ios - 为什么这个完成处理程序会跳过 for 循环

Question

我有这组代码，我只想运行:self.performSegue 在所有 for 循环和所有 Firebase 的异步任务完成运行之后:

    getFeaturedPost(withCompletion: startNext)

    func getFeaturedPost(withCompletion completion: () -> Void ) {
    print("Getting featured posts...")
    ref.child("featured").child("amount").observeSingleEvent(of: .value, with: { (snapshot) in
        self.numberOfPosts = snapshot.value as! Int

        print("There's \(self.numberOfPosts) posts avaliable.")

        for pos in 0..<self.numberOfPosts{
            print("Getting reference names for post: \(pos + 1)")
            self.ref.child("featured").child("post\(pos + 1)").observeSingleEvent(of: .value, with: { (snapshot) in
                let postID = (snapshot.value as? NSDictionary)?["postID"] as? String ?? ""
                let userOfPost = (snapshot.value as? NSDictionary)?["userOfPost"] as? String ?? ""
                self.customValues.append(("/users/public/\(userOfPost)/posts/\(postID)"))
            })
        }
    })
    print("Done! The posts are: \(customValues)")
    completion()
}

func startNext()
{
    getPostData(withCompletion: {
        print("Finished getting data, going to main screen.")
        self.performSegue(withIdentifier: "showHome", sender: nil)
    })
}

func getPostData(withCompletion completion: () -> Void ) {
    print("Getting idividual post data, there are \(customValues.count) posts")
    for i in 0..<customValues.count {
        print("Currently on post: \(i)")
        let encodedURL = (customValues[i] + "/postURL")
        ref.child(encodedURL).observe(.value, with: { (snapshot) in
            if let newURL = snapshot.value as? String{
                print("Sending \(newURL) to DemoSource Class")
                DemoSource.shared.add(urlString: newURL)
            }
        })
    }
    completion()
}

但是 startNext() 函数(转到下一个 View )在 getFeaturedPost 开始之前执行它是 for 循环，它打印当前正在发布的帖子在。最后，当我使用 DemoSource.shared.add(urlString: newURL) 将数据发送到 demosource 类时，newURL 为 nil，我有一个控制台日志显示您各函数打印语句顺序:

Getting featured posts...
Done! The posts are: []
Getting idividual post data, there are 0 posts
Finished getting data, going to main screen. // This line should be printed last meaning this function is being executed too early
There's 2 posts avaliable.
Getting reference names for post: 1 // These two lines should be called before the line 'Finished getting data'
Getting reference names for post: 2

Answer 1

DispatchGroup 的使用非常简单。这是一种计数器。 enter 增加计数器，leave 减少它。如果计数器达到 0，则执行 notify 中的闭包。

• 在异步 block 调用enter之前的循环中。
• 在末尾调用 leave 的异步 block 内。

• 循环后调用notify。

  func getFeaturedPost(withCompletion completion: @escaping () -> Void ) {
      print("Getting featured posts...")
      ref.child("featured").child("amount").observeSingleEvent(of: .value, with: { (snapshot) in
          self.numberOfPosts = snapshot.value as! Int
  
          print("There's \(self.numberOfPosts) posts avaliable.")
          let group = DispatchGroup()
          for pos in 0..<self.numberOfPosts{
              group.enter()
              print("Getting reference names for post: \(pos + 1)")
              self.ref.child("featured").child("post\(pos + 1)").observeSingleEvent(of: .value, with: { (snapshot) in
                  if let post = snapshot.value as? [String:Any] {
                      let postID = post["postID"] as? String ?? ""
                      let userOfPost = post["userOfPost"] as? String ?? ""
                      self.customValues.append(("/users/public/\(userOfPost)/posts/\(postID)"))
                  }
                  group.leave()
              })
          }
          group.notify(queue: .main) {
              print("Done! The posts are: \(customValues)")
              completion()
          }
      })
  }
  

在其他方法中相应地实现一个组。

旁注:不要在 Swift 中使用 NS... 集合类型。