iOS中的Copy

copy的概念特点

• copy产生一个新副本的过程,利用一个原对象产生一个新对象

  • copy：创建一个不可变的副本(NSString\NSArray\NSDictionary)
  • mutableCopy :创建一个可变的副本 (NSMutableDictionary\NSMutableArray\NSMutableString)

• 修改新文件或原文件，不会影响原文件或新文件

• 浅拷贝：如果没有生成新对象我们称为浅拷贝，本质是指针拷贝，指向的还是同一处

• 深拷贝：如果生成了新的对象，我们称为深拷贝，本质就是创建了一个新的对象

NSString * str;
NSLog(@"str = %p, str = %p",str,&str);
str = @"demo";
NSMutableString *copyStr = [str copy];
NSMutableString *mcopyStr = [str mutableCopy];
NSLog(@"str = %@, copyStr = %@, mcopyStr = %@",str,copyStr,mcopyStr);
NSLog(@"str = %p, copyStr = %p, mcopyStr = %p",str,copyStr,mcopyStr);


log:
str = 0x0, str = 0x7ffee6496a88
str = demo, copyStr = demo, mcopyStr = demo
str = 0x109768080, copyStr = 0x109768080, mcopyStr = 0x600002ca52c0

结论：
1、copy和mutableCopy拷贝出来的对象中的内容和以前内容一致
2、不可变的字符串通过copy操作，没有生成新的对象，而是指向同一内存
3、不可变的字符串通过mutableCopy操作，生成新的可变对象，所以需要生成一个新对象

NSMutableString *str = [NSMutableString stringWithFormat:@"demo"];
NSMutableString *copyStr =  [str copy];
NSMutableString *mcopyStr =  [str mutableCopy];
    
NSLog(@"str = %@, copyStr = %@, mcopyStr = %@",str,copyStr,mcopyStr);
NSLog(@"str = %p, copyStr = %p, mcopyStr = %p",str,copyStr,mcopyStr);

log:
str = demo, copyStr = demo, mcopyStr = demo
str = 0x60000397a370, copyStr = 0xc464b5846da8a473, mcopyStr = 0x60000397a880
结论：
1、copy和mutableCopy拷贝出来的对象中的内容和以前内容一致
2、可变的字符串通过copy操作，生成新的对象
3、可变的字符串通过mutableCopy操作，生成新的可变对象

copy的用途

• property中使用的copy、strong修饰的区别
  strong修饰string，外部修改会影响内部数据
  copy修饰string，可以防止外界修改内部的数据

@interface Person : NSObject
@property (nonatomic,strong) NSString *firstName;
@property (nonatomic,copy) NSString *lastName;
@end

NSMutableString *firstNameStr = [[NSMutableString alloc]initWithFormat:@"firstName-AA"];
Person *p = [[Person alloc]init];
p.firstName = firstNameStr;
NSLog(@"%@",p.firstName);
[firstNameStr appendString:@"aa"];
NSLog(@"%@",p.firstName);
NSMutableString *lastNameStr = [[NSMutableString alloc]initWithFormat:@"lastNameStr-BB"];
p.lastName = lastNameStr;
NSLog(@"%@",p.lastName);
[lastNameStr appendString:@"bb"];
NSLog(@"%@",p.lastName);

log:
firstName-AA
firstName-AAaa
lastNameStr-BB
lastNameStr-BB

自定义类的实现copy（NSCopying协议）

• 若想令自己写的类具有copy功能，则需要实现NSCopying、NSMutableCopying协议

- (id)copyWithZone:(nullable NSZone *)zone;
- (id)mutableCopyWithZone:(nullable NSZone *)zone;

看个例子

#import <Foundation/Foundation.h>
@interface Phone : NSObject<NSCopying>
@property (nonatomic,copy,readonly) NSString *name;
@property (nonatomic,assign,readonly) NSInteger price;

- (instancetype)initWithName:(NSString *)name withPrice:(NSInteger)price;
@end


#import "Phone.h"
@implementation Phone
- (instancetype)initWithName:(NSString *)name withPrice:(NSInteger)price{
    self = [super init];
    if (self) {
        _name = [name copy];
        _price = price;
    }
    return self;
}
- (id)copyWithZone:(NSZone *)zone{
    Phone *phone = [[[self class] allocWithZone:zone] initWithName:_name withPrice:_price];
    return phone;
}

Phone *p = [[Phone alloc]initWithName:@"iPhone" withPrice:999];
NSLog(@"%p--%@--%zd",p,p.name,p.price);
Phone *p1 = [p copy];
NSLog(@"%p--%@--%zd",p1,p1.name,p1.price);

log:
0x60000002cce0--iPhone--999
0x6000000371c0--iPhone--999

注：在- (id)copyWithZone:(NSZone *)zone方法中，一定要通过[self class]方法返回的对象调用allocWithZone:方法。因为指针可能实际指向的是PersonModel的子类。这种情况下，通过调用[self class]，就可以返回正确的类的类型对象。