如何从dataSnapshot获得价值？ [英] How get value from dataSnapshot?

问题描述

当我调试这段代码的时候，在databaseUser中dataSnapshot有来自User的数据库的值，但是在对象 user1 中没有添加，全为null。我不明白，因为在对象事件接收值通常。

  databaseEvent.child（getKeyEvent（））。addListenerForSingleValueEvent （
 new ValueEventListener（）{
 @Override 
 public void onDataChange（DataSnapshot dataSnapshot）{
 
 final Event event = dataSnapshot.getValue（Event.class）; 
 user = FirebaseAuth.getInstance（）。getCurrentUser（）; $ b $ if（user！= null）{
 
 databaseUser.orderByChild（userEmail）。equalTo（user.getEmail ））。addListenerForSingleValueEvent（new ValueEventListener（）{
 @Override 
 public void onDataChange（DataSnapshot dataSnapshot）{$ b $ if（dataSnapshot.exists（））{
 //用户已经存在
 User user1 = dataSnapshot.getValue（User.class）; 
} 
 } 
}）; 
  

我的数据库用户只能用于现在的测试：

  public class User {
 
 String userId; 
字符串userEmail; 
布尔userHasInterest; 
字符串eventId; 
 
 public String getUserId（）{
 return userId; 
} 
 public String getUserEmail（）{
 return userEmail; 
 
 $ b $ public Boolean getUserHasInterest（）{
 return userHasInterest; 
} 
 
 public String getEventId（）{
 return eventId; 
 
 $ b $ public User（String userId，String eventId，String userEmail，Boolean userHasInterest）{
 this.userId = userId; 
 this.userEmail = userEmail; 
 this.userHasInterest = userHasInterest; 
 this.eventId = eventId; 
} 
  

和方法一样，只是挑选另一个活动事件的关键：

  public String getKeyEvent（）{
 Intent intent = getIntent（）; 
 Bundle bundle = intent.getExtras（）; 
 String key = bundle.getString（Evento）; 
返回键; 
 
  

解决方案

Firebase数据库，可能会有多个结果。通过附加 ValueListener ，您可以在一个快照中获得这些结果，因此快照将包含这些结果的列表。即使只有一个结果，快照也会包含一个结果列表。

您的代码没有考虑到这个结果列表。通过循环 DataSnapshot.getChildren（）：

  databaseUser.orderByChild（userEmail）。equalTo（user.getEmail（））。addListenerForSingleValueEvent（new ValueEventListener（）{
 @Override 
 public void onDataChange（DataSnapshot dataSnapshot）{
 for DataSnapshot userSnapshot：dataSnapshot.getChildren（））{
 User user1 = userSnapshot.getValue（User.class）; 
  

When I debug this code, in databaseUser dataSnapshot has the values from User in database, but in the object user1, did not add, it's all null. And I don't understand, because in the object event receives values normally.

databaseEvent.child(getKeyEvent()).addListenerForSingleValueEvent( 
      new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

               final Event event = dataSnapshot.getValue(Event.class);
               user = FirebaseAuth.getInstance().getCurrentUser();
               if(user != null) {

                   databaseUser.orderByChild("userEmail").equalTo(user.getEmail()).addListenerForSingleValueEvent(new ValueEventListener() {
                     @Override
                     public void onDataChange(DataSnapshot dataSnapshot) {
                         if (dataSnapshot.exists()) {
                             //User already exists
                             User user1 = dataSnapshot.getValue(User.class);
              }
       }
});

My Database User, only for test for now:

public class User {

String userId;
String userEmail;
Boolean userHasInterest;
String eventId;

public String getUserId() {
    return userId;
}
public String getUserEmail() {
    return userEmail;
}

public Boolean getUserHasInterest() {
    return userHasInterest;
}

public String getEventId() {
    return eventId;
}

public User(String userId, String eventId, String userEmail, Boolean userHasInterest) {
    this.userId = userId;
    this.userEmail = userEmail;
    this.userHasInterest = userHasInterest;
    this.eventId = eventId;
}

And the method, just to pick the key of the event of another activity :

public String getKeyEvent(){
    Intent intent = getIntent();
    Bundle bundle = intent.getExtras();
    String key = bundle.getString("Evento");
    return key;
} 

解决方案

When you execute a query against the Firebase Database, there will potentially be multiple results. By attaching a ValueListener you get those results in one snapshot, so the snapshot contains a list of those results. Even if there is only a single result, the snapshot will contain a list of one result.

Your code fails to take this "list of results" into account. It's easy to fix by looping over the DataSnapshot.getChildren():

databaseUser.orderByChild("userEmail").equalTo(user.getEmail()).addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot dataSnapshot) {
     for (DataSnapshot userSnapshot: dataSnapshot.getChildren()) {
         User user1 = userSnapshot.getValue(User.class);

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