ios – 何时使用Semaphore而不是Dispatch Group？

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        performUsingGroup()
    }

    func performUsingGroup() {
        let dq1 = dispatchQueue.global(qos: .userInitiated)
        let dq2 = dispatchQueue.global(qos: .userInitiated)

        let group = dispatchGroup()

        group.enter()
        dq1.async {
            for i in 1...3 {
                print("\(#function) dispatchQueue 1: \(i)")
            }
            group.leave()
        }

        group.wait()

        dq2.async {
            for i in 1...3 {
                print("\(#function) dispatchQueue 2: \(i)")
            }
        }

        group.notify(queue: dispatchQueue.main) {
            print("done by group")
        }
    }
}

结果 – 预期 – 是：

performUsingGroup() dispatchQueue 1: 1
performUsingGroup() dispatchQueue 1: 2
performUsingGroup() dispatchQueue 1: 3
performUsingGroup() dispatchQueue 2: 1
performUsingGroup() dispatchQueue 2: 2
performUsingGroup() dispatchQueue 2: 3
done by group

为了使用信号量,我实现了：

func performUsingSemaphore() {
    let dq1 = dispatchQueue.global(qos: .userInitiated)
    let dq2 = dispatchQueue.global(qos: .userInitiated)

    let semaphore = dispatchSemaphore(value: 1)

    dq1.async {
        semaphore.wait()
        for i in 1...3 {
            print("\(#function) dispatchQueue 1: \(i)")
        }
        semaphore.signal()
    }

    dq2.async {
        semaphore.wait()
        for i in 1...3 {
            print("\(#function) dispatchQueue 2: \(i)")
        }
        semaphore.signal()
    }
}

并在viewDidLoad方法中调用它.结果是：

performUsingSemaphore() dispatchQueue 1: 1
performUsingSemaphore() dispatchQueue 1: 2
performUsingSemaphore() dispatchQueue 1: 3
performUsingSemaphore() dispatchQueue 2: 1
performUsingSemaphore() dispatchQueue 2: 2
performUsingSemaphore() dispatchQueue 2: 3

从概念上讲,dispachGroup和Semaphore都有同样的目的(除非我误解了一些东西).

老实说,我不熟悉：什么时候使用信号量,尤其是在与dispachGroup合作时 – 可能 – 处理问题.

我错过了什么部分？