Flutter 应用程序中 firebase 数据库上的 ValueEventListener

Question

I am learning android app development in flutter. I have started from flutter chat app example. In the Enable data syncing section , It is explained to use Firebase Database like below.

final DBreference = FirebaseDatabase.instance.reference().child('messages');

Also the push() and set() methods are working fine.

When i try to listen to events on child using ValueEventListener . The DBreference created above has no methods like addListenerForSingleValueEvent or addValueEventListener .

My main goal is to retrieve value of child as expalined in SO answers Retrieving child value -firebase- or Checking if a particular value exists in the firebase database

I m getting undefined class ValueEventListener
if i create a new ValueEventListener , i tried by importing

import 'com.google.firebase.database.ValueEventListener';

I m unable to import this path as well. Same error for addListenerForSingleValueEvent or addValueEventListener .

I am using android studio 3.1

Answer 1

Use

var subscription = FirebaseDatabase.instance
.reference()
.child('messages')
.onXxx
.listen((event) {
  // process event
});

where onXxx is one of

• value
• onChildAdded
• onChildRemoved
• onChildChanged

To end the subscription you can use

subscription.cancel();