AnimatedOpacity class - widgets library

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.

link

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.

Constructors

AnimatedOpacity({Key? key, Widget? child, required double opacity, Curve curve = Curves.linear, required Duration duration, VoidCallback? onEnd, bool alwaysIncludeSemantics = false}): Creates a widget that animates its opacity implicitly.
const

Properties

alwaysIncludeSemantics → bool: Whether the semantic information of the children is always included.
final
child → Widget?: The widget below this widget in the tree.
final
curve → Curve: The curve to apply when animating the parameters of this container.
finalinherited
duration → Duration: The duration over which to animate the parameters of this container.
finalinherited
hashCode → int: The hash code for this object.
no setterinherited
key → Key?: Controls how one widget replaces another widget in the tree.
finalinherited
onEnd → VoidCallback?: Called every time an animation completes.
finalinherited
opacity → double: The target opacity.
final
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited

Methods

createElement() → StatefulElement: Creates a StatefulElement to manage this widget's location in the tree.
inherited
createState() → ImplicitlyAnimatedWidgetState<AnimatedOpacity>: Creates the mutable state for this widget at a given location in the tree.
override
debugDescribeChildren() → List<DiagnosticsNode>: Returns a list of DiagnosticsNode objects describing this node's children.
inherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void: Add additional properties associated with the node.
override
noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) → DiagnosticsNode: Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
inherited
toString({DiagnosticLevel minLevel = DiagnosticLevel.info}) → String: A string representation of this object.
inherited
toStringDeep({String prefixLineOne = '', String? prefixOtherLines, DiagnosticLevel minLevel = DiagnosticLevel.debug, int wrapWidth = 65}) → String: Returns a string representation of this node and its descendants.
inherited
toStringShallow({String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String: Returns a one-line detailed description of the object.
inherited
toStringShort() → String: A short, textual description of this widget.
inherited

Operators

operator ==(Object other) → bool: The equality operator.
inherited