Flutter 导航到新路由时释放小部件

我的应用程序中有两个屏幕.

屏幕A在打开时运行计算量很大的操作，并在调用dispose()时通过取消对数据库的动画/订阅来正确处理，以防止内存泄漏.

从屏幕A可以打开另一个屏幕(屏幕B).

当我使用Navigator.pushNamed时，屏幕A保留在内存中，并且不调用dispose()，即使现在显示了屏幕B.

有没有办法在屏幕A不可见时强制将其丢弃？

从不释放第一个路由的示例代码:

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    title: 'Navigation Basics',
    home: FirstRoute(),
  ));
}

class FirstRoute extends StatefulWidget {
  @override
  _FirstRouteState createState() => _FirstRouteState();
}

class _FirstRouteState extends State<FirstRoute> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('First Route'),
      ),
      body: RaisedButton(
        child: Text('Open route'),
        onPressed: () {
          Navigator.push(
            context,
            MaterialPageRoute(builder: (context) => SecondRoute()),
          );
        },
      ),
    );
  }

  @override
  void dispose() {
    // Never called
    print("Disposing first route");
    super.dispose();
  }
}

class SecondRoute extends StatefulWidget {
  @override
  _SecondRouteState createState() => _SecondRouteState();
}

class _SecondRouteState extends State<SecondRoute> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Second Route"),
      ),
      body: RaisedButton(
        onPressed: () {
          Navigator.pop(context);
        },
        child: Text('Go back!'),
      ),
    );
  }

  @override
  void dispose() {
    print("Disposing second route");
    super.dispose();
  }
}