iOS Swift中的多线程

Question

I am trying to run two threads in parallel. But unfortunately , it sometime works sometime not. Here is my code

 let firstQueue = DispatchQueue(label: "queue1", qos: DispatchQoS.userInitiated)
 let secondQueue = DispatchQueue(label: "queue2", qos: DispatchQoS.userInitiated)
//let firstQueue = DispatchQueue(label: "queue1", qos: DispatchQoS.default , attributes: .concurrent)

    firstQueue.async {
        for i in 0..<10 {
            print("🔷", i)
        }
    }


    secondQueue.async {
        for i in 20..<30 {
            print("⚪️", i)
        }
    }

I have tried different everything but not achieve perfect parallelism. First Time Output: 0 ⚪️ 20 🔷 1 ⚪️ 21 🔷 2 ⚪️ 22 🔷 3 ⚪️ 23 🔷 4 ⚪️ 24 🔷 5 ⚪️ 25 🔷 6 ⚪️ 26 🔷 7 ⚪️ 27 🔷 8 ⚪️ 28 🔷 9 ⚪️ 29

SecondTime Output :

🔷 0 🔷 1 🔷 2 🔷 3 🔷 4 🔷 5 🔷 6 🔷 7 🔷 8 🔷 9 ⚪️ 20 ⚪️ 21 ⚪️ 22 ⚪️ 23 ⚪️ 24 ⚪️ 25 ⚪️ 26 ⚪️ 27 ⚪️ 28 ⚪️ 29

Answer 1

Your code works well, but the thing you should know is that, you haven't any choice to decide which part of your code execute first, and since your code is just printing a character and its a light job to do for cpu you see some times your characters printing ordered and some times unordered. Try to give some more heavy job to cpu and then see the results!

Answer 2

You can use DispatchGroup , also you should read more about GCD, the code below work as expected and it will fix your issue

let dispatchGroup = DispatchGroup()
let firstQueue = DispatchQueue(label: "queue1", qos: DispatchQoS.userInitiated)
let secondQueue = DispatchQueue(label: "queue2", qos: DispatchQoS.userInitiated)

firstQueue.async(group: dispatchGroup) {
    for i in 0..<10 {
        print("🔷", i)
    }
    dispatchGroup.leave()
}

secondQueue.async(group: dispatchGroup) {
    dispatchGroup.wait()
    for i in 20..<30 {
        print("⚪️", i)
    }
}

the output always will be like this 🔷 0 🔷 1 🔷 2 🔷 3 🔷 4 🔷 5 🔷 6 🔷 7 🔷 8 🔷 9 ⚪️ 20 ⚪️ 21 ⚪️ 22 ⚪️ 23 ⚪️ 24 ⚪️ 25 ⚪️ 26 ⚪️ 27 ⚪️ 28 ⚪️ 29

Answer 3

The point of multithreading is that operations will be done parallel and independent from one another. That does not guaranty any order at all, there is no synchronization, no nothing. If you need to have things synchronized then you will need to use a single thread or use a third thread that dispatches work:

func foo() {
    let firstQueue = DispatchQueue(label: "queue1")
    let secondQueue = DispatchQueue(label: "queue2")

    let mainQueue = DispatchQueue(label: "queueMain")

    var firstQueueOperationsCount: Int = 10
    var secondQueueOperationsCount: Int = 10

    func performOperations() {
        guard max(firstQueueOperationsCount, secondQueueOperationsCount) > 0 else {
            return
        }
        mainQueue.async {
            if firstQueueOperationsCount > secondQueueOperationsCount {
               firstQueueOperationsCount -= 1
                firstQueue.async {
                    print("🔷")
                    performOperations()
                }
            } else {
                secondQueueOperationsCount -= 1
                secondQueue.async {
                    print("⚪️")
                    performOperations()
                }
            }
        }
    }

    performOperations()
}

There are many ways on how to serialize tasks dispatched on multiple threads and this being just one of them. You might try searching/reading toward that direction but just be clear: What you seem to expect from multithreading is NOT WHAT MULTITHREADING IS. It does not work this way and it does not supposed to work this way. How they do work is how we want it to work.

If you have a more specific situation I am sure community here may help you identify a good approach to solve it. But from what you have written the result is not multithreading but:

for i in 0..<10 {
    print("🔷")
    print("⚪️")
}

It might be dispatched on another thread but that really makes no difference.