For循环中的DispatchGroup [英] DispatchGroup in For Loop

问题描述

因此，我花了一些时间试图让DispatchGroup在长时间的异步操作完成之前阻止for循环迭代.我发现的大多数示例都相当简单明了，但是我似乎无法像我期望的那样使简单的测试用例正常工作.

So, I'm having a bit of a time trying to get DispatchGroup to keep a for loop from iterating before a long asynchronous operation has completed. Most examples I've found are fairly straight forward and clear, but I can't seem to get my simple test case to work as I would expect.

let group = DispatchGroup()

    for i in 1...3 {
        group.enter()
        print("INDEX \(i)")
        asynchronousOperation(index: i, completion: {
            print("HELLO \(i)")
            self.group.leave()

        })
        print("OUTSIDE \(i)")
    }


func asynchronousOperation(index: Int, completion: @escaping () -> ()) {
    DispatchQueue.main.asyncAfter(deadline: DispatchTime.now()+5) {
        print("DONE \(index)")
        completion()

    }
}

这最终会打印

START
INDEX 1
OUTSIDE 1
INDEX 2
OUTSIDE 2
INDEX 3
OUTSIDE 3
DONE 1
HELLO 1
DONE 2
HELLO 2
DONE 3
HELLO 3

我希望它能打印出更多类似的东西

I would've expected it to print something more like

START
INDEX 1
OUTSIDE 1
HELLO 1
INDEX 2
OUTSIDE 2
HELLO 2
INDEX 3
OUTSIDE 3
HELLO 3

在OUTPIDE之后的下一个"INDEX"不会被打印，直到在异步操作()内部调用了group.leave()

Insofar as the next "INDEX" following an OUTSIDE wouldn't be printed until group.leave() had been called inside the asynchronousOperation()

可能是我误会了一些简单的东西-有什么想法吗?

Probably something simple I'm misunderstanding — any ideas?

推荐答案

执行以下代码以获得正确的输出:

execute the below code to get the proper output:

for i in 1...3 {
    let semaphore = DispatchSemaphore(value: 0) // create a semaphore with a value 0. signal() will make the value 1.
    print("INDEX \(i)")
    asynchronousOperation(index: i, completion: {
        print("HELLO \(i)")
        semaphore.signal() // once you make the signal(), then only next loop will get executed.
    })
    print("OUTSIDE \(i)")
    semaphore.wait() // asking the semaphore to wait, till it gets the signal.
}

   func asynchronousOperation(index: Int, completion: @escaping () -> ()) {
    DispatchQueue.global().asyncAfter(deadline: DispatchTime.now()+5) {
        print("DONE \(index)")
        completion()
    }
}

输出:

索引1 外面1 完成1 HELLO 1

INDEX 1 OUTSIDE 1 DONE 1 HELLO 1

索引2 外2 完成2 HELLO 2

INDEX 2 OUTSIDE 2 DONE 2 HELLO 2

索引3 外3 完成3 HELLO 3

INDEX 3 OUTSIDE 3 DONE 3 HELLO 3

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