Flutter - CircularProgressIndicator不会停止执行

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  bool isLoading = true;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            if (isLoading)
              CircularProgressIndicator(),
            RaisedButton(
              child: Text('完成任务'),
              onPressed: () {
                setState(() {
                  isLoading = false;
                });
              },
            ),
          ],
        ),
      ),
    );
  }
}