dispatch_async并在原始队列上调用完成处理程序 [英] dispatch_async and calling a completion handler on the original queue

问题描述

我已经看到一些相关的问题，但似乎没有一个答案.我想写一个在后台做一些工作的方法.我需要此方法在用于原始方法调用的同一线程/队列上调用完成回调.

I've seen some related questions but none seem to answer this case. I want to write a method that will do some work in the background. I need this method to call a completion callback on the same thread / queue used for the original method call.

- (void)someMethod:(void (^)(BOOL result))completionHandler {
    dispatch_queue_t current_queue = // ???

    // some setup code here
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        BOOL ok = // some result

        // do some long running processing here

        dispatch_async(current_queue, ^{
            completionHandler(ok);
        });
    });

这里需要什么魔咒，以便在与sameMethod调用相同的队列或线程上调用完成处理程序?我不想承担主线程.当然，不使用dispatch_get_current_queue.

What magic incantation is needed here so the completion handler is called on the same queue or thread as the call to sameMethod? I don't want to assume the main thread. And of course dispatch_get_current_queue is not to be used.

推荐答案

如果查看Apple文档，似乎有两种模式.

If you look through the Apple docs, there appear to be two patterns.

如果假定完成处理程序将在主线程上运行，则无需提供任何队列.一个示例是UIView的animations方法:

If it is assumed that the completion handler is to be run on the main thread, then no queue needs to be provided. An example would be UIView's animations methods:

+ (void)animateWithDuration:(NSTimeInterval)duration animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion

否则，API通常会要求调用者提供一个队列:

Otherwise, the API usually asks the caller to provide a queue:

[foo doSomethingWithCompletion:completion targetQueue:yourQueue];

我的建议是遵循这种模式.如果尚不清楚应调用完成处理程序的队列，则调用者应明确将其作为参数提供.

My suggestion is to follow this pattern. If it is unclear which queue the completion handler should be called, the caller should supply it explicitly as a parameter.

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