ListenEffect class

An effect that calls a callback function with its current animation value between begin and end.

By default, the callback will only be called while this effect is "active" (ie. after delay, and before duration) and will return a value between 0-1 (unless the curve transforms it beyond this range). If clamp is set to false, the callback will be called on every tick while the enclosing Animate is running, and may return values outside its nominal range (ex. it will return a negative value before delay).

This example will print the current animation value (which matches the value of the preceding fade effect's opacity value):

Text("Hello").animate().fadeIn(curve: Curves.easeOutExpo)
 .listen(callback: (value) => print('current opacity: $value'))

This can easily be used to drive a ValueNotifier:

ValueNotifier<double> notifier = ValueNotifier<double>(0);
Text("Hello").animate()
  .fadeIn(delay: 400.ms, duration: 900.ms)
  .listen(callback: (value) => notifier.value)

See also: CustomEffect and CallbackEffect.

Inheritance
Annotations

Constructors

ListenEffect({Duration? delay, Duration? duration, Curve? curve, double? begin, double? end, required ValueChanged<double> callback, bool clamp = true})
const

Properties

begin → double?
The begin value for the effect. If null, effects should use a reasonable default value when appropriate.
finalinherited
callback → ValueChanged<double>
final
clamp → bool
final
curve → Curve?
The specified easing curve for the effect. If null, will inherit the curve from the previous effect, or use Animate.defaultCurve if this is the first effect.
finalinherited
delay → Duration?
The specified delay for the effect. If null, will inherit the delay from the previous effect, or use Duration.zero if this is the first effect.
finalinherited
duration → Duration?
The specified duration for the effect. If null, will inherit the duration from the previous effect, or use Animate.defaultDuration if this is the first effect.
finalinherited
end → double?
The end value for the effect. If null, effects should use a reasonable default value when appropriate.
finalinherited
hashCode → int
The hash code for this object.
no setterinherited
runtimeType → Type
A representation of the runtime type of the object.
no setterinherited

Methods

build(BuildContext context, Widget child, AnimationController controller, EffectEntry entry) → Widget
Builds the widgets that implement the effect on the target child, based on the provided AnimationController and EffectEntry.
override
buildAnimation(AnimationController controller, EffectEntry entry) → Animation<double>
Returns an animation based on the controller, entry, and begin/end values.
inherited
getBeginRatio(AnimationController controller, EffectEntry entry) → double
Returns a ratio corresponding to the beginning of the specified entry.
inherited
getEndRatio(AnimationController controller, EffectEntry entry) → double
Returns a ratio corresponding to the end of the specified entry.
inherited
getOptimizedBuilder<U>({required ValueListenable<U> animation, Widget? child, required TransitionBuilder builder}) → AnimatedBuilder
Returns an optimized AnimatedBuilder that doesn't rebuild if the value hasn't changed.
inherited
getToggleBuilder({required ValueListenable<double> animation, required Widget child, required bool toggle(), required ToggleEffectBuilder builder}) → AnimatedBuilder
Returns an AnimatedBuilder that rebuilds when the boolean value returned by the toggle function changes.
inherited
isAnimationActive(Animation animation) → bool
Check if the animation is currently running / active.
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
inherited
toString() → String
A string representation of this object.
inherited

Operators

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