Flutter: Get passed arguments from Navigator in Widget's state's initState

Question

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>,     ); 

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) 

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?

Answer 1

use MaterialApp.onGenerateRoute property like this:

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)); }, 

now you can simply use NavigatorState.pushNamed:

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

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,         ),       ),     );   } }