How to get a value once of a firebase database (Android)

问题

I just started using firebase and I'm amazed of how simple are complex things, and how complex simple things can be.

I need to check if a user exists in my Firebase database, and if exists, I need to get its key and value.

I have the key and I try to find it in the DB by going to my user subtree and loking for a child with the same key (ID)

DatabaseReference root_firebase = FirebaseDatabase.getInstance().getReference();
DatabaseReference users_firebase = root_firebase.child("Users");
DatabaseReference friend_found = users_firebase.child(key_searched_friend);

once I have it I try to call some method like

friend_found.getValue(); 

or

friend_found.child("user_name").getValue();

But it does not exist,

this seems weird since I can do both

friend_found.getKey();
friend_found.child("user_name").getKey();

I can't do this with the overriden method onDataChanged() since the data does not change when I query for this value.

This is my firebase database structure:

{
  "RoomNames" : {
    "Room-KM5cof0jcoMN8a4g6vC" : {
      "room_name" : "TESTrOOM"
    },
    "Room-KM5dg_WPRdEOT4_oJ1r" : {
      "room_name" : "testRoom2"
    }
  },
  "Users" : {
    "User-KM5ZaGq0xvjQis05CPF" : {
      "user_name" : "Enrique"
    }
  }
}

回答1:

You say: I can't do this with the overriden method onDataChanged() since the data does not change when I query for this value

The guide for how to Retrieve Data explains that:

Firebase data is retrieved by attaching an asynchronous listener to a FirebaseDatabase reference. The listener is triggered once for the initial state of the data and again anytime the data changes.

So when you attach a listener to a location, onDataChanged() fires and gives you the current value.

In the section titled Read Data Once, the guide states:

In some cases you may want a callback to be called once and then immediately removed, such as when initializing a UI element that you don't expect to change. You can use the addListenerForSingleValueEvent() method to simplify this scenario: it triggers once and then does not trigger again.

回答2:

Use as below :

friend_found.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

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

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Log.e(TAG,"Error while reading data");
            }
        });

回答3:

For a database reference object, the same way one can add an event listener, it can also be removed, using removeEventListener.

Instead of creating an anonymous object like this

    friend_found.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

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

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                    Log.e(TAG,"Error while reading data");
            }
    });

you can create a named object of ValueEventListener and remove it from the database reference object using removeEventListener, at the end of the onDataChange method

    ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                    String value = dataSnapshot.getValue(String.class);

                    friend_found.removeEventListener(valueEventListener);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                    Log.e(TAG,"Error while reading data");
            }
    });

    friend_found.addValueEventListener(valueEventListener);

The code inside onDataChange method gets executed only once as the ValueEventListener object is removed as soon as the last line of the method gets executed.

来源：https://stackoverflow.com/questions/38255513/how-to-get-a-value-once-of-a-firebase-database-android