In flutter,

1. How can a parent widget know if a child among many children widgets has received focus? For example, Can we know if a child in a Row widget's children has received focus?

2. Can I detect this focus before the child widget receives it?

CodePudding user response：

It actually depends on your take and which architecture you wanna follow. This snippet that I'm posting uses NotificationListener, a custom notification and a custom child widget. This might work for an application like a print or a callback, but you might need to change the architecture and use a state management tool to achieve greater things.

Parent Widget class:

class MyParentWidget extends StatelessWidget {
  const MyParentWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return NotificationListener<FocusNotification>(
      onNotification: (notification) {
        print("New widget focused: ${notification.childKey.toString()}");
        return true;
      },
      child: Row(
        children: List.generate(
          5,
          (index) => MyChildWidget(
            Key('widget-$index'),
          ),
        ),
      ),
    );
  }
}

Child Widget class:

class MyChildWidget extends StatefulWidget {
  const MyChildWidget(Key key) : super(key: key);

  @override
  _MyChildWidgetState createState() => _MyChildWidgetState();
}

class _MyChildWidgetState extends State<MyChildWidget> {
  final node = FocusNode();

  @override
  initState() {
    node.addListener(() {
      if (node.hasFocus) {
        final notification = FocusNotification(widget.key!);
        notification.dispatch(context);
      }
    });
    super.initState();
  }

  @override
  dispose() {
    node.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return TextField(
      focusNode: node,
    );
  }
}

Custom Notification class:

class FocusNotification extends Notification {
  const FocusNotification(this.childKey);
  final Key childKey;
}