找出firebase上的子事件侦听器是否完全加载所有数据 [英] find out if child event listener on firebase completely load all data

问题描述

我正在使用firebase存储和检索我的Android应用程序的数据，在我的活动中，我使用childEventListener从firebase检索所有数据（例如：列出用户数据）。

我的问题是：

只要数据没有完全检索，我想显示一个进度条从firebase使用childEventListener。如何检查所有数据是否完全检索？因为我想在数据完全检索后关闭进度条。

任何答案都不胜感激，谢谢！

解决方案

有一种常见的方法来检测Firebase是否完成同步指定位置的初始化数据。此方法利用 Firebase事件保证之一：

值事件始终始终触发，并保证包含在拍摄快照之前发生的任何其他事件的更新

所以如果你同时拥有一个 ValueEventListener 和一个 ChildEventListener 在给定的位置， ValueEventListener.onDataChange（）保证在 后被调用 code> onChildAdded（）发生呼叫。您可以使用它来知道初始数据加载完成时间：

  ref.addListenerForSingleValueEvent（new ValueEventListener（）{
 public void onDataChange（DataSnapshot dataSnapshot）{
 System.out.println（我们完成加载初始+ dataSnapshot.getChildrenCount（）+items）; 
} 
 public void onCancelled（FirebaseError firebaseError）{} 
}）; 
 ref.addChildEventListener（new ChildEventListener（）{
 public void onChildAdded（DataSnapshot dataSnapshot，String previousKey）{
 System.out.println（Add+ dataSnapshot.getKey（）+ $ previous_ b $ b public void onChildMoved（DataSnapshot dataSnapshot，String s）{
} 
 public void onCancelled（FirebaseError firebaseError）{} 
}）; 
  

在我的测试运行中，结果是：

 将-K2WLjgH0es40OGWp6Ln添加到UI后null 
将-K2YyDkM4lUotI12OnOs添加到UI之后-K2WLjgH0es40OGWp6Ln 
将-K2YyG4ScQMuRDoFogA9添加到-K2YyDkM4lUotI12OnOs 
 ... 
将-K4BPqs_cdj5SwARoluP添加到-K4A0zkyITWOrxI9-2On 
之后添加-K4BaozoJDUsDP_X2sUu到UI -K4BPqs_cdj5SwARoluP 
将-K4uCQDYR0k05Xqyj6jI添加到UI之后-K4BaozoJDUsDP_X2sUu 
我们完成了加载初始的121个项目
  

所以你可以使用 onDataChanged（）事件隐藏进度条。

但要注意一点：Firebase不仅仅是加载数据。它不断地将数据从服务器同步到所有连接的客户端。因此，没有真正的任何时候，数据是完全检索。

I'm using firebase for storing and retrieving data for my android application, in my activity i'm retrieve all data (example: list user data) from firebase using childEventListener.

My problem is:

I want to show a progress bar as long as the data is not completely retrieved from firebase using childEventListener. How to check if all data is completely retrieved? because i want to close the progress bar after the data is completely retrieved.

Any answer would be appreciated, thanks!

解决方案

There is a common way to detect when Firebase is done synchronizing the initial data on a given location. This approach makes use of one of the Firebase event guarantees:

Value events are always triggered last and are guaranteed to contain updates from any other events which occurred before that snapshot was taken.

So if you have both a ValueEventListener and a ChildEventListener on a given location, the ValueEventListener.onDataChange() is guaranteed to be called after all the onChildAdded() calls have happened. You can use this to know when the initial data loading is done:

ref.addListenerForSingleValueEvent(new ValueEventListener() {
    public void onDataChange(DataSnapshot dataSnapshot) {
        System.out.println("We're done loading the initial "+dataSnapshot.getChildrenCount()+" items");
    }
    public void onCancelled(FirebaseError firebaseError) { }
});
ref.addChildEventListener(new ChildEventListener() {
    public void onChildAdded(DataSnapshot dataSnapshot, String previousKey) {
        System.out.println("Add "+dataSnapshot.getKey()+" to UI after "+previousKey);
    }
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
    }
    public void onChildRemoved(DataSnapshot dataSnapshot) {
    }
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
    }
    public void onCancelled(FirebaseError firebaseError) { }
});

In my test run this results in:

Add -K2WLjgH0es40OGWp6Ln to UI after null
Add -K2YyDkM4lUotI12OnOs to UI after -K2WLjgH0es40OGWp6Ln
Add -K2YyG4ScQMuRDoFogA9 to UI after -K2YyDkM4lUotI12OnOs
...
Add -K4BPqs_cdj5SwARoluP to UI after -K4A0zkyITWOrxI9-2On
Add -K4BaozoJDUsDP_X2sUu to UI after -K4BPqs_cdj5SwARoluP
Add -K4uCQDYR0k05Xqyj6jI to UI after -K4BaozoJDUsDP_X2sUu
We're done loading the initial 121 items

So you could use the onDataChanged() event to hide the progress bar.

But one thing to keep in mind: Firebase doesn't just load data. It continuously synchronizes data from the server to all connected clients. As such, there is not really any moment where the data is completely retrieved.

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