聆听多个数据库引用 [英] Listen to multiple database references

问题描述

  {
 A：[data-a]，
B：[data-b]，
C：[data-c] 
} 
  A ， > B 或 C 我想要显示Android中 ListView  。
 
 
对于一个位置，我已经能够通过将下面的代码添加到我的项目中实现此目的。
  DatabaseReference databaseReference = FirebaseDatabase.getInstance（）。getReference（A）; 
 databaseReference.addChildEventListener（new ChildEventListener（）{
 @Override $ b $ public void onChildAdded（DataSnapshot dataSnapshot，String s）{
 //添加的子元素
} 
 
 @Override 
 public void onChildChanged（DataSnapshot dataSnapshot，String s）{
 // Child got changed 
} 
 
 @Override 
 public void onChildRemoved（DataSnapshot dataSnapshot）{
 // Child remove b $ b} 
 $ b $ @Override 
 public void onChildMoved（DataSnapshot dataSnapshot，String s）{
 // Child got move 
} 
 
 @Override $ b $ public void onCancelled（DatabaseError databaseError）{
 //发生了一些错误
} 
}）; 
  
到我的问题。我是否需要为每个位置使用多个侦听器，或者可以以某种方式以更简单的方式得到有关所有更改的通知？
将需要添加多个引用和子监听器。 Firebase没有联盟，SQL数据库中的其他操作符的概念。您要检索的数据集合与用来访问它们的引用/侦听器之间存在1：1的关系。您不能使用通配符或其他机制一次访问多个数据集合。
 
I created a project in which I need to listen to multiple database locations at once.
{
  "A": [data-a],
  "B": [data-b],
  "C": [data-c]
}
Whenever anything gets changed, added or deleted at location A, B or C I want to display the action which happened inside a ListView in Android.

For one location I have been able to achieve this by adding the following code to my project.
DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("A");
databaseReference.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
    //Child got added
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
    //Child got changed
    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
    //Child got removed
    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
    //Child got moved
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
    //Some error happened
    }
});
Now to my question. Do I need multiple listeners for each location or can I somehow get notified about all changes in a simpler way?
解决方案
Yes, you would need to add multiple references and child listeners. Firebase has no concept of unions, joins, or other operators present in SQL databases. There is a 1:1 relationship between collections of data you want to retrieve and the references / listeners you use to access them. You cannot use wildcards or other mechanisms to access more than one data collection at a time from a single call.

                        
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