Nsryjdtyk

In my simple project , there is a button to change theme form light to dark and vice versa as below :

import 'package:flutter/material.dart';

import 'package:shared_preferences/shared_preferences.dart';

import 'dart:async';



void main() => runApp(MyApp());



class MyApp extends StatefulWidget {

  @override

  MyAppState createState() {

    return new MyAppState();

  }

}



class MyAppState extends State<MyApp> {

  Future<bool> load(String key, bool defaultValue) async {

    SharedPreferences prefs = await SharedPreferences.getInstance();

    bool state = prefs.getBool(key) ?? defaultValue;

    return state;

  }



  Future<void> save(String key, bool value) async {

    SharedPreferences prefs = await SharedPreferences.getInstance();

    prefs.setBool(key, value);

  }



  bool state;



  @override

  Widget build(BuildContext context) {

    load("themeKey", false).then((bool value) {

      state = value;

    });

    debugPrint(

        "..............state is : " + state.toString() + "...............");

    return MaterialApp(

      theme: state ? ThemeData.light() : ThemeData.dark(),

      home: Scaffold(

        body: Container(

          child: RaisedButton(

            child: Text("change"),

            onPressed: () {

              setState(() {

                state = !state;

              });

              save("themeKey", state);

            },

          ),

        ),

      ),

    );

  }

}

I use shared_preferences plugin to save the current theme state for the next app launch , but there is a problem it seems that app loads before shared preference loads so i am getting this exception:

Launching libmain.dart on C1905 in debug mode...

Built buildappoutputsapkdebugapp-debug.apk.

I/flutter ( 8045): ..............state is : null...............

I/flutter ( 8045): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════

I/flutter ( 8045): The following assertion was thrown building MyApp(dirty, state: MyAppState#6774f):

I/flutter ( 8045): Failed assertion: boolean expression must not be null

I/flutter ( 8045): Either the assertion indicates an error in the framework itself, or we should provide substantially

I/flutter ( 8045): more information in this error message to help you determine and fix the underlying cause.

I/flutter ( 8045): In either case, please report this assertion by filing a bug on GitHub:

I/flutter ( 8045):   https://github.com/flutter/flutter/issues/new?template=BUG.md

I/flutter ( 8045): When the exception was thrown, this was the stack:

I/flutter ( 8045): #0      MyAppState.build (package:test_shared_preferenced/main.dart:36:14)

I/flutter ( 8045): #1      StatefulElement.build (package:flutter/src/widgets/framework.dart:3787:27)

I/flutter ( 8045): #2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3699:15)

I/flutter ( 8045): #3      Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)

I/flutter ( 8045): #4      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3679:5)

I/flutter ( 8045): #5      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3826:11)

I/flutter ( 8045): #6      ComponentElement.mount (package:flutter/src/widgets/framework.dart:3674:5)

I/flutter ( 8045): #7      Element.inflateWidget (package:flutter/src/widgets/framework.dart:2950:14)

I/flutter ( 8045): #8      Element.updateChild (package:flutter/src/widgets/framework.dart:2753:12)

I/flutter ( 8045): #9      RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:909:16)

I/flutter ( 8045): #10     RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:880:5)

I/flutter ( 8045): #11     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:826:17)

I/flutter ( 8045): #12     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2266:19)

I/flutter ( 8045): #13     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:825:13)

I/flutter ( 8045): #14     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:712:7)

I/flutter ( 8045): #15     runApp (package:flutter/src/widgets/binding.dart:756:7)

I/flutter ( 8045): #16     main (package:test_shared_preferenced/main.dart:5:16)

I/flutter ( 8045): #17     _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:289:19)

I/flutter ( 8045): #18     _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)

I/flutter ( 8045): ════════════════════════════════════════════════════════════════════════════════════════════════════

so what should i do in this situation , please help

The load method is returns a Future that will run asynchronously from the rest of your code. When execution reaches debugPrint or the subsequent return statement, you cannot be guaranteed that the then block has completed its execution. The error you are receiving confirms this. The state boolean has not been set to a value yet - it is still null.

Using a FutureBuilder can help you handle cases where you are waiting on a Future to build your widget tree.

https://www.dartlang.org/tutorials/language/futures is worthwhile reading.