AnimatedOpacity class

Animated version of Opacity which automatically transitions the child's opacity over a given duration whenever the given opacity changes.

Animating an opacity is relatively expensive because it requires painting the child into an intermediate buffer.

Here's an illustration of what using this widget looks like, using a curve of Curves.fastOutSlowIn.

class LogoFade extends StatefulWidget {
  const LogoFade({super.key});

  @override
  State<LogoFade> createState() => LogoFadeState();
}

class LogoFadeState extends State<LogoFade> {
  double opacityLevel = 1.0;

  void _changeOpacity() {
    setState(() => opacityLevel = opacityLevel == 0 ? 1.0 : 0.0);
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        AnimatedOpacity(
          opacity: opacityLevel,
          duration: const Duration(seconds: 3),
          child: const FlutterLogo(),
        ),
        ElevatedButton(
          onPressed: _changeOpacity,
          child: const Text('Fade Logo'),
        ),
      ],
    );
  }
}

Hit testing

Setting the opacity to zero does not prevent hit testing from being applied to the descendants of the AnimatedOpacity widget. This can be confusing for the user, who may not see anything, and may believe the area of the interface where the AnimatedOpacity is hiding a widget to be non-interactive.

With certain widgets, such as Flow, that compute their positions only when they are painted, this can actually lead to bugs (from unexpected geometry to exceptions), because those widgets are not painted by the AnimatedOpacity widget at all when the opacity animation reaches zero.

To avoid such problems, it is generally a good idea to use an IgnorePointer widget when setting the opacity to zero. This prevents interactions with any children in the subtree when the child is animating away.

See also:

• AnimatedCrossFade, for fading between two children.
• AnimatedSwitcher, for fading between many children in sequence.
• FadeTransition, an explicitly animated version of this widget, where an Animation is provided by the caller instead of being built in.
• SliverAnimatedOpacity, for automatically transitioning a sliver's opacity over a given duration whenever the given opacity changes.