iOS dispatch_after延迟执行导致延迟dealloc

清墨

发布于 2021-04-15 17:04:07

1.8K0

发布于 2021-04-15 17:04:07

我们新建一个NewViewController，在开始的ViewController写如下代码

- (void)viewDidLoad {
    [super viewDidLoad];
    
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
    btn.frame = CGRectMake(100, 100, 100, 50);
    btn.backgroundColor = [UIColor brownColor];
    [btn addTarget:self action:@selector(jump) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn];
    
}

-(void)jump{
    NewViewController *newVC = [[NewViewController alloc]init];
    [self presentViewController:newVC animated:YES completion:nil];
}

然后在NewViewController里：

-(void)dealloc{
    NSLog(@"--------------dealloc");
}

- (void)viewDidLoad {
    [super viewDidLoad];
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
    btn.frame = CGRectMake(100, 300, 100, 50);
    btn.backgroundColor = [UIColor brownColor];
    [btn addTarget:self action:@selector(jump) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn];
    
    __weak typeof(self) weakSelf = self;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self log];
        NSLog(@"--------------after");
    });
    
}

-(void)log{
    NSLog(@"--------------log");
}

-(void)jump{
    NSLog(@"--------------dismiss");
    [self dismissViewControllerAnimated:YES completion:nil];
}

运行之后，我们一跳进newVC里面返回，这时候：

image.png

结果说明我们dismiss的时候，newVC还没有被释放，dealloc方法在dispatch_after延迟方法执行之后才会走，原因就是dispatch_after强引用了self(即newVC)，等强引用过去，self才能得到释放走dealloc。

接下来，我们在dispatch_after里把 self 用 __weak修饰，block里把self改为weakself,我们还是一样的操作流程，看看结果:

- (void)viewDidLoad {
    [super viewDidLoad];
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
    btn.frame = CGRectMake(100, 300, 100, 50);
    btn.backgroundColor = [UIColor brownColor];
    [btn addTarget:self action:@selector(jump) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn];
    
    __weak typeof(self) weakSelf = self;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [weakSelf log];
        NSLog(@"--------------after");
    });
    
}

image.png

当我们用weak修饰self时，dispatch_after并没有强引用self,所以我们dissmiss时，dealloc立马就会走，然后10s后，dispatch_after的执行函数还是会执行，输出了after，但是没有输出log，这是因为用的weakSelf，dissmiss后，newVC已经被释放，这时候代码 [weakSelf log];等同于[nil log];，所以才不会输出log。