Listening to a variable change in flutter
OP already got an answer in the comments. I'm just typing it here so the question is correctly marked as answered.
Just notify the listener:
countdown.reset.notifyListeners();
// OR
countdown.reset.value = true;
Credit to @pskink
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Rajdeep
Updated on August 19, 2020Comments
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Rajdeep almost 4 years
I'm trying to listen to a variable change to execute some code. So the variable is a bool named
reset
. I want to execute something (say reset the animation controller) once the animation ends OR a button (from another widget) is pressed. Executing something when the animation ends works as once it endsAnimationStatus.dismissed
will be its state and the listener will be called. This then allows me to use a callback functiononCountdownexpire
in order to set the variablereset
accordingly and based on what it is set, execute some code in theif(widget.reset)
block. So there is no need to listen for this case.Problem:
However, lets say a button is pressed (in another widget) and I set the variable
reset
to true. I want the animation to stop and reset. I cannot now depend on theAnimationStatus
listener as it only executes when there is a state change. I want it to reset during its state. So I have to somehow listen to the variablewidget.reset
.I have done some research on this and found out that
ValueNotifier
might be a way to go but there are not a lot of examples on how to go about using it. Like how do I go about listening to it ?Code:
class Countdown extends StatefulWidget { final VoidCallback onCountdownExpire; bool reset; Countdown(this.onCountdownExpire); @override CountdownState createState() => CountdownState(); } class CountdownState extends State<Countdown> with TickerProviderStateMixin { AnimationController controller; String get timerString { Duration duration = controller.duration * controller.value; return '${duration.inMinutes}:${(duration.inSeconds % 60).toString()}'; } @override void initState() { super.initState(); controller = AnimationController( vsync: this, duration: Duration(seconds: 2), )..addStatusListener((AnimationStatus status){ if (status == AnimationStatus.dismissed) { debugPrint("Animation.dismissed"); widget.onCountdownExpire(); if (widget.reset) { widget.reset = false; controller.reverse(from: 1.0); } } }); controller.reverse(from: 1.0); } ... // omitted code }
What I have tried but it does not seem to be working as expected:
class Countdown extends StatefulWidget { final VoidCallback onCountdownExpire; Countdown(this.onCountdownExpire); ValueNotifier reset = ValueNotifier(false); @override CountdownState createState() => CountdownState(); } class CountdownState extends State<Countdown> with TickerProviderStateMixin { AnimationController controller; @override void initState() { widget.reset.addListener(() { debugPrint("value notifier is true"); controller.reverse(from: 1.0); }); super.initState(); controller = AnimationController( vsync: this, duration: Duration(seconds: 2), )..addStatusListener((AnimationStatus status){ if (status == AnimationStatus.dismissed) { debugPrint("Animation.dismissed"); widget.onCountdownExpire(); } }); controller.reverse(from: 1.0); } ... // omitted code }
Update: The solution (above) was actually working, I just had to use something like this to notify the listener:
countdown.reset.notifyListeners(); // OR countdown.reset.value = true;
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pskink about 5 yearssorry, i mean
countdown.reset.value = ...
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