How to call provider on condition? - flutter

Question

On app homepage I set up Model2 which make API call for data. User can then navigate to other page (Navigator.push). But I want make API call from Model2 when user press back (_onBackPress()) so can refresh data on homepage.
Issue is Model2 is not initialise for all user. But if I call final model2 = Provider.of<Model2>(context, listen: false); for user where Model2 is not initialise, this will give error.
How I can call Provider only on condition? For example: if(user == paid)
StatefulWidget in homepage:
#override
Widget build(BuildContext context) {
return ChangeNotifierProxyProvider<Model1, Model2>(
initialBuilder: (_) => Model2(),
builder: (_, model1, model2) => model2
..string = model1.string,
),
child: Consumer<Model2>(
builder: (context, model2, _) =>
...
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondRoute(context: context)),
In Page2:
Future<void> _onBackPress(context) async {
// if(user == paid)
final model2 = Provider.of<Model2>(context, listen: false);
return showDialog<void>(
context: context,
barrierDismissible: false,
builder: (BuildContext context) {
return
// if(user == paid)
Provider.value(value: model2, child:
AlertDialog(
title: Text('Back'),
content: SingleChildScrollView(
child: ListBody(
children: <Widget>[
Text('Go back'),
],
),
),
actions: <Widget>[
FlatButton(
child: Text('OK'),
onPressed: () async {
// if(user == paid)
await model2.getData();
Navigator.of(context).pop();
},
),
],
),
);
},
);
}
Alternative method (maybe more easy): How to call provider on previous page (homepage) on Navigator.of(context).pop();?
TLDR: What is best solution for call API so can refresh data when user go back to previous page (but only for some user)?

Answer 1

You can wrap your second page interface builder in a WillPopScope widget, and then, pass whatever method you want to call to the onWillPop callback of the WillPopScope widget. This way, you can make your API call when user presses the back button. Find more about the WillPopScope widget on this WillPopScope Flutter dev documentation article.

Answer 2

tldr; Establish and check your single point of truth before the call to the Provider
that may result in a null value or evaluate as a nullable reference.
Perhaps you can change the architecture a bit to establish a single (nullable or bool) reference indicating whether the user has paid. Then use Darts nullability checks (or just a bool) to implement the behavior you want. This differs from your current proposal in that there would be no need to call on the Provider to instantiate the model. Just add a single point of truth to your User object that is initialized to null or false, and then change that logic only when the User has actually paid.
Toggling widgets/behavior in this way could be a solution.
Alternatives considered:
Packaging critical data points into a separate library so that the values can be imported where needed.
Other state management methods for key/value use.
If you want to simply hide/show parts of a page consider using the OffStage class or the Visibility class
Ref
Dart null-checking samples