Objective-C基础回顾 1. 复制--NSCopying

让我们复习一下内存管理的规则：“如果你使用alloc、copy或new方法获取了一个对象，则该对象的保留计数器的值为1，而且你要释放它。”既然叫copy方法，肯定就能创建对象的副本。copy消息会告诉对象创建一个全新的对象，并让新对象与接收copy消息的原对象完全一样。

一个对象如果想调用copy方法，需要实现NSCopying协议，该协议如下：

The NSCopying protocol declares a method for providing functional copies of an object. The exact meaning of “copy” can vary from class to class, but a copy must be a functionally independent object with values identical to the original at the time the copy was made. A copy produced with NSCopying is implicitly retained by the sender, who is responsible for releasing it.

NSCopying declares one method, copyWithZone:, but copying is commonly invoked with the convenience methodcopy. Thecopy method is defined for all objects inheriting fromNSObject and simply invokescopyWithZone: with the default zone.

Your options for implementing this protocol are as follows:

• Implement NSCopying using alloc and init... in classes that don’t inheritcopyWithZone:.
• Implement NSCopying by invoking the superclass’scopyWithZone: whenNSCopying behavior is inherited. If the superclass implementation might use theNSCopyObject function, make explicit assignments to pointer instance variables for retained objects.
• Implement NSCopying by retaining the original instead of creating a new copy when the class and its contents are immutable.

If a subclass inherits NSCopying from its superclass and declares additional instance variables, the subclass has to overridecopyWithZone: to properly handle its own instance variables, invoking the superclass’s implementation first.

下面通过一个例子来说明：

定义一个Cat类，初始化一个对象cat1，然后通过cat1调用copy方法来创建新的对象cat2

Cat.h文件

#import <Foundation/Foundation.h>@interface Cat : NSObject<NSCopying>{    NSString *_name;   //名称    NSString *_color;  //颜色};- (void) setName:(NSString*) name andColor:(NSString *) color;- (NSString *) getName;- (NSString *) getColor;@end

Cat.m文件

#import "Cat.h"@implementation Cat- (void) setName:(NSString*) name andColor:(NSString *) color{    _name = name;    _color = color;}- (NSString *) getName{    return _name;}- (NSString *) getColor{    return _color;}- (id) copyWithZone:(NSZone *)zone{    Cat *newCat = [[Cat allocWithZone:zone] init];    [newCat setName:_name andColor:_color];    return newCat;}@end

main.m文件

#import <Foundation/Foundation.h>#import "Cat.h"int main(int argc, const char * argv[]){    @autoreleasepool {        //通过alloc初始化创建对象        Cat *cat1 = [[Cat alloc] init];        [cat1 setName:@"Mimi" andColor:@"red"];           //通过copy方法创建新对象cat2        Cat *cat2 = [cat1 copy];        [cat2 setName:@"JJ" andColor:@"blue"];                NSLog(@"cat1 %@ %@ %@ ", cat1, [cat1 getName], [cat1 getColor]);        NSLog(@"cat2 %@ %@ %@ ", cat2, [cat2 getName], [cat2 getColor]);    }    return 0;}

输出结果：

cat1 <Cat: 0x10010b5e0> Mimi red 

cat2 <Cat: 0x10010bc30> JJ blue