Triggering page controller animation from ViewModel - flutter

Question

i'm trying to implement a View with a PageView widget, that is being controlled by the ViewModel.
In my view i have buttons that trigger the following method in the ViewModel:
#override
nextPage() {
if (_index < 4) {
_index++;
increaseProgress();
currentIndex.add(_index);
}
}
currentIndex is just a sink of my _currentIndexController, and outputCurrentIndex is a stream of the controller:
#override
Sink get currentIndex => _currentIndexController.sink;
#override
Stream<int> get outputCurrentIndex => _currentIndexController.stream.map((currentIndex) => currentIndex);
it looks like the value is added to the stream successfully, but i can't get it to trigger changing pages, i've set up this listener in the initState() method of the view:
_viewModel.outputCurrentIndex.listen((index) {
_pageController.animateToPage(index, duration: const Duration(milliseconds: 1000), curve: Curves.ease);
});
but it is not triggered for some reason.. what am i doing wrong?
here is a full code of my View:
class RegisterView extends StatefulWidget {
const RegisterView({Key? key}) : super(key: key);
#override
_RegisterViewState createState() => _RegisterViewState();
}
class _RegisterViewState extends State<RegisterView> {
final RegisterViewModel _viewModel = getIt<RegisterViewModel>();
final PageController _pageController = PageController(initialPage: 0);
final FixedExtentScrollController _weightScrollController = FixedExtentScrollController(initialItem: 80);
final FixedExtentScrollController _ageScrollController = FixedExtentScrollController(initialItem: 13);
final FixedExtentScrollController _heightScrollController = FixedExtentScrollController(initialItem: 13);
#override
void initState() {
_bind();
super.initState();
}
#override
void dispose(){
_viewModel.dispose();
super.dispose();
}
_bind() {
_viewModel.start();
_viewModel.outputCurrentIndex.listen((index) {
_pageController.animateToPage(index, duration: const Duration(milliseconds: 1000), curve: Curves.ease);
});
}
#override
Widget build(BuildContext context) {
List<Widget> pagesList = [
const SexPage(),
AgePage(
scrollController: _ageScrollController,
),
WeightPage(scrollController: _weightScrollController),
HeightPage(scrollController: _heightScrollController),
];
return MultiProvider(
providers: [
StreamProvider.value(value: _viewModel.outputProgress, initialData: 0.25),
StreamProvider.value(value: _viewModel.outputCurrentIndex, initialData: 0),
],
child: Scaffold(
backgroundColor: ColorManager.backgroundColor,
appBar: AppBar(
systemOverlayStyle: SystemUiOverlayStyle(
statusBarColor: ColorManager.backgroundColor,
statusBarBrightness: Brightness.dark,
statusBarIconBrightness: Brightness.dark,
),
centerTitle: true,
title: AppBarWidget(),
elevation: AppSize.s0,
),
body: PageView(
reverse: true,
controller: _pageController,
physics: NeverScrollableScrollPhysics(),
children: [...pagesList],
),
),
);
}
}
class AppBarWidget extends StatefulWidget {
const AppBarWidget({
Key? key,
}) : super(key: key);
#override
State<AppBarWidget> createState() => _AppBarWidgetState();
}
class _AppBarWidgetState extends State<AppBarWidget> {
final RegisterViewModel _viewModel = getIt<RegisterViewModel>();
#override
Widget build(BuildContext context) {
return Row(
mainAxisAlignment: MainAxisAlignment.center,
mainAxisSize: MainAxisSize.max,
children: [
Expanded(
flex: 1,
child: InkWell(
child: Text(
AppStrings.skip,
style: Theme.of(context).textTheme.labelMedium,
),
onTap: () => _viewModel.nextPage(),
),
),
Expanded(
flex: 4,
child: Padding(
padding: const EdgeInsets.symmetric(horizontal: AppPadding.p60),
child: Transform(
alignment: Alignment.center,
transform: Matrix4.rotationY(pi),
child: Progresso(
progress: Provider.of<double>(context),
progressStrokeCap: StrokeCap.round,
backgroundStrokeCap: StrokeCap.round,
progressColor: ColorManager.primary,
backgroundColor: ColorManager.progressBarBackgroundGrey,
progressStrokeWidth: 10.0,
backgroundStrokeWidth: 10.0,
),
),
),
),
Expanded(
flex: 1,
child: Provider.of<int>(context) > 1
? InkWell(
child: Row(
children: [
Text(
AppStrings.back,
style: Theme.of(context).textTheme.labelMedium,
),
Icon(
Icons.arrow_forward_ios,
color: ColorManager.subtitleGrey,
),
],
),
onTap: () => _viewModel.previousPage(),
)
: Container(),
),
],
);
}
}
and my ViewModel:
class RegisterViewModel extends BaseViewModel with RegisterViewModelInputs, RegisterViewModelOutputs {
final StreamController _progressBarController = StreamController<double>.broadcast();
final StreamController _currentIndexController = StreamController<int>.broadcast();
final StreamController _isBackEnabled = StreamController<bool>.broadcast();
double _progress = 0.25;
int _index = 0;
#override
void dispose() {
_progressBarController.close();
_currentIndexController.close();
_isBackEnabled.close();
}
#override
void start() {
// TODO: implement start
}
#override
Sink get currentIndex => _currentIndexController.sink;
#override
Stream<int> get outputCurrentIndex => _currentIndexController.stream.map((currentIndex) => currentIndex);
#override
Stream<double> get outputProgress => _progressBarController.stream.map((progress) => progress);
#override
Sink get progress => _progressBarController.sink;
#override
setCurrentIndex(int index) {
currentIndex.add(index);
}
#override
increaseProgress() {
if (_progress <= 1.0) {
_progress += 0.25;
progress.add(_progress);
}
}
#override
decreaseProgress() {
if (_progress > 0) {
_progress -= 0.25;
progress.add(_progress);
}
}
#override
Sink get isBackEnabled => _isBackEnabled.sink;
#override
Stream<bool> get outputIsBackEnabled => outputIsBackEnabled.map((isEnabled) => isEnabled);
#override
setIsBackEnabled(int index) {
_isBackEnabled.add(index > 0 ? true : false);
}
#override
nextPage() {
if (_index < 4) {
_index++;
increaseProgress();
setCurrentIndex(_index);
}
}
#override
previousPage() {
if (_index > 0) {
_index--;
decreaseProgress();
setCurrentIndex(_index);
}
}
}
abstract class RegisterViewModelInputs {
register();
increaseProgress();
decreaseProgress();
nextPage();
previousPage();
setIsBackEnabled(int index);
Sink get currentIndex;
Sink get isBackEnabled;
Sink get progress;
}
abstract class RegisterViewModelOutputs {
Stream<int> get outputCurrentIndex;
Stream<double> get outputProgress;
Stream<bool> get outputIsBackEnabled;
}
EDIT: I have moved the stream listener to the _AppBarWidgetState build method, and it seems to work now, but i don't fully understand why it hasn't worked before..
is it because the PageController wasn't assigned to a view yet? where is the correct place for the listener? it doesn't makes sense to me for it to be in a child widget.