如何在没有死锁的情况下同步调度主队列？ [英] How to dispatch on main queue synchronously without a deadlock?

问题描述

我需要同步在主队列上调度一个块。我不知道我目前是在主线程上运行还是不运行。天真的解决方案如下所示：

I need to dispatch a block on the main queue, synchronously. I don’t know if I’m currently running on the main thread or no. The naive solution looks like this:

dispatch_sync(dispatch_get_main_queue(), block);

但如果我目前在主队列上运行的块内，此调用会造成死锁。 （同步调度等待块完成，但块甚至没有开始运行，因为我们正在等待当前块完成。）

But if I’m currently inside of a block running on the main queue, this call creates a deadlock. (The synchronous dispatch waits for the block to finish, but the block does not even start running, since we are waiting for the current one to finish.)

显而易见的下一个步骤是检查当前队列：

The obvious next step is to check for the current queue:

if (dispatch_get_current_queue() == dispatch_get_main_queue()) {
    block();
} else {
    dispatch_sync(dispatch_get_main_queue(), block);
}

这很有效，但很难看。在我至少把它隐藏在一些自定义功能之前，是不是有更好的解决方案来解决这个问题？我强调我不能异步调度块 - 应用程序处于异步调度块将执行得太晚的情况。

This works, but it’s ugly. Before I at least hide it behind some custom function, isn’t there a better solution for this problem? I stress that I can’t afford to dispatch the block asynchronously – the app is in a situation where the asynchronously dispatched block would get executed "too late".

推荐答案

我需要在我的Mac和iOS应用程序中经常使用这样的东西，所以我使用以下帮助函数（最初在这个答案）：

I need to use something like this fairly regularly within my Mac and iOS applications, so I use the following helper function (originally described in this answer):

void runOnMainQueueWithoutDeadlocking(void (^block)(void))
{
    if ([NSThread isMainThread])
    {
        block();
    }
    else
    {
        dispatch_sync(dispatch_get_main_queue(), block);
    }
}

您可以通过

runOnMainQueueWithoutDeadlocking(^{
    //Do stuff
});

这几乎是你在上面描述的过程，我和其他几位开发人员谈过为自己独立制作这样的东西。

This is pretty much the process you describe above, and I've talked to several other developers who have independently crafted something like this for themselves.

我使用 [NSThread isMainThread] 而不是检查 dispatch_get_current_queue（），因为警告部分对于该功能一旦警告不要使用它进行身份测试和 iOS 6中不推荐使用该呼叫。

I used [NSThread isMainThread] instead of checking dispatch_get_current_queue(), because the caveats section for that function once warned against using this for identity testing and the call was deprecated in iOS 6.

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