Flutter开发应用。我们经常会用到路由传参，从一个视图，跳转到另外一个视图，并带上参数。

当然实际应用中，这个参数可能为null，也可能是正常的。

如果你只看官方教程，你是会被误导，因为官方教程只讲了正常的情况，没有讲为null的情况。这个时候，我们需要处理Null的情况。

参见代码

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {

    final args = ModalRoute.of(context)!.settings.arguments as HomeScreenArguments?;

    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
             Text(
              args?.name ?? "welcome",
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
            ElevatedButton(
              // Within the `FirstScreen` widget
              onPressed: () {
                // Navigate to the second screen using a named route.
                Navigator.pushNamed(context, '/booking');
              },
              child: const Text('开始记账'),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

 

我们需要注意args参数定义的地方

    final args = ModalRoute.of(context)!.settings.arguments as HomeScreenArguments?;

后面的问号，表示这个参数可能是null

然后我们看实际用这个参数的地方

              args?.name ?? "welcome",

 

用了 ?? 操作符，当前面左边为null，我们用一个默认值来作为显示。

 

这样我们就解决了当没有传参，我们该做什么的问题。