Question

我正在尝试从Firebase读取数据，但是尽管我正在使用本教程，但android studio无法读取

我试图将android studio发送的链接复制到firebase：

DatabaseReference myRef = database.getReference（“ USERS”）。child（uID）.child（“ DeviceID”）; textview.setText（myRef.toString（））;

并在浏览器中显示结果，它向我显示了firebase中的结果，但是当我尝试使用它来获取数据时，它没有检索任何内容。这是我试图通过调用函数来读取它的方法：

textview.setText（ReadDeviceId）;

''''''''''''''''''''''''''''''

   private String ReadDeviceId(){

    FBUser = FirebaseAuth.getInstance().getCurrentUser();
    uID = FBUser.getUid();
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("USERS").child(uID).child("DeviceID");

    myRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            r_deviceID = dataSnapshot.getValue(String.class);
        }

        @Override
        public void onCancelled(DatabaseError error) {
            // Failed to read value
            r_deviceID = "no userID";
        }
    });
    return r_deviceID;
}

''''''''''''''''''''''''''''''''''''''''' 知道我的Firebase数据库安全规则是： '''''''''''''''''''''''''''''

{

“规则”：{

“。write”：“ auth！= null”，

“。read”：是

}

'''''''''''''''''

但什么也没显示

Answer 1

您可以尝试此代码

 private String ReadDeviceId(){
      FBUser = FirebaseAuth.getInstance().getCurrentUser();
      uID = FBUser.getUid();
      FirebaseDatabase database = FirebaseDatabase.getInstance();
      DatabaseReference myRef = 
           database.getReference("USERS").child(uID).child("DeviceID");

     myRef.addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
             for(Datasnapshot snapshot : dataSnapshot.getChildren)
             {
                //try to map it with your on model class and replace the String with your model class
                r_deviceID = snapshot.getValue(String.class)
             }
             //r_deviceID = dataSnapshot.getValue(String.class);
          }

          @Override
          public void onCancelled(DatabaseError error) {
        // Failed to read value
             r_deviceID = "no userID";
           }
     });
     return r_deviceID;
 }

Answer 2

In your ReadDeviceId() function, you are returning the value of r_deviceID before it is set (event listeners are not synchronous). Therefore, your textview.setText(ReadDeviceId()); will always be textview.setText(null);, which will show nothing.

For your use case, you should change addValueEventListener to addListenerForSingleValueEvent and set the textview's value from within the handler itself like this:

private String ReadDeviceId(){
    FBUser = FirebaseAuth.getInstance().getCurrentUser();
    uID = FBUser.getUid();
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("USERS").child(uID).child("DeviceID");

    myRef.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
                textview.setText(dataSnapshot.getValue(String.class));
        }

        @Override
        public void onCancelled(DatabaseError error) {
                // Failed to read value
                textview.setText("no userID");
        }
    });
}

android studio无法从Firebase实时数据库中读取检索到的数据

2 个答案: