As I'm learning Flutter I've come to navigation. I want to pass data between screens similarly to passing data between Activities in Android and passing data between View Controllers in iOS. How do I do it in Flutter?

Passing data forward to the next screen

To send data to the next screen you do the following things:

Make the SecondScreen constructor take a parameter for the type of data that you want to send to it. In this particular example, the data is defined to be a String value and is set here with this.text.
```
class SecondScreen extends StatelessWidget {
  final String text;
  SecondScreen({Key key, @required this.text}) : super(key: key);

  ...
```
Then use the Navigator in the FirstScreen widget to push a route to the SecondScreen widget. You put the data that you want to send as a parameter in its constructor.
```
Navigator.push(
    context,
    MaterialPageRoute(
      builder: (context) => SecondScreen(text: 'Hello',),
    ));
```

The full code for main.dart is here:

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    title: 'Flutter',
    home: FirstScreen(),
  ));
}

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

class _FirstScreenState extends State<FirstScreen> {

  // this allows us to access the TextField text
  TextEditingController textFieldController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('First screen')),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [

          Padding(
            padding: const EdgeInsets.all(32.0),
            child: TextField(
              controller: textFieldController,
              style: TextStyle(
                fontSize: 24,
                color: Colors.black,
              ),
            ),
          ),

          RaisedButton(
            child: Text(
              'Go to second screen',
              style: TextStyle(fontSize: 24),
            ),
            onPressed: () {
              _sendDataToSecondScreen(context);
            },
          )

        ],
      ),
    );
  }

  // get the text in the TextField and start the Second Screen
  void _sendDataToSecondScreen(BuildContext context) {
    String textToSend = textFieldController.text;
    Navigator.push(
        context,
        MaterialPageRoute(
          builder: (context) => SecondScreen(text: textToSend,),
        ));
  }
}

class SecondScreen extends StatelessWidget {
  final String text;

  // receive data from the FirstScreen as a parameter
  SecondScreen({Key key, @required this.text}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Second screen')),
      body: Center(
        child: Text(
          text,
          style: TextStyle(fontSize: 24),
        ),
      ),
    );
  }
}

Passing data back to the previous screen

When passing data back you need to do the following things:

In the FirstScreen, use the Navigator to push (start) the SecondScreen in an async method and wait for the result that it will return when it finishes.
```
final result = await Navigator.push(
    context,
    MaterialPageRoute(
      builder: (context) => SecondScreen(),
    ));
```
In the SecondScreen, include the data that you want to pass back as a parameter when you pop the Navigator.
```
Navigator.pop(context, 'Hello');
```
Then in the FirstScreen the await will finish and you can use the result.
```
setState(() {
  text = result;
});
```

Here is the complete code for main.dart for your reference.

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    title: 'Flutter',
    home: FirstScreen(),
  ));
}

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

class _FirstScreenState extends State<FirstScreen> {

  String text = 'Text';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('First screen')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [

            Padding(
              padding: const EdgeInsets.all(32.0),
              child: Text(
                text,
                style: TextStyle(fontSize: 24),
              ),
            ),

            RaisedButton(
              child: Text(
                'Go to second screen',
                style: TextStyle(fontSize: 24),
              ),
              onPressed: () {
                _awaitReturnValueFromSecondScreen(context);
              },
            )

          ],
        ),
      ),
    );
  }

  void _awaitReturnValueFromSecondScreen(BuildContext context) async {

    // start the SecondScreen and wait for it to finish with a result
    final result = await Navigator.push(
        context,
        MaterialPageRoute(
          builder: (context) => SecondScreen(),
        ));

    // after the SecondScreen result comes back update the Text widget with it
    setState(() {
      text = result;
    });
  }
}

class SecondScreen extends StatefulWidget {
  @override
  _SecondScreenState createState() {
    return _SecondScreenState();
  }
}

class _SecondScreenState extends State<SecondScreen> {
  // this allows us to access the TextField text
  TextEditingController textFieldController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Second screen')),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [

          Padding(
            padding: const EdgeInsets.all(32.0),
            child: TextField(
              controller: textFieldController,
              style: TextStyle(
                fontSize: 24,
                color: Colors.black,
              ),
            ),
          ),

          RaisedButton(
            child: Text(
              'Send text back',
              style: TextStyle(fontSize: 24),
            ),
            onPressed: () {
              _sendDataBack(context);
            },
          )

        ],
      ),
    );
  }

  // get the text in the TextField and send it back to the FirstScreen
  void _sendDataBack(BuildContext context) {
    String textToSendBack = textFieldController.text;
    Navigator.pop(context, textToSendBack);
  }
}

回答2:

This solution is very easy by passing variables in constructor:

first page:

Navigator.of(context).push(MaterialPageRoute(builder:(context)=>SecondPage('something')));

second page:

class SecondPage extends StatefulWidget {
  String something;
  SecondPage(this.something);
  @override
  State<StatefulWidget> createState() {
    return SecondPageState(this.something);
  }
}
class SecondPageState extends State<SecondPage> {
  String something;
  SecondPageState(this.something);
  @override
  Widget build(BuildContext context) {
   return Scaffold(
    appBar: AppBar(
    //now you have passing variable
    title: Text(something),
   ),
   ...
  }

回答3:

Easiest way

FirstPage.dart

 Navigator.push(
          context,
          MaterialPageRoute(
              builder: (context) => PasswordRoute(usernameController)));

//usernameController is String value,If you want to pass multiple values add all

SecondPage.dart

class PasswordRoute extends StatefulWidget {
  final String usernameController;//if you have multiple values add here
PasswordRoute(this.usernameController, {Key key}): super(key: key);//add also..example this.abc,this...

  @override
  State<StatefulWidget> createState() => _PasswordPageState();
}

class _PasswordPageState extends State<PasswordRoute> {
 @override
  Widget build(BuildContext context) {
...child: Text(widget.usernameController);
}
}

回答4:

First Screen : //send data to second screen

 Navigator.push(context, MaterialPageRoute(builder: (context) {
                          return WelcomeUser(usernameController.text);

                          }));

Second Screen : //fetch data from first screen

 final String username;
  WelcomeUser(this.username);

//use data to display

body: Container(
    child: Center(
      child: Text("Welcome "+widget.username,
      textAlign: TextAlign.center,
      ),
    ),
  ),

回答5:

Answers above are useful for a small app, but if you want to remove the headache of continuously worrying about a widgets state, Google presented the Provider package. https://pub.dev/packages/provider

Have a look into that one, or watch these videos from Andrea Bizzotto: https://www.youtube.com/watch?v=MkFjtCov62g // Provider: The Essential Guide https://www.youtube.com/watch?v=O71rYKcxUgA&t=258s // Provider: Introduction

Learn how to use the Provider package, and you are set for life :)