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super.key,
required this.text,
});
final String text;
@override
Widget build(BuildContext context) {
return Center(
child: Text(
text,
textDirection: TextDirection.ltr,
),
);
}
}<code_end>
The previous example uses the constructor of the MyStatelessWidget
class to pass the text, which is marked as final.
This class extends StatelessWidget—it contains immutable data.The build method of a stateless widget is typically called
in only three situations:<topic_end>
<topic_start>
The StatefulWidget
A StatefulWidget is a widget that changes state.
Use the setState method to manage the
state changes for a StatefulWidget.
A call to setState() tells the Flutter
framework that something has changed in a state,
which causes an app to rerun the build() method
so that the app can reflect the change.State is information that can be read synchronously when a widget
is built and might change during the lifetime of the widget.
It’s the responsibility of the widget implementer to ensure that
the state object is promptly notified when the state changes.
Use StatefulWidget when a widget can change dynamically.
For example, the state of the widget changes by typing into a form,
or moving a slider.
Or, it can change over time—perhaps a data feed updates the UI.Checkbox, Radio, Slider, InkWell,
Form, and TextField
are examples of stateful widgets that subclass
StatefulWidget.The following example declares a StatefulWidget
that requires a createState() method.
This method creates the state object that manages the widget’s state,
_MyStatefulWidgetState.
<code_start>class MyStatefulWidget extends StatefulWidget {
const MyStatefulWidget({
super.key,
required this.title,
});
final String title;
@override
State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}<code_end>
The following state class, _MyStatefulWidgetState,
implements the build() method for the widget.
When the state changes, for example, when the user toggles
the button, setState() is called with the new toggle value.
This causes the framework to rebuild this widget in the UI.
<code_start>class _MyStatefulWidgetState extends State<MyStatefulWidget> {
bool showText = true;
bool toggleState = true;
Timer? t2;
void toggleBlinkState() {
setState(() {
toggleState = !toggleState;
});
if (!toggleState) {
t2 = Timer.periodic(const Duration(milliseconds: 1000), (t) {
toggleShowText();
});
} else {
t2?.cancel();
}
}
void toggleShowText() {
setState(() {
showText = !showText;
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Column(
children: <Widget>[
if (showText)
const Text(
'This execution will be done before you can blink.',
),
Padding(
padding: const EdgeInsets.only(top: 70),
child: ElevatedButton(
onPressed: toggleBlinkState,
child: toggleState
? const Text('Blink')
: const Text('Stop Blinking'),
),
),
],
),
),
);
}
}<code_end>