dependencies:
    rounded_loading_button: ^2.0.3

import 'package:rounded_loading_button/rounded_loading_button.dart';

final RoundedLoadingButtonController _btnController = RoundedLoadingButtonController();

void _doSomething() async {
    Timer(Duration(seconds: 3), () {
        _btnController.success();
    });
}

RoundedLoadingButton(
    child: Text('Tap me!', style: TextStyle(color: Colors.white)),
    controller: _btnController,
    onPressed: _doSomething,
)