如何在Flutter中从左到右填充文本的颜色

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Container(
            padding: EdgeInsets.all(16.0),
            child: RichText(
              text: TextSpan(
                children: [
                  TextSpan(
                    text: 'Hello',
                    style: TextStyle(
                      color: Colors.red,
                    ),
                  ),
                  TextSpan(
                    text: ' World',
                    style: TextStyle(
                      color: Colors.blue,
                    ),
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}