how to access an object created in one stateful widget in another stateful widget in flutter

Question

I am stuck in my project where I have two stateful widgets created in flutter as two different dart files. Now, I have to access the instance of an object created in first widget in the second widget, but I am not so sure how I can do this in flutter when creating widgets.

One possible solution I thought of is to declare the two widgets in just one dart file instead of two dart files for two layouts, but I am curious if we can do it by declaring in two separate dart files.

I have posted the files just to re-create the issue.

main.dart

 import 'package:flutter/material.dart';
 import 'package:untitled2/models.dart';
 import 'package:untitled2/secondwidget.dart';

void main() {
 runApp(new MaterialApp(
     home: new MyApp(),
 ),);
}

class MyApp extends StatefulWidget {
@override
_MyAppState createState() => new _MyAppState();
  }

      class _MyAppState extends State<MyApp> {
 final TextEditingController _nameController = new TextEditingController();
 final TextEditingController _emailIdController = new 
 TextEditingController();
 final TextEditingController _passwordController = new 
 TextEditingController();
 final TextEditingController _confirmPasswordController = new 
 TextEditingController();

 MyModel myModel = new MyModel();

 @override
 Widget build(BuildContext context) {
  return new MaterialApp(
  home: new Scaffold(
    body: new ListView(

      children: <Widget>[
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
              labelText: 'Enter your Name'
            ),
            controller: _nameController,
            onSaved: (String value){myModel.name = value;} ,
          ),
        ),
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'EmailId'
            ),
              controller: _emailIdController,
              onSaved: (String value){myModel.emailId = value;}
          ),
        ),
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'Password'
            ),
              controller: _passwordController,
              onSaved: (String value){myModel.password = value;}
          ),
        ),
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'Confirm Password'
            ),
            controller: _confirmPasswordController,

          ),
        ),
        new Container(
          child: new FlatButton(
            onPressed: ((){

              Navigator.push(context, new MaterialPageRoute(builder: (_) => 
         new SecondScreen(),),);

            }),
            child: new Text('Save'),),
        ),



      ],


      ),
     ),
    );
   }
  }

models.dart

class MyModel {

String name;
String emailId;
String password;

  }

secondwidget.dart

 import 'package:flutter/material.dart';


 class SecondScreen extends StatefulWidget {
 @override
 _SecondScreenState createState() => new _SecondScreenState();
}

class _SecondScreenState extends State<SecondScreen> {
@override
Widget build(BuildContext context) {
return new MaterialApp(
  home: new Scaffold(
    body: new ListView(

      children: <Widget>[
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'Enter address'
            ),
          ),
        ),
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'Address Line 2'
            ),
          ),
        ),
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'Address Line 3'
            ),
          ),
        ),
        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'POST CODE'
            ),
          ),
        ),

        new Container(
          child: new TextFormField(
            decoration: new InputDecoration(
                labelText: 'Mobile Number'
            ),
          ),
        ),
        new Container(
          child: new FlatButton(
              onPressed: ((){


                //I want to push the data captured from main.dart and from 
                            //secondwidget.dart into database
     // I want to use the instance of MyModel from main.dart here to save 
      // the data captured from the first screen and this one into database

              }),
              child: new Text('Save'),),
        ),


      ],


    ),
  ),
);
}
}

Answer 1

There are lots of ways to do this depending on your use case. Here are a few options:

1. You can expose the created object as public member of your State. Then use the currentState property of a GlobalKey in one State to get a reference to the other State. Now you can access the created object via the public member. (Note: This pattern limits the testability and encapsulation of your State, so use it sparingly.)
2. Both widgets can have a ancestor widget that extends InheritedWidget that they use to look up the created object.
3. Both widgets can be passed a model argument in their a constructor, such as a ValueNotifier. They can use this object to read and write the value.

If you go into more detail on your use case we can help you pick a pattern that makes sense.

Here is some code implementing option #1.

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

final key = new GlobalKey<MyStatefulWidget1State>();

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        body: new Column(
          mainAxisAlignment: MainAxisAlignment.spaceAround,
          children: <Widget>[
            new MyStatefulWidget1(key: key),
            new MyStatefulWidget2(),
          ],
        ),
      ),
    );
  }
}

class MyStatefulWidget1 extends StatefulWidget {
  MyStatefulWidget1({ Key key }) : super(key: key);
  State createState() => new MyStatefulWidget1State();
}

class MyStatefulWidget1State extends State<MyStatefulWidget1> {
  String _createdObject = "Hello world!";
  String get createdObject => _createdObject;

  @override
  Widget build(BuildContext context) {
    return new Center(
      child: new Text(_createdObject),
    );
  }
}

class MyStatefulWidget2 extends StatefulWidget {
  State createState() => new MyStatefulWidget2State();
}

class MyStatefulWidget2State extends State<MyStatefulWidget2> {
  String _text = 'PRESS ME';

  @override
  Widget build(BuildContext context) {
    return new Center(
      child: new RaisedButton(
        child: new Text(_text),
        onPressed: () {
          setState(() {
            _text = key.currentState.createdObject;
          });
        },
      ),
    );
  }
}

Answer 2

Here is how I pass parameter to the child widget.

First Widget dart file

class FirstWidget extends StatefulWidget {
    _FirstWidgetState createState() => _FirstWidgetState() 
}

class _FirstWidgetState extends State<FirstWidget> {
    String param = "My parameter";
    @override
    Widget build(BuildContext context) {
        return Container(
                    child: 
                        SecondWidget(param), //pass parmater here
                );
    }
}

Second Widget dart file

class SecondWidget extends StatefulWidget {
    final String param; 
    SecondWidget(this.param); //passed paramter
    _SecondWidgetState createState() => _SecondWidgetState() 
}

class _SecondWidgetState extends State<SecondWidget> {
    @override
    Widget build(BuildContext context) {
        return Container(
                    child: 
                        Text(widget.param), //output paramter
                );
    }
}

Answer 3

You can pass the model when you construct SecondScreen widget.

1. Add model to SecondScreen constructor:

   class SecondScreen extends StatefulWidget {
     final MyModel myModel;
   
     SecondScreen(MyModel myModel, {Key key}):
         super(key: key);
   ...
   }
   

2. Pass model when you construct SecondScreen in main.dart

   Navigator.push(context, new MaterialPageRoute(builder: (_) => 
     new SecondScreen(model)));
   

3. Now you can access model in _SecondScreenState via widget.myModel