Flutter provider in initState - flutter

Question

I'm currently trying Provider as a state management solution, and I understand that it can't be used inside the initState function.
All examples that I've seen call a method inside a derived ChangeNotifier class upon user action (user clicks a button, for example), but what if I need to call a method when initialising my state?
Motivation:
Creating a screen which loads assets (async) and shows progress
An example for the ChangeNotifier class (can't call add from initState):
import 'package:flutter/foundation.dart';
class ProgressData extends ChangeNotifier {
double _progress = 0;
double get progress => _progress;
void add(double dProgress) {
_progress += dProgress;
notifyListeners();
}
}

Answer 1

You can call such methods from the constructor of your ChangeNotifier:
class MyNotifier with ChangeNotifier {
MyNotifier() {
someMethod();
}
void someMethod() {
// TODO: do something
}
}

Answer 2

Change your code to this
class ProgressData extends ChangeNotifier {
double _progress = 0;
double get progress => _progress;
void add(double dProgress) async {
// Loading Assets maybe async process with its network call, etc.
_progress += dProgress;
notifyListeners();
}
ProgressData() {
add();
}
}

Answer 3

In initState all the of(context) things don't work correctly, because the widget is not fully wired up with every thing in initState.
You can use this code:
Provider.of<ProgressData>(context, listen: false).add(progress)
Or this code:
Future.delayed(Duration.zero).then(_){
Provider.of<ProgressData>(context).add(progress)
}):

Answer 4

So an AssetLoader class which reports on its progress will look something like this, I guess:
import 'package:flutter/foundation.dart';
class ProgressData extends ChangeNotifier {
double _progress = 0;
ProgressData() {
_loadFake();
}
Future<void> _loadFake() async {
await _delayed(true, Duration(seconds: 1));
_add(1.0);
await _delayed(true, Duration(seconds: 2));
_add(2.0);
await _delayed(true, Duration(seconds: 3));
_add(3.0);
}
// progress
double get progress => _progress;
// add
void _add(double dProgress) {
_progress += dProgress;
notifyListeners();
}
// _delayed
Future<dynamic> _delayed(dynamic returnVal, Duration duration) {
return Future.delayed(duration, () => returnVal);
}
}

Answer 5

As Fateme said:
the widget is not fully wired up with everything in initState
Also, you can use something like this in your initState
WidgetsBinding.instance?.addPostFrameCallback((timeStamp) {
Provider.of<ProgressData>(context, listen: false).add(5);
});
I think it's more standard!
Be aware that you should use the correct context! I mean the context of the Builder!

Answer 6

The problem here lies with the fact that context does not exist yet in initState as extensively explained by the other answers. It doesn't exist because it hasn't yet been made a part of the widget tree.
Calling a method
If you're not assigning any state and only calling a method then initState would be the best place to get this done.
// The key here is the listen: false
Provider.of<MyProvider>(context, listen: false).mymethod();
The code above is allowed by Flutter because it doesn't have to listen for anything. In short, it's a one off. Use it where you only want to do something instead of read/listen to something.
Listening to changes
Alternatively, if you need to listen to changes from Provider then the use of didChangeDependencies would be the best place to do so as context would exist here as in the docs.
This method is also called immediately after initState.
int? myState;
#override
void didChangeDependencies() {
// No listen: false
myState = Provider.of<MyProvider>(context).data;
super.didChangeDependencies();
}
If you've never used didChangeDependencies before, what it does is get called whenever updateShouldNotify() returns true. This in turn lets any widgets that requested an inherited widget in build() respond as needed.
I'd usually use this method in a FutureBuilder to prevent reloading data when data already exists in Provider after switching screens. This way I can just check Provider for myState and skip the preloader (if any) entirely.
Hope this helps.