dispatch_async(dispatch_get_main_queue(),^{})存在的一个坑

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    NSLog(@"before main queue : %@",[NSThread currentThread]);
    
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"main queue");
    });
    
    NSLog(@"after main queue : %@",[NSThread currentThread]);
    
    return YES;
}
    
2017-06-27 09:33:57.001 TestMain[1811:19444] before main queue : {number = 1, name = main}
2017-06-27 09:33:57.001 TestMain[1811:19444] after main queue : {number = 1, name = main}
2017-06-27 09:33:57.012 TestMain[1811:19444] main queue

可以看到在main thread中，执行顺序并不是和代码顺序一样，

dispatch_async(dispatch_get_main_queue(), ^{
    NSLog(@"main queue");
});

最后执行。

如果不注意的话，可能会导致一些问题。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    NSLog(@"before main queue : %@",[NSThread currentThread]);
    
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"main queue");
        
        self.label = [UILabel new];
    });
    
    NSLog(@"after main queue : %@",[NSThread currentThread]);
    NSLog(@"self.label: %@",self.label);
    
    return YES;
}

before main queue : {number = 1, name = main}
after main queue : {number = 1, name = main}
self.label: (null)
main queue

解决方法

1. 参考SDWebImage的宏定义，判断一下当前是不是主线程，如果是直接执行，如果不是，异步执行

    #ifndef dispatch_main_async_safe
    #define dispatch_main_async_safe(block)\
    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
    block();\
    } else {\
    dispatch_async(dispatch_get_main_queue(), block);\
    }
    #endif

2. 将需要顺序的执行的代码放到同一个作用域里面

原因

应该是和runloop有关的，但是具体的解释还没想明白。

dispatch_async(dispatch_get_main_queue(), ^{
    NSLog(@"main queue");
    
    self.label = [UILabel new];
});

dispatch_async将block提交给了queue然后立即返回，但是block什么时候执行，由queue说了算。然后，就不知道了……

更新

block 什么时候执行由Runloop说了算，不是由queue说了算，实际就是下一个runloop循环会执行，因为runloop在唤醒后会去处理相关的任务

调用信息

void
_dispatch_main_queue_callback_4CF(
        void *ignored DISPATCH_UNUSED)
{
    // the main queue cannot be suspended and no-one looks at this bit
    // so abuse it to avoid dirtying more memory

    if (_dispatch_main_q.dq_side_suspend_cnt) {
        return;
    }
    _dispatch_main_q.dq_side_suspend_cnt = true;
    _dispatch_main_queue_drain(&_dispatch_main_q);
    _dispatch_main_q.dq_side_suspend_cnt = false;
}