iOS 浅谈：深.浅拷贝与copy.strong

浅谈：深.浅拷贝与copy.strong

深.浅拷贝

• copy mutableCopy NSString

NSString *string = @"汉斯哈哈哈";
// 没有产生新对象
NSString *copyString = [string copy];
// 产生新对象
NSMutableString *mutableCopyString = [string mutableCopy];

NSLog(@"string = %p copyString = %p mutableCopyString = %p", string, copyString, mutableCopyString);

• copy mutableCopy NSMutableString

NSMutableString *string = [NSMutableString stringWithString:@"汉斯哈哈哈"];

// 产生新对象
NSString *copyString = [string copy];
// 产生新对象
NSMutableString *mutableCopyString = [string mutableCopy];

NSLog(@"string = %p copyString = %p mutableCopyString = %p", string, copyString, mutableCopyString);

结论：

注意：其它对象NSArray、NSMutableArray 、NSDictionary、NSMutableDictionary一样适用

• copy NSObject

HSPerson *p = [[HSPerson alloc] init];
p.age = 20;
p.height = 170.0;

HSPerson *copyP = [p copy]; // 这里崩溃

崩溃：

看崩溃信息HSPerson应该先实现：

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

測试：

#import "HSPerson.h"

@interface HSPerson()<NSCopying>

@end

@implementation HSPerson

- (id)copyWithZone:(NSZone *)zone
{

    return @"汉斯哈哈哈";
}

@end

HSPerson *p = [[HSPerson alloc] init];
p.age = 20;
p.height = 170.0;

HSPerson *copyP = [p copy];
NSLog(@"copyP: %@", copyP);

能够看出copyWithZone又一次分配新的内存空间。则：

- (id)copyWithZone:(NSZone *)zone
{
    HSPerson *person = [[HSPerson allocWithZone:zone] init];
    return person;

// 有些人可能以下alloc,又一次初始化空间，但这方法已给你分配了zone，自己就无需再次alloc内存空间了
//    HSPerson *person = [[HSPerson alloc] init];
}

HSPerson *p = [[HSPerson alloc] init];
p.age = 20;
p.height = 170.0;

HSPerson *copyP = [p copy];
NSLog(@"p = %p copyP = %p", p, copyP);

NSLog(@"age = %d height = %f", copyP.age, copyP.height);

尽管copy了份新的对象，然而age,height值并未copy，那么：

- (id)copyWithZone:(NSZone *)zone
{
    HSPerson *person = [[HSPerson allocWithZone:zone] init];
    person.age = self.age;
    person.height = self.height;
    // 这里self事实上就要被copy的那个对象，非常显然要自己赋值给新对象。所以这里能够控制copy的属性
    return person;
}

这时你会想。有NSMutableCopying？没错，是有这货：

- (id)mutableCopyWithZone:(NSZone *)zone
{
    HSPerson *person = [[HSPerson allocWithZone:zone] init];
    person.age = self.age;
    person.height = self.height;

    return person;
}

NSCopying、NSMutableCopying有啥差别？
事实上感觉不是必需有NSMutableCopying，由于压根就没可变的HSPerson，但假设该对象有其它行为，能够借用NSMutableCopying实现，哈哈哈

copy.strong

说完深浅拷贝。理解copy.strong就轻松多了！

• copy

#import <Foundation/Foundation.h>

@interface HSPerson : NSObject

@property (nonatomic, copy) NSString *name;

@end

NSMutableString *string = [NSMutableString stringWithFormat:@"汉斯哈哈哈"];

HSPerson *person = [[HSPerson alloc] init];
person.name = string;

// 不能改变person.name的值，由于其内部copy新的对象
[string appendString:@" hans"];

 NSLog(@"name = %@", person.name);

property copy 实际上就对name干了这个：

- (void)setName:(NSString *)name
{
    _name = [name copy];
}

如果name为NSMutableString。会发生什么事？

@property (nonatomic, copy) NSMutableString *name;

这样会挨骂哦，实际上内部还是：

- (void)setName:(NSMutableString *)name
{
    _name = [name copy];
}

copy出来的仍然是不可变字符。假设有人用NSMutableString的方法，就会崩溃:

• strong

@property (nonatomic, strong) NSString *name;

NSMutableString *string = [NSMutableString stringWithFormat:@"汉斯哈哈哈"];

HSPerson *person = [[HSPerson alloc] init];
person.name = string;

// 能够改变person.name的值，由于其内部没有生成新的对象
[string appendString:@" hans"];

NSLog(@"name = %@", person.name);

总结：用copy与strong取决于需求。假设不希望被外界更改用copy。反之用strong