在 MBProgressHud 文档中，它声明在调度内部使用它，如下所示:

[MBProgressHUD showHUDAddedTo:self.view animated:YES];
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    // Do something...
    dispatch_async(dispatch_get_main_queue(), ^{
        [MBProgressHUD hideHUDForView:self.view animated:YES];
    });
});

考虑到您不希望它使主线程陷入困境，这是完全可以理解的。但是我可以在使用 block 时改为这样做吗:

[MBProgressHUD showHUDAddedTo:self.view animated:YES];
    [object deleteInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
                if (error)
                {

                }
                else
                {
                   [MBProgressHUD hideHUDForView:self.view animated:YES];
                }
            }];

或者我仍然需要使用 dispatch 吗？

最佳答案

把你的代码改成这样:

 [MBProgressHUD showHUDAddedTo:self.view animated:YES];
 [object deleteInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
         NSLog(@"Am I running on the main thread: %@", [NSThread isMainThread] ? @"YES": @"NO");
         if (error)
         {

         }
         else
         {

         }
   }];

如果它记录“YES”那么你不需要在主线程上运行[MBProgressHUD hideHUDForView:self.view animated:YES];，否则你需要使用

dispatch_async(dispatch_get_main_queue(), ^{
    [MBProgressHUD hideHUDForView:self.view animated:YES];
});

更新:

block 在调用它们的任何线程上运行，请注意以下示例:

- (void)viewDidLoad {
    [super viewDidLoad];

    [self longRunningProcessWithCompletionBlock:^{
        NSLog(@"Is running on the main thread? %@", [NSThread isMainThread] ? @"YES" : @"NO");
    }];
}



- (void)longRunningProcessWithCompletionBlock:(void (^)(void))completionBlock {

    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    //this is running on the concurrent thread, so does completionBlock() as it has been called on a concurrent thread.
    dispatch_async(concurrentQueue, ^{
        [NSThread sleepForTimeInterval:3];
        completionBlock();
    });
}

所以基本上上面的结果将是 "Is running on the main thread? NO"

我再次对 viewDidLoad 进行了完全相同的调用:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    [self longRunningProcessWithCompletionBlock:^{
        NSLog(@"Is running on the main thread? %@", [NSThread isMainThread] ? @"YES" : @"NO");
    }];
}

但是这一次，我在主线程上调用 longRunningProcessWithCompletionBlock 的 completionBlock 如下:

- (void)longRunningProcessWithCompletionBlock:(void (^)(void))completionBlock {

    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(concurrentQueue, ^{

        [NSThread sleepForTimeInterval:3];

        //notice the difference, this time we are calling the completionBlock on the main thread, so that block will be running on the main thread
        dispatch_async(dispatch_get_main_queue(), ^{
            completionBlock();
        });

    });
}

这次因为我们已经在主线程上调用了完成 block ，所以结果会是 Is running on the main thread?是的

所以简而言之， block 不能保证它们在主线程上执行!但它们可以保证它们将在调用它们的任何线程上执行。

在您的情况下，Parse.com 开发人员正在主线程上调用 deleteInBackgroundWithBlock 的完成处理程序 block ，这就是您看到该日志为"is"的原因。所以您只需要调用 [MBProgressHUD hideHUDForView:self.view animated:YES]; 而无需 dispatch_async(dispatch_get_main_queue(), ^{ }); (因为它已经在主线程上了线程，这是一个额外的不必要步骤)

关于ios - 我会在 MBProgressHud block 中使用分派(dispatch)吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26557320/