  return car;
}

- (id)mutableCopyWithZone:(NSZone *)zone {
  EXMutableCar* mutableCar = [[EXMutableCar allocWithZone:zone] init];
  mutableCar.name = [_name mutableCopyWithZone:zone];
  return mutableCar;
}

@end

// EXMutableCar.h
#import "EXCar.h"

@interface EXMutableCar : EXCar

@property (nonatomic, strong) NSString* name;

@end

// EXMutableCar.m
#import "EXMutableCar.h"

@implementation EXMutableCar

@synthesize name = _mutableName;

- (id)copyWithZone:(NSZone *)zone {
  EXMutableCar* car = [super copyWithZone:zone];
  car->_mutableName = [_mutableName copyWithZone:zone];
  return car;
}

- (id)mutableCopyWithZone:(NSZone *)zone {
  EXMutableCar* car = [super mutableCopyWithZone:zone];
  car->_mutableName = [_mutableName mutableCopyWithZone:zone];
  return car;
}

说明:

• EXCar 接口实现"复制"协议;
• 子类EXMutableCar重写相同的属性,使其成为可能readwrite.

实现中的第一件事EXMutableCar:手动,@synthesize name因为Xcode给我们一个警告,因为我们的超类中有相同的属性(但具有不同的访问说明符).

注意我们可以给我们的实例变量赋予相同的名称_name,但重要的是要理解我们在子类中声明一个不同的变量,因为_name从超类对我们来说是不可访问的.

接下来,Apple文档说明:

如果子类从其超类继承NSCopying并声明其他实例变量,则子类必须重写copyWithZone:以正确处理其自己的实例变量,首先调用超类的实现.

同样适用于NSMutableCopying:

如果一个子类从其超类继承NSMutableCopying并宣布额外的实例变量,子类必须覆盖mutableCopyWithZone:妥善处理自己的实例变量,首先调用超类的实现.

我们确实声明了其他实例变量,因此我们也在子类中重写这些方法.

结果:

EXCar* car = [[EXCar alloc]init]; // car.name is (null)
EXCar* carCopy = [car copy]; // we can do this
EXMutableCar* mutableCar = [car mutableCopy]; // and this
mutableCar.name = @"BMW";
car = [mutableCar copy]; // car.name is now @"BMW"
EXMutableCar* anotherMutableCar = [car mutableCopy]; //anotherMutableCar.name is @"BMW"