What is the difference between ChildEventListener and ValueEventListener Firebase interfaces?

Question

The documentation says they both listen to changes at a Firebase database location.

Answer 1

these are the key differences between the two

if your database has following records:

  -LDU4T1buanVuJrpOYxW
      message:"hi stack"
      user: "john"
  -LDasdfa1buanVuJrpOYxW
      message:"hi john"
      user: "stack"

1) on new entry:

• a)ChildEventListener.onChildAdded will be called once but the datasnapshot will only have the one added
• b) ValueEventListener.onDatachange will be called once but the datasnapshot will have everybody

2) because of a) getting the new record in childeventlistener is

 public void onChildAdded(DataSnapshot dataSnapshot, String s) {
    Map map =(Map) dataSnapshot.getValue();
    String message = map.get("message").toString();

because of b) getting the new record in ValueEventListener is

    public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot data : dataSnapshot.getChildren()) {
               .... loop until end
                Map map = (Map)data.getValue();
                String message = map.get("message").toString();

so for example if you don't care what happen when somebody delete the first in ValueEventListener you always get notified but in ChildEventListener you only get notified if you override onChildRemoved.

so it depends on what you want to do. for example in a chat app. you will only care about new messages, you don't want to be reinserting all messages again in your chat room.