等待多个街区完成 [英] Waiting for multiple blocks to finish

问题描述

我有这些方法从互联网上检索一些对象信息：

I have those methods to retrieve some object information from the internet:

- (void)downloadAppInfo:(void(^)())success
                failure:(void(^)(NSError *error))failure;
- (void)getAvailableHosts:(void(^)())success
                  failure:(void(^)(NSError *error))failure;
- (void)getAvailableServices:(void(^)())success
                     failure:(void(^)(NSError *error))failure;
- (void)getAvailableActions:(void(^)())success
                    failure:(void(^)(NSError *error))failure;

下载的东西存储在对象属性中，这就是成功函数不返回任何内容的原因。

The downloaded stuff gets stored in object properties, so that is why the success functions return nothing.

现在，我想要一个这样的方法：

Now, I want to have one method like this:

- (void)syncEverything:(void(^)())success
               failure:(void(^)(NSError *error))failure;

除了调用上面的所有方法之外什么也没做，只有在每个方法执行完之后才返回成功或失败阻止。

Which does nothing else than calling all the methods above, and returning only after every single method has performed its success or failure block.

我该怎么做？

提示：我知道级联方法在彼此调用成功块会起作用。但是，当后来的实现包括更多方法时，这既不干净也不有用。

Hint: I am aware that cascading the methods calls in each others success block would work. But this is neither 'clean' nor helpful when later implementations include further methods.

尝试：

我试过在 NSOperation 中运行每个调用，并将 NSOperations 添加到 NSOperationQueue 后跟完成操作，它取决于前面的每一个操作。

I tried running each of the calls in an NSOperation and adding those NSOperations to an NSOperationQueue followed by a "completion operation" which depends on every one of the preceding operations.

这不起作用。由于即使在各自的成功/失败块返回之前，操作也被认为已完成。

This won't work. Since the operations are considered completed even before their respective success/failure blocks return.

我也尝试使用 dispatch_group 。但我不清楚我是以正确的方式做到这一点。不幸的是，它无法正常工作。

I also tried using dispatch_group. But it is not clear to me wether I am doing it the right way. Unfortunately, it is not working.

推荐答案

你几乎就在那里，问题很可能是那些方法是异步的，所以你需要一个额外的同步步骤。只需尝试以下修复：

You were almost there, the problem is most likely to be that those methods are asynchronous, so you need an extra synchronization step. Just try with the following fix:

for(Appliance *appliance in _mutAppliances) {
  dispatch_group_async(
     group,
     dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
       dispatch_semaphore_t sem = dispatch_semaphore_create( 0 );

       NSLog(@"Block START");

       [appliance downloadAppInfo:^{
          NSLog(@"Block SUCCESS");
            dispatch_semaphore_signal(sem);
       }
       failure:^(NSError *error){
         NSLog(@"Block FAILURE");
         dispatch_semaphore_signal(sem);
       }];

       dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

       NSLog(@"Block END");
 });

 dispatch_group_notify(
   group,
   dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
     NSLog(@"FINAL block");
     success();
 });
}

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