Flutter: shared preferences - dart

Question

I have this function:
Future<String> load(SharedPreferences prefs, String fileName) async {
prefs = await SharedPreferences.getInstance();
String jsonString = prefs.getString(fileName) ?? "";
if (jsonString.isNotEmpty) {
return jsonString;
}else{
return ...
}
}
What should I return in the else case? I tried with "" but it doesn't work.

Answer 1

Shared Preferences
In Flutter, Shared Preferences are used to store primitive data (int, double, bool, string, and stringList). This data is associated with the app, so when the user uninstalls your app, the data will also be deleted.
Get the plugin
The shared_preferences plugin from pub is a wrapper around Android SharedPreferences and iOS NSUserDefaults. You can get this plugin by adding the shared_preferences line to your pubspec.yaml file in the dependencies section.
dependencies:
shared_preferences: '>=0.5.12+2 <2.0.0'
You can change the version number to whatever the current one is, but anything less than 2.0 should be compatible.
Import the package
In whichever file you need the Shared Preferences, add the following import:
import 'package:shared_preferences/shared_preferences.dart';
Reading and writing data
To get the shared preferences object you can do the following:
final prefs = await SharedPreferences.getInstance();
This will be used for all of the following examples.
int
read: final myInt = prefs.getInt('my_int_key') ?? 0;
write: prefs.setInt('my_int_key', 42);
double
read: final myDouble = prefs.getDouble('my_double_key') ?? 0.0;
write: prefs.setDouble('my_double_key', 3.14);
bool
read: final myBool = prefs.getBool('my_bool_key') ?? false;
write: prefs.setBool('my_bool_key', true);
string
read: final myString = prefs.getString('my_string_key') ?? '';
write: prefs.setString('my_string_key', 'hello');
stringList
read: final myStringList = prefs.getStringList('my_string_list_key') ?? [];
write: prefs.setStringList('my_string_list_key', ['horse', 'cow', 'sheep']);
Removing data
You can remove any saved data by supplying the key name:
prefs.remove('my_int_key');
I rarely find a need to do that, though. I just overwrite the old data or ignore it. You shouldn't store any sensitive data in Shared Preferences.
See also
Shared Preferences Service in Flutter for Code Maintainability
Documentation: Storing key-value data on disk
What are the ?? double question marks in Dart?
How to create an empty list in Dart

Answer 2

The answer is "it depends". Namely, it depends on what exactly you are doing with the result of this function, and what a good empty default value means in that context.
Assuming you're decoding the returned JSON string into a Map<String, dynamic>, then a good default value might be the empty map. In that case, you could reformulate your function as follows:
Future<String> loadJSON(final String fileName) async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
final String jsonString = prefs.getString(fileName);
if (jsonString != null && jsonString.isNotEmpty) {
return jsonString;
}
return "{}"; // default value
}
final String jsonString = await loadJSON("test.json");
final Map<String, dynamic> jsonData = json.decode(jsonString);
However, it probably makes more sense to reformulate this procedure as a slightly higher-level function returning actual map values:
Future<Map<String, dynamic>> loadData(final String fileName) async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
final String jsonString = prefs.getString(fileName);
if (jsonString != null && jsonString.isNotEmpty) {
return json.decode(jsonString);
}
return Map(); // default value
}
final Map<String, dynamic> jsonData = await loadData("test.json");