Stateful Widgets

Widget which can change also after the creation, allowing the User Interactivity.
To save state and respond to user interaction and events, we use StatefulWidget.

Creating StatefulWidget 

class MyStatefulWidget extends StatefulWidget{
  final String text;   MyStatefulWidget({
    this.text,     });  @override  createState() => _MyStatefulWidgetState();}

class _MyStatefulWidgetState extends State<MyStatefulWidget>{
  var color = Colors.purple;  @override  Widget build(BuildContext context) {
    // TODO: implement build    return Container(
      child: Text(widget.text),      color: color,    );  }
}

While the StatefulWidget instance itself is also immutable, it create a state object using createState.
The State Object stores the WidgetState and can change throughout the widgets lifetime. The text is passed in during instantiation and is not changed.
The color which is in the state object can be changed.
Note * State object is _MyStatefulWidgetState class
our convention of using the underscore indicates that the class is private. Since the only widget that has access to it is the MyStatefulWidget class
For consistency we give the state objects the same name as the StatefulWidget's name.

State Object 

 It contains a set state function that you call each time you want to change any state.
Similarly, you can always override other methods in StatefulWidget such as dispose.
initState()
dispose()
or only setState() can be called