iOS block内部为什么要加__strong

block的循环引用这里就不说了,现在是有一种场景,比如在当前的VC中,延迟执行了block方法,但是当block执行的时候,当前的VC已经不存在了,获取不到到VC的属性了,我们该怎么解决

定义一个block还有全局的字符串属性

@property (nonatomic,copy)Block block;
@property (nonatomic,strong)NSString *str;

然后我们点击,vc返回了

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"-----%@",weakSelf.str);
        });
    };
    self.block();
    [self.navigationController popViewControllerAnimated:YES];
}

2018-12-27 15:42:47.773924+0800 TestDemo[33437:595246] dealloc
2018-12-27 15:42:49.267220+0800 TestDemo[33437:595246] -----(null)

可以从控制台看出,在block执行之前vc已经被销货了,所以再去向一个空的对象发送消息,肯定是发送失败的.
那么我们为了保证block的正常执行,可以尝试使用在block内部使用: __strong,防止self提前释放

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        //在block的执行过程中,使用强对象对弱对象进行引用
        __strong TwoViewController *strongSelf = weakSelf;
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"-----%@",strongSelf.str);
        });
    };
    self.block();
    [self.navigationController popViewControllerAnimated:YES];
}

2018-12-27 15:45:20.427785+0800 TestDemo[33486:599182] -----123
2018-12-27 15:45:20.427937+0800 TestDemo[33486:599182] dealloc

可以发现,可以获取的到str属性,并且在打印完成之后VC才会被销毁
那么问题来了,也就是本文的主题,为什么加了__strong就可以获取的到了呢?
分析:既然获取的到,那就是没有销货,在iOS中控制一个视图是否被销货是由RetainCount来判断的,那么我们是否可以打印看下RetainCount来看下呢:
首先百度了一个在ARC中获取RetainCount的方法:

CFGetRetainCount((__bridge CFTypeRef)(weakSelf))

我们先看下在没有加__strong的时候:

    printf("点击时 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self)));
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            //此处不可以打印,因为weakSelf已经被销货了,否则会崩溃
            printf("block 内部retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
            NSLog(@"-----%@",weakSelf.str);
        });
    };
    self.block();
    [self.navigationController popViewControllerAnimated:YES];

点击时 retain count = 3
dealloc时 retain count = 1
2018-12-27 18:08:03.368435+0800 TestDemo[35790:805937] dealloc
2018-12-27 18:08:05.059696+0800 TestDemo[35790:805937] -----(null)

加了__strong:

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    printf("点击时 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self)));
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        //在block的执行过程中,使用强对象对弱对象进行引用
        printf("strongSelf赋值之前 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
        __strong TwoViewController *strongSelf = weakSelf;
        printf("strongSelf赋值之后 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            printf("block 内部retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
            NSLog(@"-----%@",strongSelf.str);
        });
    };
    self.block();
    [self.navigationController popViewControllerAnimated:YES];
}

点击时 retain count = 3
strongSelf赋值之前 retain count = 4
strongSelf赋值之后 retain count = 5
block 内部retain count = 2
2018-12-27 18:09:12.434576+0800 TestDemo[35823:808094] -----123
dealloc时 retain count = 1
2018-12-27 18:09:12.434785+0800 TestDemo[35823:808094] dealloc

从以上我们可以得出结论 :

在__strong修饰了weakSelf之后,VC的Retaincount增加了1,block代码块是存放在栈区的,执行完成之后,会被系统自动回收,这时候strongSelf会被释放掉,此时self的引用计数才恢复正常了.