How to detect orientation change in layout in Flutter?

How to find out Orientation is portrait or landscape in Flutter

if(portrait){
  return ListView.builder()
}else{
  return GridView.count()
}

In order to determine the Orientation of the screen, we can use the OrientationBuilder Widget. The OrientationBuilder will determine the current Orientation and rebuild when the Orientation changes.

new OrientationBuilder(
  builder: (context, orientation) {
    return new GridView.count(
      // Create a grid with 2 columns in portrait mode, or 3 columns in
      // landscape mode.
      crossAxisCount: orientation == Orientation.portrait ? 2 : 3,
    );
  },
);

you can find the complete example here: https://flutter.io/cookbook/design/orientation/