从有状态的父级修改有状态的子小部件中的值

Question

I have two widgets. Both are stateful widgets.I put a button and Child widget in HomePage widget. I want to change counter variable in Child widget from HomePage widget when click on button in HomePage widget. I should use setState method for that but I can't call it outside from Child widget.What is the proper way to do that?

Parent

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: [
          Child(),
          FlatButton(
            onPressed: () {

              //Increase Child widget's counter variable
              
            },
            child: Text("Button In Parent"),
            color: Colors.amber,
          )
        ],
      ),
    );
  }
}

Child

class Child extends StatefulWidget {
  @override
  _ChildState createState() => _ChildState();
}

class _ChildState extends State<Child> {
  int counter = 0;

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.green,
      child: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              'Text In Child: ',
              style: TextStyle(fontSize: 30),
            ),
            Text(
              //Increase this variable from parent
              counter.toString(),
              style: TextStyle(fontSize: 50),
            ),
          ],
        ),
      ),
    );
  }
}

Answer 1

You have to use FlatButton and setState in same class otherwise you can not access the class variable, Use the code below and try to understand logic of setState. Have a good day.

class Child extends StatefulWidget {
  @override
  _ChildState createState() => _ChildState();
}

class _ChildState extends State<Child> {
  int counter = 0;

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.green,
      child: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              'Text In Child: ',
              style: TextStyle(fontSize: 30),
            ),
            Text(
              //Increase this variable from parent
              counter.toString(),
              style: TextStyle(fontSize: 50),
            ),
          Expanded(
            child:Container(
              color:Colors.white,
              child:FlatButton(
                onPressed(){
                  setState((){
                    counter=counter+1;
                });
              };
            ),
          ),          
        ),         
      ],
    );
  }
}

Answer 2

You can use the setState of the parent widget to rebuild the child widget. Initialize a counter variable in the parent widget and increment it and call setState .

In the onTap of your FlatButton in parent widget

setState(() {
   counter++;
});

Let the Child widget take this as a parameter Child(counts: counter)

class Child extends StatefulWidget {
  final int counts;

  Child({this.counts});
  @override
  _ChildState createState() => _ChildState();
}

and display it in the Text in your Child

Text(
  widget.counts.toString(),
  style: TextStyle(fontSize: 50),
),

This is one way of doing it.