正确放置 reloadData

Question

所以我有一个 tableView，其中有一个 header 元素和一些单元格。我正在从服务器中提取单元格，然后在完成后重新加载 tableview。好吧，这至少是现在的目标，我在每次附加到数组后重新加载 tavleView。我最终只想在所有数据基本上都拉一次之后重新加载它。函数

self.fetchEventsFromServer()

处理提取数据的工作。我阅读了 dispatchGroups 并认为这是正确的方法，但我不知道该怎么做。

import UIKit
import Firebase

class FriendsEventsView: UITableViewController{
    var cellID = "cellID"
    var friends = [Friend]()
    var followingUsers = [String]()
    //label that will be displayed if there are no events
    var currentUserName: String?
    var currentUserPic: String?
    var currentEventKey: String?

    override func viewDidLoad() {
        super.viewDidLoad()
        self.title = "Friends Events"
        view.backgroundColor = .white
        // Auto resizing the height of the cell
        tableView.estimatedRowHeight = 44.0
        tableView.rowHeight = UITableViewAutomaticDimension
        self.navigationItem.rightBarButtonItem = UIBarButtonItem(image: #imageLiteral(resourceName: "close_black").withRenderingMode(.alwaysOriginal), style: .done, target: self, action: #selector(self.goBack))
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: cellID)
        self.tableView.tableFooterView = UIView(frame: CGRect.zero)
        fetchEventsFromServer { (error) in
            if error != nil {
                print(error)
                return
            } else {
                DispatchQueue.main.async {
                    self.tableView.reloadData()
                }
            }
        }

    }

    @objc func goBack(){
        dismiss(animated: true)
    }
    override func numberOfSections(in tableView: UITableView) -> Int {
        print(friends.count)
        return friends.count
    }
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
//        return friends[section].collapsed ? 0 : friends[section].items.count
        return 1
    }
    func tableView(_ tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat {
        return UITableViewAutomaticDimension
    }
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: cellID, for: indexPath)
        cell.textLabel?.text = "Something to fill Section: \(indexPath.section) Row: \(indexPath.row)"
        return cell
    }

    override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
        let header = tableView.dequeueReusableHeaderFooterView(withIdentifier: "header") as? CollapsibleTableViewHeader ?? CollapsibleTableViewHeader(reuseIdentifier: "header")
        print(section)
        header.arrowLabel.text = ">"
        header.setCollapsed(friends[section].collapsed!)
        print(friends[section].collapsed!)
        header.section = section
       // header.delegate = self
        header.friendDetails = friends[section]
        return header
    }
    override func tableView(_ tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
        return 50
    }

   func fetchEventsFromServer(_ completion: @escaping (_ error: Error?) -> Void ){
        //will grab the uid of the current user

        guard let myUserId = Auth.auth().currentUser?.uid else {
            return
        }
        let ref = Database.database().reference()
        //checking database for users that the current user is following
        ref.child("following").child(myUserId).observeSingleEvent(of: .value, with: { (followingSnapshot) in
           //handling potentail nil or error cases
            guard let following = followingSnapshot.children.allObjects as? [DataSnapshot]
                else {return}

            //validating if proper data was pulled
            for followingId in following {
                print(followingId.key)
                ref.child("users").child(followingId.key).observeSingleEvent(of: .value, with: { (userInfoSnapShot) in
                    guard let followingUserInfo = userInfoSnapShot.children.allObjects as? [DataSnapshot] else {
                        return
                    }
                    //validating if proper data was pulled for each follower
                    for currentUserInfo in followingUserInfo {
                        if currentUserInfo.key == "username"{
                            self.currentUserName = currentUserInfo.value as! String
                            print(self.currentUserName)
                            var friend = Friend(friendName: self.currentUserName!, imageUrl: self.currentUserPic!)
                            self.friends.append(friend)
                        }
                        if currentUserInfo.key == "profilePic"{
                            self.currentUserPic = currentUserInfo.value as! String
                            print(self.currentUserPic)
                        }



                    }

                }, withCancel: { (err) in
                    completion(err)
                    print("Couldn't grab info for the current list of users: \(err)")
                })
                completion(nil)
            }

        }) { (err) in
            completion(err)
            print("Couldn't grab people that you are currently following: \(err)")
        }
    completion(nil)

    }
}

任何关于我将如何快速完成这项工作的想法真的让我很烦恼

Snapshot of following strucutre 
 "following" : {
    "Iwr3EWqFBmS6kYRjuLW0Pw0CRJw2" : {
      "CW1AIDxM43Ot3C1GtsyhQ0Zzwof2" : true
    },
    "OYWgNjYHEtX6EatkolPO5YXt6Rw2" : {
      "nlSbmr1CXPbtuaUALNnftdHrbSt1" : true,
      "qYSDao0zhFbLrzd0IJBafi7qdis2" : true
    },
    "nRrGzLFt3TeN4OOrwTe0RjHQoF13" : {
      "CW1AIDxM43Ot3C1GtsyhQ0Zzwof2" : true,
      "XV62sIs7anaGaoo0Wr9kooC8FDP2" : true,
      "r51UQXn4Q2WcPWIhXIG3dhZTHkX2" : true
    },
    "nbmheFEPmBerm5avZwnriGJkaK12" : {
      "nlSbmr1CXPbtuaUALNnftdHrbSt1" : true
    },
    "nlSbmr1CXPbtuaUALNnftdHrbSt1" : {
      "OYWgNjYHEtX6EatkolPO5YXt6Rw2" : true,
      "qYSDao0zhFbLrzd0IJBafi7qdis2" : true
    },
    "qYSDao0zhFbLrzd0IJBafi7qdis2" : {
      "nlSbmr1CXPbtuaUALNnftdHrbSt1" : true
    },
    "wdciX8B2LeUy2NyDwU5cjLog5xx2" : {
      "72297UgQllfrEaAQnUCPKuQMv933" : true,
      "nbmheFEPmBerm5avZwnriGJkaK12" : true
    }
  }

Answer 1

您想向func fetchEventsFromServer() 函数添加一个完成处理程序，并在完成后调用reloadData()。

func fetchEventsFromServer(_ completion: @escaping (_ error: Error?) -> Void ) {
......... 
    withCancel: { (err) in
                completion(err)
                print("Couldn't grab info for the current list of users: \(err)")
             })
         } completion(nil) // Right here I would think
     }) { (err) in
}

然后当你打电话时

fetchEventsFromServer { (error) in 
    if error != nil {
        print(error)
        return
    } else {
        self.tableView.reloadData()
    }
}

此外，您不需要将self. 添加到您在代码中添加的很多内容中。只有当它嵌套在闭包之类的东西中时。

Answer 2

这是一个极其简化的示例作为指导（我的 20 循环是任意的，仅作为示例）：

let group = DispatchGroup()
for _ in 0...20
{
    group.enter()

    // Do your server call and in it's completion block call..
    group.leave()
}
let result = group.wait(timeout: .now() + 60)

基本思想是创建组，在 for 循环开始时“进入”组，每次从服务器收到响应时“离开”组，然后在循环外“等待” " 命令基本上会阻止该方法继续执行，直到您“离开”的次数与您“进入”组的次数一样多（或者您花费的时间太长）。

注意 - 由于这会阻塞它所在的线程，因此您绝对不想在 viewDidLoad 中执行此操作，因为它会锁定您的整个 UI，直到收到所有数据。在非主线程中启动它。

Answer 3

如果您事先不知道您的数据应该更新多少次（您将拥有多少条记录），您可以使用debouncer 而不是每次都调用reloadData。对于debouncer 实现，请查看How can I debounce a method call?

例如使用这个答案https://stackoverflow.com/a/40634366/7132300你可以做

let debouncedFunction = debounce(interval: 500, queue: DispatchQueue.main, action: { (_: String) in
    self.tableView.reloadData()
})

然后调用debouncedFunction("dummy") 而不是reloadData。随意调整timeout 参数以满足您的需要。此外，由于您不需要它，因此应该很容易摆脱那个虚拟字符串参数。