List.map returns List<Future> - flutter

Question

I got a class for handling SharePreferences
class SharedPreferencesUtils {
static Future<String> getSharedPreference(String key) async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
return prefs.getString(key);
}
}
I try to use this class from another class to get all my sharedPreferences with this method:
void getAllPrefs() async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
var keyList = prefs.getKeys().toList();
var valueList = keyList.map((key) async {
String value = await SharedPreferencesUtils.getSharedPreference(key);
return value;
}).toList();
print("KEY LIST IS $keyList");
print("VALUE LIST IS $valueList");
}
And, while the keyList works well, the valueList just returns:
VALUE LIST IS [Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>', Instance of 'Future<String>']
I don't really get why I am not getting the actual String value corresponding to the key, as I understood Futures, in this case, the execution should await until the var value gets the String value that I am asking for.....am I wrong?
Note: there are values stored in SharedPreferences, that is for sure.

Answer 1

This is a good one :)
I will only need to mention one key concept and you will see why this is happening: Any async function returns a Future.
In your case, the map call uses an async callback and hence the values in your lists are Futures.
There is a helper in the Future class: Future.wait
You can simply pass your Iterable to it and it will return a list with resolved futures:
Future<void> getAllPrefs() async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
final keyList = prefs.getKeys().toList();
final valueList = await Future.wait(keyList.map((key) async {
String value = await SharedPreferencesUtils.getSharedPreference(key);
return value;
}));
print("KEY LIST IS $keyList");
print("VALUE LIST IS $valueList");
}
How do you do it without the helper? Well, not use map because it requires a callback, but you need to stay in the same scope if you want to get rid of Future values as any outside function would need to be async. So here you go:
Future<void> getAllPrefs() async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
final keyList = prefs.getKeys().toList();
final valueList = List<String>(keyList.length);
for (int i = 0; i < valueList.length; i++)
valueList[i] = await SharedPreferencesUtils.getSharedPreference(keyList[i]);
print("KEY LIST IS $keyList");
print("VALUE LIST IS $valueList");
}
One practice that helps you to remember that all async functions return futures is using Future<void> as the return type instead.
If you use Future.wait, i.e. still use your map call, you can make it a lot more concise like this:
await Future.wait(keyList.map(SharedPreferencesUtils.getSharedPreferences));
You can just use keyList.map(SharedPreferencesUtils.getSharedPreferences) because getSharedPreferences already takes a String and returns a Future<String>, which is equivalent to what you were doing before :)