Flutter - create Instance of Shared Preferences in class - flutter

Question

I have a class called ThemeUtil
import 'package:shared_preferences/shared_preferences.dart';
class ThemeUtil {
SharedPreferences prefs;
bool initialized = false;
ThemeUtil() {
SharedPreferences.getInstance().then((val) {
prefs = val;
initialized = true;
});
}
bool getBrightness() {
if (initialized) {
try {
return prefs.getBool("dark") ?? false;
} catch (error) {
return false;
}
} else {
return null;
}
}
setBrightness(bool _dark) {
try {
prefs.setBool("dark", _dark);
} catch (error) {
return;
}
}
}
but my problem is when I use it in main.dart it always returns false which results in a white theme becuase shared prefs in ThemeUtil has not been created. Does anyone know how to fix that?

Answer 1

Problem you are facing is called accessing class instance. Each time you are accessing your utils class, you are creating another class instance of that class, where in the constructor of it, you constantly generate asynchronously new SharedPreferences instance. That's why you are always getting false values. What I would propose is to create singleton Utils class, where you always access already assigned prefs.
class LocalStorage {
static final LocalStorage _instance = LocalStorage._privateConstructor();
factory LocalStorage() {
return _instance;
}
SharedPreferences _prefs;
LocalStorage._privateConstructor() {
SharedPreferences.getInstance().then((prefs) {
_prefs = prefs;
});
}
}
And now make sure you initialise this singleton at app launch:
void main() {
LocalStorage();
runApp(MyApp());
}
After initial creation of SharedPreferences object, you can synchronously access it's content anywhere in the app by calling:
LocalStorage().getBrightness();