wait()会等待线程执行完载执行之后的逻辑，阻塞当前线程

    let myQuenue = DispatchQueue(label: "myQuneue")
    let myGroup = DispatchGroup()
    
    myGroup.enter()
    myQuenue.async {
        Thread.sleep(forTimeInterval: 5)
        print("哈哈哈哈哈哈哈哈哈哈")
        myGroup.leave()
    }
    
    myGroup.enter()
    myQuenue.async {
        Thread.sleep(forTimeInterval: 1)
        print("kkkkkkkk")
        myGroup.leave()
    }
    
    myGroup.wait()
    print("咔咔咔咔咔咔扩扩扩扩扩")
    结果：哈哈哈哈哈哈哈哈哈哈
         kkkkkkkk
         咔咔咔咔咔咔扩扩扩扩扩

notify()不会阻塞当前线程，先执行notify后面的同步方法

    let myQuenue = DispatchQueue(label: "myQuneue")
    let myGroup = DispatchGroup()
    
    myGroup.enter()
    myQuenue.async {
        Thread.sleep(forTimeInterval: 5)
        print("哈哈哈哈哈哈哈哈哈哈")
        myGroup.leave()
    }
    
    myGroup.enter()
    myQuenue.async {
        Thread.sleep(forTimeInterval: 1)
        print("kkkkkkkk")
        myGroup.leave()
    }
    
    myGroup.notify(queue: myQuenue) {
        print("k啦啦啦啦啦啦啦啦")
    }
    print("呜呜呜呜呜呜呜呜无无")
  结果：呜呜呜呜呜呜呜呜无无
       哈哈哈哈哈哈哈哈哈哈
       kkkkkkkk
       咔咔咔咔咔咔扩扩扩扩扩