Swift 何时使用 Semaphore 而不是 Dispatch Group

我假设我知道如何使用DispatchGroup，为了理解这个问题，我try 过:

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")
        }
    }
}

正如预期的那样，结果是:

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都有相同的用途(除非我误解了什么).

老实说，我不熟悉:什么时候使用信号量，尤其是在使用DispachGroup时——可能——处理这个问题.

我错过了什么？