线程中处理 UI 需要注意

-[UILabel setText:] must be used from main thread only

解决方法:

    [self performSelectorOnMainThread:@selector(<#selector#>:)     withObject:<#你要处理的数据#> waitUntilDone:YES];


- (void)<#selector#>:(<#你要处理的数据#> *)<#name#>
{
}

    [self performSelector:action withObject:obj afterDelay:0.3];

dispatch_async(dispatch_get_global_queue(0, 0), ^{ 
    // 处理耗时操作的代码块... 
    // 通知主线程刷新 
    dispatch_async(dispatch_get_main_queue(), ^{ 
    //回调或者说是通知主线程刷新， 
    }); 
});

        dispatch_semaphore_t sem = dispatch_semaphore_create(0);
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            XPFLog(@"任务一：");
            
        });
        
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            XPFLog(@"任务二：");
            
        });
        
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            XPFLog(@"任务三：");
            dispatch_semaphore_signal(sem);
            
        });