如何在颤动的另一个有状态小部件中访问在一个有状态小部件中创建的对象

Question

我被困在我的项目中，我在颤振中创建了两个有状态的小部件作为两个不同的 dart 文件。现在，我必须在第二个小部件中访问在第一个小部件中创建的对象的实例，但我不太确定在创建小部件时如何在颤振中做到这一点。

我想到的一种可能的解决方案是在一个 dart 文件中声明两个小部件，而不是在两个布局中声明两个 dart 文件，但我很好奇我们是否可以通过在两个单独的 dart 文件中声明来做到这一点。

我发布这些文件只是为了重现问题。

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

根据您的用例，有很多方法可以做到这一点。这里有几个选项：

1. 您可以将创建的对象公开为State 的公共成员。然后在一个State 中使用GlobalKey 的currentState 属性来获取对另一个State 的引用。现在您可以通过公共成员访问创建的对象。 （注意：这种模式限制了 State 的可测试性和封装性，因此请谨慎使用。）
2. 两个小部件都可以有一个祖先小部件，它扩展了InheritedWidget，它们用于查找创建的对象。
3. 两个小部件都可以在其构造函数中传递模型参数，例如ValueNotifier。他们可以使用此对象来读取和写入值。

如果您更详细地了解您的用例，我们可以帮助您选择一种有意义的模式。

这是一些实现选项 #1 的代码。

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

您可以在构造SecondScreen小部件时传递模型。

1. 将模型添加到SecondScreen构造函数：

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

2. 在 main.dart 中构造 SecondScreen 时传递模型

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

3. 现在您可以通过widget.myModel 访问_SecondScreenState 中的模型

Answer 3

这是我将参数传递给子小部件的方式。

第一个 Widget dart 文件

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
                );
    }
}

第二个 Widget dart 文件

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
                );
    }
}