Synchronizing issue with delays in loops

问题

I'm trying to delay specific threads inside a loop and I am not getting the behavior I want. I would like 1B to run only once 1A has been completed, while 2A runs in parallel on a separate thread. My implementation runs 1A then 1B then 2A instead. Any idea how I can fix this?

Implementation

override func viewDidLoad() {
    super.viewDidLoad()

    // 1
    DispatchQueue.main.async {
        self.loopManager(printable: "1A") // 1A
        self.loopManager(printable: "1B") // 1B
    }

    // 2
    DispatchQueue.main.async {
        self.loopManager(printable: "2A") // 2A
    }
}

func loopManager(printable: String) {
    for i in 0...3 {
        doABC(printable: String(i) + ", " + printable)
        sleep(1)
    }
}

func doABC(printable: String) {
    print(printable)
}

Logs

0, 1A
1, 1A
2, 1A
3, 1A
0, 1B
1, 1B
2, 1B
3, 1B
0, 2A
1, 2A
2, 2A
3, 2A

回答1:

You are running your code in the same thread. You need to run 2A in another thread.

DispatchQueue.global().async {
    self.loopManager(printable: "2A") // 2A
}

Output sample:

0, 1A
0, 2A
1, 2A
1, 1A
2, 1A
2, 2A
3, 1A
3, 2A
0, 1B
1, 1B
2, 1B
3, 1B

来源：https://stackoverflow.com/questions/45527755/synchronizing-issue-with-delays-in-loops