Question

我要做的是顺序执行for循环，在开始下一次迭代之前我等待completionHandler（）。

我必须等待完成处理程序在迭代之前返回
保证返回完成处理程序
我正在尝试同步等待优先级较低的队列

代码：

// we're on the main queue
for index in 0..<count {
    var outcome: Any?
    let semaphore = DispatchSemaphore(value: 0)
    let queue = DispatchQueue(label: "\(index) iteration")
    // this will access a UI component and wait for user to
    // enter a value that's passed to the completion handler
    funcWithCompletionHandler() { [weak self] (result) in
        outcome = result
        semaphore.signal()
    }
    // wait here for the completion handler to signal us
    queue.sync {
        semaphore.wait()
        if let o = outcome {
            handleOutcome(outcome)
        }
    }
    // now, we iterate
}

我已经尝试了很多其他的解决方案，我看到这里似乎没什么用。

Answer 1

我更喜欢使用后台组，您可以在类上创建它的实例，如下所示： var group = DispatchGroup()

    DispatchQueue.global(qos: .background).async {
        self.group.wait()

       // this part will execute after the last one left
       // .. now, we iterate part
    }


    for index in 0..<count {
        var outcome: Any?
        let queue = DispatchQueue(label: "\(index) iteration")



        funcWithCompletionHandler() { [weak self] (result) in
            outcome = result
            self.group.enter() // group count = 1
        }

        queue.sync {
            if let o = outcome {
                handleOutcome(outcome)
                self.group.leave()
       // right here group count will be 0 and the line after wait will execute
            }
        }
    }

你如何等待主队列完成处理程序？

1 个答案: