实现<NSCopying>协议

如果你尝试复制自己的一个类，例如分数类

#import "Fraction.h"

@implementation Fraction

- (instancetype)initWithFirst:(NSInteger)first second:(NSInteger)second
{
self = [self init];
if (self) {
[self setFirst:first];
[self setSecond:second];
}
return self;
}

+ (instancetype)fractionWithFirst:(NSInteger)first second:(NSInteger)second
{
Fraction *f = [[Fraction alloc] initWithFirst:first second:second];
return f;
}

- (void)printFraction
{
NSLog(@"%ld / %ld", [self first], [self second]);
}

@end

这样去复制

newFraction = [myFraction mutableCopy];

你会得到这样的错误

*** -[AddressBook copyWithZone:]: selector not recognized
*** Uncaught exception:
*** -[AddressBook copyWithZone:]: selector not recognized

为了让自定义类实现复制方法，需要根据< NSCopying >协议实现一到两个方法

- (id)copyWithZone:(NSZone *)zone
{
Fraction *f = [[Fraction alloc] init];
[f setFirst:_first];
[f setSecond:_second];
return f;
}

现在来测试下

Fraction *f1 = [Fraction fractionWithFirst:2 second:3];
Fraction *f2 = [f1 copy];
[f2 setFirst:1];
[f2 setSecond:2];
[f1 printFraction];
NSLog(@"%p", f1);
[f2 printFraction];
NSLog(@"%p", f2);

控制台输出

2 / 3
0x100200b80
1 / 2
0x100202ba0

可见f2是f1的成功复制，且地址不是一样的