realtime database syntax in flutter

1. initilitation_declaration.dart

// declaration and inisialitation variable
  int _counter;
  DatabaseReference _counterRef;
  DatabaseReference _messagesRef;
  StreamSubscription<Event> _counterSubscription;
  StreamSubscription<Event> _messagesSubscription;
  bool _anchorToBottom = false;

  String _kTestKey = 'Hello';
  String _kTestValue = 'world!';
  DatabaseError _error;

2. inside_initState_explanation.dart

@override
void initState() {
  super.initState();
  /*** Create variable deafult in realtime database ***/
  _counterRef = FirebaseDatabase.instance.reference().child('counter'); 
  
  /*** Create the FirebaseDatabase Object ***/
  final FirebaseDatabase database = FirebaseDatabase();
  
  /*** Create new table in realtime database ***/
  _messagesRef = database.reference().child('messages');
  
  /*** Calling the variable in realtime. Get the value and print it ***/
  database.reference().child('counter').once().then((DataSnapshot snapshot) {
    print('Connected to second database and read ${snapshot.value}');
  });
  
  /*** Attempts to sets the database persistence to [enabled]. ***/
  database.setPersistenceEnabled(true);
  
  /*** Attempts to set the size of the persistence cache. ***/
  database.setPersistenceCacheSizeBytes(10000000);
  
  /*** To set automatic download and kept in sync ***/
  _counterRef.keepSynced(true);
  
  /*** fill the _counterSubscription to changes _counter's variable ***/
  _counterSubscription = _counterRef.onValue.listen((Event event) {
    setState(() {
      _error = null;
      _counter = event.snapshot.value ?? 0;
    });
  }, onError: (Object o) {
    final DatabaseError error = o;
    setState(() {
      _error = error;
    });
  });
  
  /*** fill the _messageSubscription to print value after add new data in table and limit 10 ***/
  _messagesSubscription =
      _messagesRef.limitToLast(10).onChildAdded.listen((Event event) {
    print('Child added: ${event.snapshot.value}');
  }, onError: (Object o) {
    final DatabaseError error = o;
    print('Error: ${error.code} ${error.message}');
  });
}

3. dispose_incrementAction.dart

  @override
  void dispose() {
    super.dispose();
    _messagesSubscription.cancel();
    _counterSubscription.cancel();
  }

  Future<void> _increment() async {
    /*** Updating values in _counterRef with realtime ***/
    /*** runTransaction Performs an optimistic-concurrency transactional update to the data at this Firebase Database location. ***/
    final TransactionResult transactionResult =
        await _counterRef.runTransaction((MutableData mutableData) async {
      mutableData.value = (mutableData.value ?? 0) + 1;
      return mutableData;
    });      
    /*** mutableData will be a value's variable from _conterRef(DatabaseReference). Then add 1 to current value ***/

    if (transactionResult.committed) {
      _messagesRef.push().set(<String, String>{
        _kTestKey: '$_kTestValue ${transactionResult.dataSnapshot.value}'
      });
    } else {
      print('Transaction not committed.');
      if (transactionResult.error != null) {
        print(transactionResult.error.message);
      }
    }
    /*** if the update true / committed, the system will send new data to add in table ***/
  }

4. build_widget.dart

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Flutter Database Example'),
      ),
      body: Column(
        children: <Widget>[
        
          /*** Text to show _counter's value ***/
          Flexible(
            child: Center(
              child: _error == null
                  ? Text(
                      'Button tapped $_counter time${_counter == 1 ? '' : 's'}.\n\n'
                      'This includes all devices, ever.',
                    )
                  : Text(
                      'Error retrieving button tap count:\n${_error.message}',
                    ),
            ),
          ),
          
          /*** CheckBox for changes position List to under layout ***/
          ListTile(
            leading: Checkbox(
              onChanged: (bool value) {
                setState(() {
                  _anchorToBottom = value;
                });
              },
              value: _anchorToBottom,
            ),
            title: const Text('Anchor to bottom'),
          ),

          /*** FirebaseAnimatedList to create List with firebase reference with animation***/
          Flexible(
            child: FirebaseAnimatedList(
              key: ValueKey<bool>(_anchorToBottom),
              query: _messagesRef,
              reverse: _anchorToBottom,
              sort: _anchorToBottom
                  ? (DataSnapshot a, DataSnapshot b) => b.key.compareTo(a.key)
                  : null,
              itemBuilder: (BuildContext context, DataSnapshot snapshot,
                  Animation<double> animation, int index) {

                return SizeTransition(
                  sizeFactor: animation,
                  child: ListTile(
                    trailing: IconButton(
                      onPressed: () =>
                          _messagesRef.child(snapshot.key).remove(),
                      icon: Icon(Icons.delete),
                    ),
                    title: Text(
                      "$index: ${snapshot.value.toString()}",
                    ),
                  ),
                );

              },
            ),
          ),
        ],
      ),
      
      /*** Call increment action ***/
      floatingActionButton: FloatingActionButton(
        onPressed: _increment,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }

Full_code.dart

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter;
  DatabaseReference _counterRef;
  DatabaseReference _messagesRef;
  StreamSubscription<Event> _counterSubscription;
  StreamSubscription<Event> _messagesSubscription;
  bool _anchorToBottom = false;

  String _kTestKey = 'Hello';
  String _kTestValue = 'world!';
  DatabaseError _error;

  @override
  void initState() {
    super.initState();
    _counterRef = FirebaseDatabase.instance.reference().child('counter');
    final FirebaseDatabase database = FirebaseDatabase();
    _messagesRef = database.reference().child('messages');
    database.reference().child('counter').once().then((DataSnapshot snapshot) {
      print('Connected to second database and read ${snapshot.value}');
    });
    database.setPersistenceEnabled(true);
    database.setPersistenceCacheSizeBytes(10000000);
    _counterRef.keepSynced(true);
    _counterSubscription = _counterRef.onValue.listen((Event event) {
      setState(() {
        _error = null;
        _counter = event.snapshot.value ?? 0;
      });
    }, onError: (Object o) {
      final DatabaseError error = o;
      setState(() {
        _error = error;
      });
    });
    _messagesSubscription =
        _messagesRef.limitToLast(10).onChildAdded.listen((Event event) {
      print('Child added: ${event.snapshot.value}');
    }, onError: (Object o) {
      final DatabaseError error = o;
      print('Error: ${error.code} ${error.message}');
    });
  }

  @override
  void dispose() {
    super.dispose();
    _messagesSubscription.cancel();
    _counterSubscription.cancel();
  }

  Future<void> _increment() async {
    // Increment counter in transaction.
    final TransactionResult transactionResult =
        await _counterRef.runTransaction((MutableData mutableData) async {
      mutableData.value = (mutableData.value ?? 0) + 1;
      return mutableData;
    });

    if (transactionResult.committed) {
      _messagesRef.push().set(<String, String>{
        _kTestKey: '$_kTestValue ${transactionResult.dataSnapshot.value}'
      });
    } else {
      print('Transaction not committed.');
      if (transactionResult.error != null) {
        print(transactionResult.error.message);
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Flutter Database Example'),
      ),
      body: Column(
        children: <Widget>[
          Flexible(
            child: Center(
              child: _error == null
                  ? Text(
                      'Button tapped $_counter time${_counter == 1 ? '' : 's'}.\n\n'
                      'This includes all devices, ever.',
                    )
                  : Text(
                      'Error retrieving button tap count:\n${_error.message}',
                    ),
            ),
          ),
          ListTile(
            leading: Checkbox(
              onChanged: (bool value) {
                setState(() {
                  _anchorToBottom = value;
                });
              },
              value: _anchorToBottom,
            ),
            title: const Text('Anchor to bottom'),
          ),
          Flexible(
            child: FirebaseAnimatedList(
              key: ValueKey<bool>(_anchorToBottom),
              query: _messagesRef,
              reverse: _anchorToBottom,
              sort: _anchorToBottom
                  ? (DataSnapshot a, DataSnapshot b) => b.key.compareTo(a.key)
                  : null,
              itemBuilder: (BuildContext context, DataSnapshot snapshot,
                  Animation<double> animation, int index) {
                return SizeTransition(
                  sizeFactor: animation,
                  child: ListTile(
                    trailing: IconButton(
                      onPressed: () =>
                          _messagesRef.child(snapshot.key).remove(),
                      icon: Icon(Icons.delete),
                    ),
                    title: Text(
                      "$index: ${snapshot.value.toString()}",
                    ),
                  ),
                );
              },
            ),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _increment,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}