How to call nested conditional void function with onPressed [flutter] - flutter

Question

I have a customized flutter stepper widget and I want to having conditional back button when I press the backbutton in appbar, it will be back on previous step, just like this
onPressed: () {
if (currentStep != 0) {
onStepCancel;
} else {
Navigator.pop(context);
}
},
and somehow onStepCancel is can't be call because it has a value of final VoidCallback? onStepCancel, I put another function inside another function A.K.A nested, I want to use this widget in another class, so it can be simplify by only putting the void function inside onStepCancel
CustomStepper(
. . .
currentStep: controller.currentStep.value,
onStepContinue: controller.increment,
onStepCancel: controller.decrement,
);
the void function that fill with decrement function of currentStep will be proceed inside onStepCancel and when user click on back button with condition currentStep != 0 it will show the previous step and when user is reaching currentStep == 0 it will back to previous page Navigator.pop(context);, but the problem is onStepCancel can't be VoidCallback? because the conditional function is already the void function itself and it can't be returned VoidCallBack? inside it, so how can I call onStepCancel function inside conditional case with onPressed, this is the widget function where I put it:
Widget _buttonBack(int stepIndex, BuildContext context) {
return IconButton(
onPressed: () {
if (currentStep != 0) {
onStepCancel;
} else {
Navigator.pop(context);
}
},
icon: SvgPicture.asset(
Images.backArrowButton,
color: ColorResources.brandHeavy,
),
);
}
and I want to put Widget _buttonBack inside:
Scaffold(
appBar: AppBar(
. . .
leading: _buttonBack(currentStep, context),
. . .
);
edit:
onStepCancel is fullfil with void function that I call from controller:
void decrement() => currentStep--;
void decrement() is the function for getting back to previous Step and I call decrement() function in onStepCancel just like this:
onStepCancel: controller.decrement
Inside my CustomStepper class, onStepCancel will pass decrement to onPressed(), I have try few way to put conditional onPressed(), with this:
onPressed: () => currentStep != 0 ? onStepCancel : Navigator.pop(context),
and this
onPressed: () {
if (currentStep != 0) {
onStepCancel;
} else {
Navigator.pop(context);
}
},
both conditional doesn't work, but the funny thing is when I call onStepCancel without conditional case, like this:
onPressed: onStepCancel
it show no problem and it works well, so the point is, on step cancel will only work well without nested function, I only could call it with onStepCancel without any conditional function, how to call nested function? is it needed to be in any other form instead of function or else?

Answer 1

if onStepCancel is a callback, you should call it instead of returning it:
if (currentStep != 0) {
onStepCancel();
} else {
Navigator.pop(context);
}