如何在同一 lib 文件夹中的其他 screen.dart 文件中使用 main.dart 中的 flutter 变量？

Question

I wanted to create an app that randomly generates my lucky number when i press floatingActionButton. I wanted to do it within 2 dart files.. let me show you the code in dart.main and first_screen.dart.

dart.main

import 'package:demo/app_screens/first_screen.dart';
import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(
  debugShowCheckedModeBanner: false,
  home: MyFlutterApp()
));

class MyFlutterApp extends StatefulWidget {
  @override
  _MyFlutterAppState createState() => _MyFlutterAppState();
}

class _MyFlutterAppState extends State<MyFlutterApp> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.yellow,
        centerTitle: true,
        title: Text('My first App',style: TextStyle(color: Colors.black, fontSize: 25.0),),
      ),
      body: FirstScreen(),
     floatingActionButton: FloatingActionButton(
        onPressed: (){
          setState(() {
          int thatNum = generateLuckyNumber();
        });},
        child: Icon(Icons.add),
     ),
    );
  }
}

and first_screen.dart in the lib/screens directory

import 'dart:math';

import 'package:flutter/material.dart';

class FirstScreen extends StatefulWidget{
  @override
  _FirstScreenState createState() => _FirstScreenState();
}

class _FirstScreenState extends State<FirstScreen> {
  @override
  Widget build(BuildContext context) {
    return Material(color: Colors.lightBlue,
      child: Center(
        child: Text('My lucky number is ${thatNum}',style: TextStyle(
            fontSize: 28,color: Colors.black,backgroundColor:Colors.white)),
      ),
    );
  }


}

int generateLuckyNumber() {
  var random= Random();
  int luckyNumber= random.nextInt(10);
  return luckyNumber;
}

I would like to use the varibale thatNum declared in main.dart file in the first_screen.dart file.. How do you do that?

Answer 1

The easiest way to do this, pass the thatNum in FirstScreen constructor. Do the required changes given below

class _MyFlutterAppState extends State<MyFlutterApp> {
  int thatNum;   // <- declare thatNum in the class
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.yellow,
        centerTitle: true,
        title: Text('My first App',style: TextStyle(color: Colors.black, fontSize: 25.0),),
      ),
      body: FirstScreen(thatNum:thatNum),  // <-- pass thatNum in constructor
     floatingActionButton: FloatingActionButton(
        onPressed: (){
          setState(() {
          thatNum = generateLuckyNumber();  /* <- generateLuckyNumber and assign to thatNum */
        });},
        child: Icon(Icons.add),
     ),
    );
  }
}

In the FirstScreen declare thatNum

class FirstScreen extends StatefulWidget{
  final thatNum;   // <- declare thatNum
  FirstScreen({this.thatNum});
  @override
  _FirstScreenState createState() => _FirstScreenState();
}

In _FirstScreenState check whether the widget.thatNum is null or not. If it is null assign Loading text or show thatNum if not widget.thatNum is not null.

class _FirstScreenState extends State<FirstScreen> {
  @override
  Widget build(BuildContext context) {
    return Material(color: Colors.lightBlue,
      child: Center(
        child: Text('My lucky number is ${widget.thatNum??"Loading"}',style: TextStyle(
            fontSize: 28,color: Colors.black,backgroundColor:Colors.white)),
      ),
    );
  }
}

Note: You can also use state management solution, but the above solution solves the issue easily. Still, you can check state management solution here

Answer 2

You can do this thing in two ways:

1. Pass the variables in dependency of FirstScreen.
2. Use InheritedWidget.

1. Pass the variables in dependency of FirstScreen.

body: FirstScreen(thatNum), // in Scaffold of main.dart file.

Use it like:

class FirstScreen extends StatefulWidget{
  final thatNum;
  FirstScreen(this.thatNum);
  @override
  _FirstScreenState createState() => _FirstScreenState();
}

class _FirstScreenState extends State<FirstScreen> {
  @override
  Widget build(BuildContext context) {
    return Material(color: Colors.lightBlue,
      child: Center(
        child: Text('My lucky number is ${widget.thatNum} ??"Loading"}',style: TextStyle(
            fontSize: 28,color: Colors.black,backgroundColor:Colors.white)),
      ),
    );
  }
}

2. Use InheritedWidget.

Create an InheritedWidget as follows.

class MyInheritedWidget extends InheritedWidget {
  const MyInheritedWidget({
    Key key,
    @required this.thatNum,
    @required Widget child,
  }) : assert(color != null),
       assert(child != null),
       super(key: key, child: child);

  final thatNum;

  static MyInheritedWidget of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<MyInheritedWidget>();
  }

  @override
  bool updateShouldNotify(MyInheritedWidget old) => thatNum!= old.thatNum;
}

Change: body: FirstScreen(), to: body: MyInheritedWidget(child: FirstScreen(), thatNum:thatNum),

Now all the decendent of MyInheritedWidget will be able to access thatNum by using context like:

class _FirstScreenState extends State<FirstScreen> {
  @override
  Widget build(BuildContext context) {
    final thatNum = MyInheritedWidget.of(context).thatNum;
    return Material(color: Colors.lightBlue,
      child: Center(
        child: Text('My lucky number is $thatNum ??"Loading"}',style: TextStyle(
            fontSize: 28,color: Colors.black,backgroundColor:Colors.white)),
      ),
    );
  }
}

If you have a child Widget that needs thatNum and that child widget doesn't need to pass thatNum to any further widget then thatNum should be passed in dependency.

If you have a long hierarchy of Widget that need this data then InheritedWidget must be used to avoid passing data in each and every child's constructor.

I hope this helps, in case of any doubt please comment. If this answer helps you then please accept and up-vote it.