查明Firebase上的childEventListener是否已完成加载所有数据 [英] Find out if childEventListener on Firebase has completed loading all data

问题描述

我正在使用Firebase实时数据库为我的Android应用程序存储和检索数据.在我的活动中，我使用childEventListener从Firebase检索所有数据(例如:用户数据列表).

I'm using Firebase Realtime Database for storing and retrieving data for my Android application. In my activity I retrieve all data (example: list of user data) from Firebase using a childEventListener.

只要数据没有从数据库中完全检索出来，我想显示一个进度条.如何检查是否已完全检索所有数据，以便在加载数据后关闭进度栏?

I want to show a progress bar as long as the data is not completely retrieved from the database. How do I check if all data is completely retrieved so that I can close the progress bar after the data is loaded?

推荐答案

有一种通用方法可以检测Firebase在给定位置上完成 initial 数据同步的时间.这种方法利用了 Firebase事件保证之一:

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:

值事件总是最后触发，并保证包含快照创建之前发生的任何其他事件的更新.

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

因此，如果在给定位置上同时具有ValueEventListener和ChildEventListener，则在所有onChildAdded()调用都发生之后，保证ValueEventListener.onDataChange()被称为.您可以使用它来了解何时完成初始数据加载:

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) { }
});

在我的测试中，运行结果为:

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

因此您可以使用onDataChanged()事件隐藏进度栏.

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

但要记住一件事:Firebase不仅加载数据.它将服务器中的数据持续同步到所有连接的客户端.因此，实际上根本没有任何时间完全检索数据.

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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