DispatchGroup顺序执行问题

Question

我正在尝试实现 dispath 组，以执行一系列操作，这些操作应该一个接一个地执行。 但问题是添加到组中的所有任务都是并行执行的。 请分享您的想法，以下是我正在尝试的示例。

    let group = DispatchGroup()
    
    group.enter()
    print("Enter Activity One")
    self.performActivityOne(param: []) {
        group.leave()
        print("leave 1")
    }
    
    group.enter()
    print("Enter Activity two")
    self. self.performActivityTwo(param: []) {
        group.leave()
        print("leave 2")
    }
    
    group.notify(queue: DispatchQueue.main) {
        // This block will be executed once all above threads completed and call dispatch_group_leave
        print("Prepare completed. I'm readyyyy")
    }

我得到的 output 是

Enter Activity One
Enter Activity two
leave 2

Answer 1

感谢所有支持和宝贵意见，我使用信号量解决了这个问题：

let semaphore = DispatchSemaphore(value: 1)
    DispatchQueue.global(qos: .userInitiated).async {
         let dispatchGroup = DispatchGroup()

         dispatchGroup.enter()
        semaphore.wait()
        self.performActivityOne(param: []) {
            dispatchGroup.leave()
            print("leave")
            semaphore.signal()
        }
        
        dispatchGroup.enter()
        semaphore.wait()
        self. self.performActivityTwo(param: []) {
            dispatchGroup.leave()
            print("leave 2")
            semaphore.signal()
        }
        
    dispatchGroup.wait()
      DispatchQueue.main.async {
         print("Completed all"))
      }
    }