How to detect orientation change in layout in Flutter?

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/

@override
Widget build(BuildContext context) {
  return Scaffold(
    body: OrientationBuilder(builder: (_, orientation) {
      if (orientation == Orientation.portrait)
        return _buildPortraitLayout(); // if orientation is portrait, show your portrait layout
      else
        return _buildLandscapeLayout(); // else show the landscape one
    }),
  );
}

it's quite easy

if (MediaQuery.of(context).orientation == Orientation.portrait){
    // is portrait
}else{
// is landscape
}

You can use MediaQuery to check orientation:

var isPortrait = MediaQuery.of(context).orientation == Orientation.portrait