iOS-Block 中 为何使用 weakSelf 和 strongSelf

island沐月
3,690 阅读13分钟
原文链接： www.jianshu.com
目录

本篇幅代码会有些多，需要读者耐心看完。

一 block 循环引用讲解

详细请移步至 iOS-底层原理(11)-block-循环引用详解，该文章详细讲解了有关 block 引用的原理及解决思路。

原理就是相互引用

image.png
二 为何使用 weakSelf 可以解决循环引用
  • __weak：不会产生强引用，指向的对象销毁时，会自动让指针置为nil
  • __unsafe_unretained：不会产生强引用，不安全，指向的对象销毁时，指针存储的地址值不变
image.png

代码佐证 - 在 MRC 环境下

  • 1 直接在 block里面使用 self
- (void)test1 {
    // 引用计数
    NSLog(@"self1 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
    
    // 循环引用
    self.myBlock = ^{
        [self test];
    };
    self.myBlock();
}

- (void)test {
    NSLog(@"test 方法 self 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    NSLog(@"touchesBegan self = %lu",(unsigned long)[self retainCount]);
}
  • 运行结果
2019-06-09 14:44:32.975002+0800 WeakStrongSelfDemo[6897:172037] self1 内存地址:0x7fc69ef14cc0 引用计数:9
2019-06-09 14:44:32.975150+0800 WeakStrongSelfDemo[6897:172037] test方法 self 内存地址:0x7fc69ef14cc0 引用计数:10
2019-06-09 14:44:34.638030+0800 WeakStrongSelfDemo[6897:172037] touchesBegan self 内存地址:0x7fc69ef14cc0 引用计数:5

总结：直接在 block 中使用 self，发现其引用计数+1，执行完touchesBegan方法后，引用计数变成5.

  • 2 在 block 中使用 weakSelf
typedef void(^MyBlock)(void);

@interface TestViewController ()
/** myBlock */
@property(nonatomic, copy)MyBlock myBlock;
@end

- (void)test2 {
    // 引用计数
    NSLog(@"self1 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
    
    __weak typeof(self) weakSelf = self;
    NSLog(@"weakSelf 内存地址:%p 引用计数:%lu",weakSelf,(unsigned long)[weakSelf retainCount]);
    
    NSLog(@"self2 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
    
    // 循环引用
    self.myBlock = ^{
        [weakSelf test];
    };
    self.myBlock();
}
  • 运行结果
2019-06-09 14:49:46.690693+0800 WeakStrongSelfDemo[6952:174797] self1 内存地址:0x7ffd7dc1e520 引用计数:9
2019-06-09 14:49:46.690852+0800 WeakStrongSelfDemo[6952:174797] weakSelf 内存地址:0x7ffd7dc1e520 引用计数:11
2019-06-09 14:49:46.690931+0800 WeakStrongSelfDemo[6952:174797] self2 内存地址:0x7ffd7dc1e520 引用计数:11
2019-06-09 14:49:46.691016+0800 WeakStrongSelfDemo[6952:174797] test方法 self 内存地址:0x7ffd7dc1e520 引用计数:12
2019-06-09 14:49:49.165380+0800 WeakStrongSelfDemo[6952:174797] touchesBegan self 内存地址:0x7ffd7dc1e520 引用计数:4

总结：self和weakSelf的内存地址相同，在没有执行 block之前，他两的引用计数值相同。执行完 block 后，执行touchesBegan屏幕点击，发现引用计数变为4。注意前面直接使用self的时候，引用计数是5。即得出结论

  1. 直接在block中使用self，会造成循环引用， block 执行完后，self的引用计数不正常（应该为4的，但实际上是 5，多 1是因为被block引用着）
  2. 如果使用weakSelf不会造成循环引用，self的引用计数正常（至于为啥是 4，应该是系统其它变量引用着 self）。
三 为何使用 strongSelf

在 doSomething 中，weakSelf 不会变成 nil，不过在 doSomething 执行完成，调用第二个方法 doOtherThing 的时候，weakSelf 有可能被释放，于是，strongSelf 就派上用场了：

__weak __typeof__(self) weakSelf = self;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    __strong __typeof(self) strongSelf = weakSelf;
    [strongSelf doSomething];
    [strongSelf doOtherThing];
});
  • 代码例子佐证
- (void)test3 {
    // 引用计数
    NSLog(@"self1 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
    
    __weak typeof(self) weakSelf = self;
    NSLog(@"weakSelf 内存地址:%p 引用计数:%lu",weakSelf,(unsigned long)[weakSelf retainCount]);
    
    NSLog(@"self2 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
    
    NSLog(@"------------------1---------------------");
    
    // 循环引用
    self.myBlock = ^{
        NSLog(@"block self = %lu",(unsigned long)[self retainCount]);
        
        NSLog(@"block weakSelf = %lu",(unsigned long)[weakSelf retainCount]);
        
        NSLog(@"---------------------2------------------");
        
        __strong typeof (weakSelf) strongSelf = weakSelf;
        
        NSLog(@"block self = %lu",(unsigned long)[self retainCount]);
        NSLog(@"block weakSelf = %lu",(unsigned long)[weakSelf retainCount]);
        NSLog(@"block strongSelf = %lu",(unsigned long)[strongSelf retainCount]);;
        
        NSLog(@"-----------------3----------------------");
        
        for (int i = 0; i < 5; i++) {
            NSLog(@"block self = %lu",(unsigned long)[self retainCount]);
        }
        
        for (int i = 0; i < 5; i++) {
            NSLog(@"block weakSelf = %lu",(unsigned long)[weakSelf retainCount]);
        }
        
        for (int i = 0; i < 5; i++) {
            NSLog(@"block strongSelf = %lu",(unsigned long)[strongSelf retainCount]);
        }
    };
    self.myBlock();
}
  • 运行结果
2019-06-09 15:02:38.065125+0800 WeakStrongSelfDemo[7175:182681] self1 内存地址:0x7ff4ddc41390 引用计数:9
2019-06-09 15:02:38.065280+0800 WeakStrongSelfDemo[7175:182681] weakSelf 内存地址:0x7ff4ddc41390 引用计数:11
2019-06-09 15:02:38.065368+0800 WeakStrongSelfDemo[7175:182681] self2 内存地址:0x7ff4ddc41390 引用计数:11
2019-06-09 15:02:38.065441+0800 WeakStrongSelfDemo[7175:182681] ------------------1---------------------
2019-06-09 15:02:38.065530+0800 WeakStrongSelfDemo[7175:182681] block self = 12
2019-06-09 15:02:38.065604+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 13
2019-06-09 15:02:38.065681+0800 WeakStrongSelfDemo[7175:182681] ---------------------2------------------
2019-06-09 15:02:38.065848+0800 WeakStrongSelfDemo[7175:182681] block self = 13
2019-06-09 15:02:38.066290+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 14
2019-06-09 15:02:38.066566+0800 WeakStrongSelfDemo[7175:182681] block strongSelf = 15
2019-06-09 15:02:38.066844+0800 WeakStrongSelfDemo[7175:182681] -----------------3----------------------
2019-06-09 15:02:38.067101+0800 WeakStrongSelfDemo[7175:182681] block self = 15
2019-06-09 15:02:38.067378+0800 WeakStrongSelfDemo[7175:182681] block self = 15
2019-06-09 15:02:38.067701+0800 WeakStrongSelfDemo[7175:182681] block self = 15
2019-06-09 15:02:38.067955+0800 WeakStrongSelfDemo[7175:182681] block self = 15
2019-06-09 15:02:38.068178+0800 WeakStrongSelfDemo[7175:182681] block self = 15
2019-06-09 15:02:38.068521+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 16
2019-06-09 15:02:38.068815+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 17
2019-06-09 15:02:38.069058+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 18
2019-06-09 15:02:38.069359+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 19
2019-06-09 15:02:38.069751+0800 WeakStrongSelfDemo[7175:182681] block weakSelf = 20
2019-06-09 15:02:38.070052+0800 WeakStrongSelfDemo[7175:182681] block strongSelf = 21
2019-06-09 15:02:38.084354+0800 WeakStrongSelfDemo[7175:182681] block strongSelf = 22
2019-06-09 15:02:38.084437+0800 WeakStrongSelfDemo[7175:182681] block strongSelf = 23
2019-06-09 15:02:38.084485+0800 WeakStrongSelfDemo[7175:182681] block strongSelf = 24
2019-06-09 15:02:38.084528+0800 WeakStrongSelfDemo[7175:182681] block strongSelf = 25
2019-06-09 15:06:01.899710+0800 WeakStrongSelfDemo[7175:182681] touchesBegan self 内存地址:0x7ff4ddc41390 引用计数:5
  • 接下来对运行结果分段进行分析
// 引用计数
for (int i = 0; i < 3; i++) {
    NSLog(@"self1 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
}

__weak typeof(self) weakSelf = self;
for (int i = 0; i < 3; i++) {
    NSLog(@"weakSelf 内存地址:%p",weakSelf);
}
NSLog(@"引用计数:%lu",(unsigned long)[weakSelf retainCount]);
NSLog(@"----------------0--------------");

for (int i = 0; i < 3; i++) {
    NSLog(@"引用计数:%lu",(unsigned long)[weakSelf retainCount]);
}

NSLog(@"self2 内存地址:%p 引用计数:%lu",self,(unsigned long)[self retainCount]);
NSLog(@"weakSelf 内存地址:%p 引用计数:%lu",weakSelf,(unsigned long)[weakSelf retainCount]);
  • 运行结果
2019-06-09 15:13:07.694736+0800 WeakStrongSelfDemo[7373:189509] self1 内存地址:0x7f93c9f065f0 引用计数:9
2019-06-09 15:13:07.694883+0800 WeakStrongSelfDemo[7373:189509] self1 内存地址:0x7f93c9f065f0 引用计数:9
2019-06-09 15:13:07.694978+0800 WeakStrongSelfDemo[7373:189509] self1 内存地址:0x7f93c9f065f0 引用计数:9
2019-06-09 15:13:07.695075+0800 WeakStrongSelfDemo[7373:189509] weakSelf 内存地址:0x7f93c9f065f0
2019-06-09 15:13:07.695166+0800 WeakStrongSelfDemo[7373:189509] weakSelf 内存地址:0x7f93c9f065f0
2019-06-09 15:13:07.695252+0800 WeakStrongSelfDemo[7373:189509] weakSelf 内存地址:0x7f93c9f065f0
2019-06-09 15:13:07.695336+0800 WeakStrongSelfDemo[7373:189509] 引用计数:13
2019-06-09 15:13:07.695410+0800 WeakStrongSelfDemo[7373:189509] ----------------0--------------
2019-06-09 15:13:07.695494+0800 WeakStrongSelfDemo[7373:189509] 引用计数:14
2019-06-09 15:13:07.695577+0800 WeakStrongSelfDemo[7373:189509] 引用计数:15
2019-06-09 15:13:07.695651+0800 WeakStrongSelfDemo[7373:189509] 引用计数:16
2019-06-09 15:13:07.695821+0800 WeakStrongSelfDemo[7373:189509] self2 内存地址:0x7f93c9f065f0 引用计数:16
2019-06-09 15:13:07.695990+0800 WeakStrongSelfDemo[7373:189509] weakSelf 内存地址:0x7f93c9f065f0 引用计数:18

通过运行结果可知，每次访问变量weakSelf（比如打印其内存地址，读取其引用计数[weakSelf retainCount]），都会使其引用计数+1。但是self不会。

  • 运行结果段 2 分析
weakSelf 内存地址:0x7f8e05e22ae0 引用计数:11
self2 内存地址:0x7f8e05e22ae0 引用计数:11  // 因为 self 和 weakSelf 指向同一块内存，所以他们的引用计数值相同
------------------1---------------------
block self = 12  // 因为 block 也强引用了 self，所以 self 的引用计数+1
block weakSelf = 13  // 访问了 [weakSelf retainCount]，引用计数+1
---------------------2------------------
block self = 13  // 即weakSelf的引用计数 13
block weakSelf = 14  // 访问了 [weakSelf retainCount]，引用计数+1
block strongSelf = 15  // 原理同weakSelf一样
  • 运行结果段 3 分析
2019-06-09 15:20:03.540679+0800 WeakStrongSelfDemo[7446:193548] block self = 15
2019-06-09 15:20:03.541845+0800 WeakStrongSelfDemo[7446:193548] block self = 15
2019-06-09 15:20:03.542679+0800 WeakStrongSelfDemo[7446:193548] block self = 15
2019-06-09 15:20:03.543429+0800 WeakStrongSelfDemo[7446:193548] block self = 15
2019-06-09 15:20:03.544483+0800 WeakStrongSelfDemo[7446:193548] block self = 15
2019-06-09 15:20:03.544844+0800 WeakStrongSelfDemo[7446:193548] block weakSelf = 16
2019-06-09 15:20:03.545132+0800 WeakStrongSelfDemo[7446:193548] block weakSelf = 17
2019-06-09 15:20:03.545381+0800 WeakStrongSelfDemo[7446:193548] block weakSelf = 18
2019-06-09 15:20:03.545757+0800 WeakStrongSelfDemo[7446:193548] block weakSelf = 19
2019-06-09 15:20:03.546010+0800 WeakStrongSelfDemo[7446:193548] block weakSelf = 20
2019-06-09 15:20:03.546266+0800 WeakStrongSelfDemo[7446:193548] block strongSelf = 21
2019-06-09 15:20:03.546596+0800 WeakStrongSelfDemo[7446:193548] block strongSelf = 22
2019-06-09 15:20:03.567575+0800 WeakStrongSelfDemo[7446:193548] block strongSelf = 23
2019-06-09 15:20:03.567672+0800 WeakStrongSelfDemo[7446:193548] block strongSelf = 24
2019-06-09 15:20:03.567725+0800 WeakStrongSelfDemo[7446:193548] block strongSelf = 25

原因在结果段 2 已经分析过了

前面都是从代码例子去佐证，接下来我们从底层原理角度去分析它

四 从底层原理角度分析
4.1 weakSelf
image.png
image.png
- (void)test5 {
    Student *stu = [[Student alloc] init];
    __weak Student *weakStu = stu;
    
    self.myBlock = ^{
        NSLog(@"%p",stu);
        NSLog(@"%p",weakStu);
    };
    self.myBlock();
}

// 在 block 内部存储数据格式
struct __main_block_impl_0 {
    NSObject *__weak weakStu;
}

从 block底层结构分析可知，一个被__weak修饰的局部变量，最终会被block捕获为自己的属性NSObject *__weak weakStu;

所以对weakSelf直白的理解就是结构体__main_block_impl_0把weakSelf这个局部变量捕获作为了自己的属性。结构体直接引用了weakSelf内存空间，想使用weakself中保存的self指针,而这个weakSelf并没有增加self的引用计数,所以self依然可以释放。

4.2 strongSelf
__strong typeof (weakSelf) strongSelf = weakSelf;

最终转换为底层代码是

TestViewController *strongSelf = weakSelf;

并且执行block里面的代码，本质是执行一个函数__TestViewController__test_block_func_0。所以TestViewController *strongSelf = weakSelf;赋值得到的是self（由 weakSelf 传过来），这使得在函数体运行期间，self不会被释放。

既然是函数，那么大括号走完其函数体重的局部变量就会被释放，也就是说函数执行完以后，strongSelf作为局部变量正常释放了，strongSelf也不属于block结构体的属性，weakSelf与strongSelf一内一外，意义是不一样的。

借鉴网上的图


image
image

__strong 确保在 Block 内，strongSelf 不会被释放。

五 总结
__weak __typeof(self)weakSelf = self;    //1

[self.context performBlock:^{      
    [weakSelf doSomething];          //2
     __strong __typeof(weakSelf)strongSelf = weakSelf;  //3
    [strongSelf doAnotherSomething];        
}];
1.使用__weak __typeof是在编译的时候,另外创建一个局部变量weak对象来操作self，引用计数不变。block 会将这个局部变量捕获为自己的属性，
访问这个属性，从而达到访问 self 的效果，因为他们的内存地址都是一样的。

2.因为weakSelf和self是两个变量,doSomething有可能就直接对self自身引用计数减到0了.
  所以在[weakSelf doSomething]的时候,你很难控制这里self是否就会被释放了.weakSelf只能看着.

3.__strong __typeof在编译的时候,实际是对weakSelf的强引用.
  指针连带关系self的引用计数会增加.但是你这个是在block里面,生命周期也只在当前block的作用域.
  所以,当这个block结束, strongSelf随之也就被释放了.不会影响block外部的self的生命周期.

总结
在 Block 内如果需要访问 self 的方法、变量，建议使用 weakSelf。
如果在 Block 内需要多次 访问 self，则需要使用 strongSelf。

相关文章参考
透彻理解block中weakSelf和strongSelf
到底什么时候才需要在ObjC的Block中使用weakSelf/strongSelf

项目链接地址 - WeakStrongSelfDemo

avatar
文章
阅读
粉丝
相关推荐
weak self 的黄金法则
1.3k阅读
 · 
3点赞
Weak self，一个关于 Swift 内存管理和闭包的故事
502阅读
 · 
2点赞
从零开始学self：Python中的self关键字解析
977阅读
 · 
1点赞
iOS 中 weak self 和 unowned self 的区别是什么？
2.8k阅读
 · 
10点赞
京东一面：post为什么会发送两次请求？🤪🤪🤪
247k阅读
 · 
3.0k点赞
iOS中的内存管理｜weak
3.2k阅读
 · 
3点赞
Self Attention 自注意力机制
891阅读
 · 
2点赞
在 React Router 中使用 JWT
302k阅读
 · 
172点赞
论文笔记：Self-Instruct: Aligning Language Model with Self Generated Instructions
3.8k阅读
 · 
6点赞
“当高启强遇到陈书婷”与TCP协议
308k阅读
 · 
207点赞

友情链接：