How to Pass Data from Child Widget to Its Parent in Flutter

In order to pass data from children to its parent, we often use a callback. In Flutter, we define a callback using typedef. A typedef is a type alias. It is a shortcut for creating a new type based on an existing type.

However, in this post, we will use Notification. A notification is a message that can be broadcast to other widgets without using a callback. It is often used for communicating between parent and child widgets. Notification is also useful when we want to communicate between two unrelated widgets.

//define a notification
class ScanResultChanged extends Notification {
  final bool correct;
  final String message;

  ScanResultChanged(this.correct, this.message);
}

//call it in an event of child widget
onPressed:(){
  ScanResultChanged(true, "Scanned completed").dispatch(context);
}

//wrap the parent widget with NotificationListener<ScanResultChanged>

NotificationListener<ScanResultChanged>(
onNotification: (n) {
  setState(() {
    _scanCorrect = n.correct;
    _scanMessage = n.message;
  });
  return true;
},
child: Container(
  width: MediaQuery.of(context).size.width,
  constraints: const BoxConstraints(minHeight: 36),
  padding: const EdgeInsets.all(MyTheme.padding),
  child: Column(
    mainAxisAlignment: MainAxisAlignment.start,
    crossAxisAlignment: CrossAxisAlignment.start,
    children: [
      if(_scanCorrect)
        Text(_scanMessage)
    ],
  ),
),
)