Access_BuildContext.dart

void withBuilder() {
  return Builder(
    builder: (context) {
      return RaisedButton(
        child: Text('Button'),
        onPressed: () {
          final renderObject = context.findRenderObject();
        }
      );
    }
  );
}

final globalKey = GlobalKey();

void withGlobalKey() {
  return RaisedButton(
    key: globalKey,
    child: Text('Button'),
    onPressed: () {
      final renderObject = globalKey.currentContext.findRenderObject();
    }
  );
}