从 Firebase 完成加载后如何使 UIRefreshControl 结束刷新

Question

以下方法是我的刷新数据方法。它将从 firebase 读取以获取最新数据。因为 firebase 是不同步的，所以我在 endRefreshing 之前等待 5 秒等待 Firebase 完成读取过程。

func refreshData() {
    //Remove old data
    self.items.removeAll()
    //Renew all data
    var ref: DatabaseReference!
    ref = Database.database().reference(withPath: "tasks")
    //Loading local drafts
    var drafts : [Task]!
    if let local_drafts = NSKeyedUnarchiver.unarchiveObject(withFile: Task.ArchiveURL.path) as? [Task] {
        drafts = local_drafts
    }
    else{
        drafts = []
    }
    //Reloading the database
    ref.observe(.value, with: { snapshot in
        var newItems: [Task] = []
        self.num_of_tasks = Int(snapshot.childrenCount)
        for item in snapshot.children {
            //let local = item as! DataSnapshot
            //let snapshotValue = local.value as! [String: AnyObject]
            //print(snapshotValue["main_content"] as! String!)
            let taskItem = Task(snapshot: item as! DataSnapshot)
            newItems.append(taskItem!)
        }
        let merged = drafts + newItems
        self.items = merged
        self.tableView.reloadData()
        //Wait for 5 seconds
        let deadlineTime = DispatchTime.now() + .seconds(3)
        DispatchQueue.main.asyncAfter(deadline: deadlineTime, execute: {
            self.refreshControl!.endRefreshing()
        })
    })
}

但这不是一个好的解决方案，因为有时所有数据都加载了但刷新仍在继续，这很奇怪。有没有办法解决这种情况？让 refreshControl 结束刷新，直到我从 firebase 加载所有数据？

Answer 1

下面是一种方法。它会一直等到异步数据下载完成，当completionHandler返回true或false时，refreshControl将停止刷新。我这里没有编译器，所以我希望它能正常工作。

func viewDidLoad(){
refreshData{ _ in
    self.refreshControl!.endRefreshing()
}
}

func refreshData(completionHandler:@escaping (Bool)->() ) {
    //Remove old data
    self.items.removeAll()
    //Renew all data
    var ref: DatabaseReference!
    ref = Database.database().reference(withPath: "tasks")
    //Loading local drafts
    var drafts : [Task]!
    if let local_drafts = NSKeyedUnarchiver.unarchiveObject(withFile: Task.ArchiveURL.path) as? [Task] {
        drafts = local_drafts
    }
    else{
        drafts = []
    }
    //Reloading the database
    ref.observe(.value, with: { snapshot in
        var newItems: [Task] = []
        self.num_of_tasks = Int(snapshot.childrenCount)
        for item in snapshot.children {
            //let local = item as! DataSnapshot
            //let snapshotValue = local.value as! [String: AnyObject]
            //print(snapshotValue["main_content"] as! String!)
            let taskItem = Task(snapshot: item as! DataSnapshot)
            newItems.append(taskItem!)
        }
        let merged = drafts + newItems
        self.items = merged
        self.tableView.reloadData()
        completionHandler(true)
    })
}