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);
// Generate a random border radius.
_borderRadius =
BorderRadius.circular(random.nextInt(100).toDouble());
});
},
child: const Icon(Icons.play_arrow),
),
),
);
}
}<code_end>
<topic_end>
<topic_start>Fade a widget in and out
UI developers often need to show and hide elements on screen.
However, quickly popping elements on and off the screen can
feel jarring to end users. Instead,
fade elements in and out with an opacity animation to create
a smooth experience.The AnimatedOpacity widget makes it easy to perform opacity
animations. This recipe uses the following steps:<topic_end>
<topic_start>
1. Create a box to fade in and out
First, create something to fade in and out. For this example,
draw a green box on screen.
<code_start>Container(
width: 200,
height: 200,
color: Colors.green,
)<code_end>
<topic_end>
<topic_start>
2. Define a StatefulWidget
Now that you have a green box to animate,
you need a way to know whether the box should be visible.
To accomplish this, use a StatefulWidget.A StatefulWidget is a class that creates a State object.
The State object holds some data about the app and provides a way to
update that data. When updating the data,
you can also ask Flutter to rebuild the UI with those changes.In this case, you have one piece of data:
a boolean representing whether the button is visible.To construct a StatefulWidget, create two classes: A
StatefulWidget and a corresponding State class.
Pro tip: The Flutter plugins for Android Studio and VSCode include
the stful snippet to quickly generate this code.
<code_start>// The StatefulWidget's job is to take data and create a State class.
// In this case, the widget takes a title, and creates a _MyHomePageState.
class MyHomePage extends StatefulWidget {
final String title;
const MyHomePage({
super.key,
required this.title,
});
@override
State<MyHomePage> createState() => _MyHomePageState();
}
// The State class is responsible for two things: holding some data you can
// update and building the UI using that data.
class _MyHomePageState extends State<MyHomePage> {
// Whether the green box should be visible.
bool _visible = true;
@override
Widget build(BuildContext context) {
// The green box goes here with some other Widgets.
}
}<code_end>
<topic_end>
<topic_start>
3. Display a button that toggles the visibility
Now that you have some data to determine whether the green box
should be visible, you need a way to update that data.
In this example, if the box is visible, hide it.
If the box is hidden, show it.To handle this, display a button. When a user presses the button,
flip the boolean from true to false, or false to true.
Make this change using setState(),
which is a method on the State class.
This tells Flutter to rebuild the widget.For more information on working with user input,
see the Gestures section of the cookbook.
<code_start>FloatingActionButton(
onPressed: () {
// Call setState. This tells Flutter to rebuild the
// UI with the changes.
setState(() {
_visible = !_visible;
});
},
tooltip: 'Toggle Opacity',
child: const Icon(Icons.flip),
)<code_end>
<topic_end>
<topic_start>
4. Fade the box in and out
You have a green box on screen and a button to toggle the visibility
to true or false. How to fade the box in and out? With an
AnimatedOpacity widget.The AnimatedOpacity widget requires three arguments:
<code_start>AnimatedOpacity(
// If the widget is visible, animate to 0.0 (invisible).
// If the widget is hidden, animate to 1.0 (fully visible).
opacity: _visible ? 1.0 : 0.0,
duration: const Duration(milliseconds: 500),
// The green box must be a child of the AnimatedOpacity widget.
child: Container(
width: 200,