Flutter:从Widget状态的initState中的Navigator获取传递的参数 [英] Flutter: Get passed arguments from Navigator in Widget's state's initState

问题描述

我有一个StatefulWidget，我想在命名路由中使用.我必须按照 https://flutter中的建议传递一些参数. dev/docs/cookbook/navigation/navigate-with-arguments 即

I have a StatefulWidget which I want to use in named route. I have to pass some arguments which I am doing as suggested in https://flutter.dev/docs/cookbook/navigation/navigate-with-arguments i.e.

Navigator.pushNamed(
      context,
      routeName,
      arguments: <args>,
    );

现在，我需要在状态的initState方法中访问这些参数，因为需要这些参数来订阅某些外部事件.如果将args = ModalRoute.of(context).settings.arguments;调用放在initState中，则会收到运行时异常.

Now, I need to access these argument's in the state's initState method as the arguments are needed to subscribe to some external events. If I put the args = ModalRoute.of(context).settings.arguments; call in initState, I get a runtime exception.

20:49:44.129 4 info flutter.tools I/flutter ( 2680): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
20:49:44.129 5 info flutter.tools I/flutter ( 2680): The following assertion was thrown building Builder:
20:49:44.129 6 info flutter.tools I/flutter ( 2680): inheritFromWidgetOfExactType(_ModalScopeStatus) or inheritFromElement() was called before
20:49:44.130 7 info flutter.tools I/flutter ( 2680): _CourseCohortScreenState.initState() completed.
20:49:44.130 8 info flutter.tools I/flutter ( 2680): When an inherited widget changes, for example if the value of Theme.of() changes, its dependent
20:49:44.131 9 info flutter.tools I/flutter ( 2680): widgets are rebuilt. If the dependent widget's reference to the inherited widget is in a constructor
20:49:44.131 10 info flutter.tools I/flutter ( 2680): or an initState() method, then the rebuilt dependent widget will not reflect the changes in the
20:49:44.131 11 info flutter.tools I/flutter ( 2680): inherited widget.
20:49:44.138 12 info flutter.tools I/flutter ( 2680): Typically references to inherited widgets should occur in widget build() methods. Alternatively,
20:49:44.138 13 info flutter.tools I/flutter ( 2680): initialization based on inherited widgets can be placed in the didChangeDependencies method, which
20:49:44.138 14 info flutter.tools I/flutter ( 2680): is called after initState and whenever the dependencies change thereafter.
20:49:44.138 15 info flutter.tools I/flutter ( 2680): 
20:49:44.138 16 info flutter.tools I/flutter ( 2680): When the exception was thrown, this was the stack:
20:49:44.147 17 info flutter.tools I/flutter ( 2680): #0      StatefulElement.inheritFromElement.<anonymous closure> (package:flutter/src/widgets/framework.dart:3936:9)
20:49:44.147 18 info flutter.tools I/flutter ( 2680): #1      StatefulElement.inheritFromElement (package:flutter/src/widgets/framework.dart:3969:6)
20:49:44.147 19 info flutter.tools I/flutter ( 2680): #2      Element.inheritFromWidgetOfExactType (package:flutter/src/widgets/framework.dart:3285:14)
20:49:44.147 20 info flutter.tools I/flutter ( 2680): #3      ModalRoute.of (package:flutter/src/widgets/routes.dart:698:46)
20:49:44.147 21 info flutter.tools I/flutter ( 2680): #4      _CourseCohortScreenState.initState.<anonymous closure> (package:esk2/cohort_screen.dart:57:23)

我不想将该逻辑放在build方法中，因为build可以被多次调用，并且初始化只需发生一次.我可以将整个逻辑放在带有boolean isInitialized标志的块中，但这似乎并不是正确的方法. 到目前为止，此要求/案例是否不受支持?

I do not want to put that logic in build method as build could be called multiple times and the initialization needs to happen only once. I could put the entire logic in a block with a boolean isInitialized flag, but that does not seem like the right way of doing this. Is this requirement/case not supported in flutter as of now?

推荐答案

使用MaterialApp.onGenerateRoute属性，如下所示:

onGenerateRoute: (RouteSettings settings) {
  print('build route for ${settings.name}');
  var routes = <String, WidgetBuilder>{
    "hello": (ctx) => Hello(settings.arguments),
    "other": (ctx) => SomeWidget(),
  };
  WidgetBuilder builder = routes[settings.name];
  return MaterialPageRoute(builder: (ctx) => builder(ctx));
},

现在您只需使用NavigatorState.pushNamed:

Navigator.of(context).pushNamed("hello", arguments: "world");

这里有一些测试Hello小部件:

here you have some test Hello widget:

class Hello extends StatelessWidget {
  final String greet;

  Hello(this.greet);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Text(
          'hello $greet',
          textScaleFactor: 5.0,
        ),
      ),
    );
  }
}

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