在继续之前，如何等待经过dispatch_async? [英] How to wait past dispatch_async before proceeding?

问题描述

我正在执行一系列的dispatch_async，我只想在完成所有操作后才更新UI.问题是dispatch_async中的方法在单独的线程中调用某些东西，因此它在数据完全加载之前返回，而在加载所有内容之前调用dispatch_group_notify.

I have a series of dispatch_async that I am performing and I would like to only update the UI when they are all done. Problem is the method within dispatch_async calls something in a separate thread so it returns before the data is fully loaded and dispatch_group_notify is called before everything is loaded.

因此，我引入了一个无限循环，使其等待直到设置了标志为止. 这是最好的方法吗?参见下面的代码.

So I introduce a infinite loop to make it wait until a flag is set. Is this the best way? See code below.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();

for (...) {
  dispatch_group_async(group, queue, ^{
    __block BOOL dataLoaded = NO;

    [thirdPartyCodeCallWithCompletion:^{
       dataLoaded = YES;
    }];

    // prevent infinite loop
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), 
    queue, ^{
          dataLoaded = YES;
    });

    // infinite loop to wait until data is loaded
    while (1) {
       if (dataLoaded) break;
    }
  }

  dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    //update UI
  });
}

推荐答案

您已经知道调度组.为什么不只使用dispatch_group_wait()(其中包括对超时的支持)呢?您可以使用dispatch_group_enter()和dispatch_group_leave()而不是dispatch_group_async()来使组不完成，直到完成调用的第三方调用的内部块为止.

You're already aware of dispatch groups. Why not just use dispatch_group_wait(), which includes support for a timeout? You can use dispatch_group_enter() and dispatch_group_leave() rather than dispatch_group_async() to make the group not done until the internal block for the third-party call with completion is finished.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();

for (...) {
  dispatch_group_enter(group);
  dispatch_async(queue, ^{
    [thirdPartyCodeCallWithCompletion:^{
       dispatch_group_leave(group);
    }];
  }
}

dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, NSECS_PER_SEC));
dispatch_async(dispatch_get_main_queue(), ^{
  //update UI
});

---

dispatch_group_wait()的使用确实使该代码同步，如果在主线程上运行，这是不好的.根据超时发生的确切情况，您可以像以前一样使用dispatch_group_notify()并使用dispatch_after()只是更新UI，而不是假装已完成的块.

---

The use of dispatch_group_wait() does make this code synchronous, which is bad if run on the main thread. Depending on what exactly is supposed to happen if it times out, you could use dispatch_group_notify() as you were and use dispatch_after() to just updates the UI rather than trying to pretend the block completed.

更新:我调整了代码以确保更新UI"出现在主队列中，以防万一此代码不在主线程上.

Update: I tweaked my code to make sure that "update UI" happens on the main queue, just in case this code isn't already on the main thread.

顺便说一句，我只在调用thirdPartyCodeCallWithCompletion:的块中使用了dispatch_async()，因为您的原始文件使用了dispatch_group_async()，而且我不确定该假设方法是否是异步的.不过，大多数带有完成块的API都是异步的.如果是这样的话，那么您可以直接调用它.

By the way, I only used dispatch_async() for the block which calls thirdPartyCodeCallWithCompletion: because your original used dispatch_group_async() and I wasn't sure that the hypothetical method was asynchronous. Most APIs which take a completion block are asynchronous, though. If that one is, then you can just invoke it directly.

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