Change parent variable and rebuild parent widget when child widget changes Flutter

You can create a filed in your CustomWidget onFlatButtonPressed. The field will be a Callback Function or VoidCallBack then call the function when the flat button is pressed in your CustomWidget and define the function in your **MyScreen*

Like this

class MyScreen extends StatefulWidget {  @override  _MyScreenState createState() => _MyScreenState();}class _MyScreenState extends State<MyScreen> {  bool isChildButtonPressed = false;  @override  Widget build(BuildContext context) {    return Container(      child: MyCustomWidget(        onFlatButtonPressed: (){          setState(() {            //Write what you want to do here          });        },      ),    );  }}class MyCustomWidget extends StatefulWidget {  final VoidCallback onFlatButtonPressed;  MyCustomWidget({@required this.onFlatButtonPressed});  @override  _MyCustomWidget createState() => _MyCustomWidget();}class _MyCustomWidget extends State<MyCustomWidget> {  @override  Widget build(BuildContext context) {    return Container(      width: 200,      height: 200,      child: Row(        children: [          FlatButton(child: Text("I am child button"), onPressed: widget.onFlatButtonPressed),        ],      ),    );  }}