How to initialize SharedPreferences using a class constructor?-CodePudding

I woudl like to initialize a private SharedPreferences field and seems a good place for it is using a constructor. But as long as SharedPreferences.getInstance() returns a Future a compiler can not use await because of The await expression can only be used in an async function.. Otherwise it is impossible to make the constructor body as async. So how to make it?

GetIt getIt = GetIt.instance;

void setupLocator() {
  getIt.registerSingleton<LocalStorageService>(LocalStorageService());
}

class LocalStorageService {
  SharedPreferences _prefs;
  LocalStorageService() : _prefs = await SharedPreferences.getInstance(); //error
  ...
}

It is possible to make an additional function to call the setupLocator() and make it async and then use it inside main() method awaiting a result and passing an instance of SharedPreferences into constructor of LocalStorageService as an argument, but I would prefer to initialize it inside the LocalStorageService class.

CodePudding user response：

You should check this question: Calling an async method from component constructor in Dart

I think you can do something like this - just keep in mind your _prefs would not be available immediately after the object is created.

GetIt getIt = GetIt.instance;

void setupLocator() {
  getIt.registerSingleton<LocalStorageService>(LocalStorageService());
}

class LocalStorageService {
  SharedPreferences _prefs;
  LocalStorageService() : SharedPreferences.getInstance().then((prefs) => _prefs=prefs); 
  ...
}

CodePudding user response：

Since you are using GetIt, I suggest you to use registerSingletonAsync method

  getIt.registerSingletonAsync<LocalStorageService>(() async {
    final pref = await SharedPreferences.getInstance();
    return LocalStorageService(pref);
  });

I personally don't like creating it in a constructor since maybe another class is dependent on it and it will get a null variable before constructing and it will cause problems.