Mindflow: Flutter deal with Future! in 6 Steps

With a Swift background, Flutter+Dart require a different thought-process. Future<T> is associated with async calls and it could be hard to re-route your brain in a one go. In this article I am sharing my experience on how the mindflow when dealing with Future<T> using SharePreferences as an example, from start to a 1-liner in 6 steps.

1. Direct mindflow:

This is how a method looks like with the consideration of existing of Future<T>, it doesn’t work since SharedPreferences.getInstance() returns Future<SharedPreferences> instead of SharedPreferences.

static int getCount(){
  
  // This will return Future<SharedPreferences>, unwarp is needed. 
  final preferences = SharedPreferences.getInstance(); 
  
  // Future<SharedPreferences> causes error 
  // because SharedPreferences has .getInt()
  // but Future<SharedPreferences> does not have .getInt()
  final value = preferences.getInt("attributeName");

  return value;
}

2. Fix the Future!

To handle Future<SharedPreferences>, add async and await. But the value can be null!

// async is needed to use the keyword await
// async also require Future<int> instead of int
static Future<int> getCount() async {
  
  // Keyword await "unwarp" Future<SharedPreferences> to SharedPreferences
  final preferences = await SharedPreferences.getInstance(); 
  
  // .getInt() works normally now,
  // but null can be return if "attributeName" does not have a value
  final value = preferences.getInt("attributeName");

  return value;
}

3. Fix the null value!

Change the final to var for value to be validated:

// async is needed to use the keyword await
static Future<int> getCount() async {
  
  // Good now
  final preferences = await SharedPreferences.getInstance(); 
  
  // Change final to var so value can be validated
  var value = preferences.getInt("attributeName");

  // Validating value
  if (value == null){
    value = 0;
  }

  return value;
}

4. Shorthand for if …

if … could be a bit blocky, we could replace it with (condition) ? (true) : (false):

static Future<int> getCount() async {
  
  // Shortent this too
  final pref = await SharedPreferences.getInstance(); 
  
  // Replace if with (condition) ? (true) : (false)
  var value = pref.getInt("attributeName") == null ? 0 : pref.getInt("attributeName");
  
  return value;
}

5. Remove more

Future<T> can be stacked with (await Future<T>) to get T within the async method.

static Future<int> getCount() async {

  // Remove this line too
  
  // Replace it with  (await Future<SharedPreferences>) to get SharedPreferences
  var value = (await SharedPreferences.getInstance()).getInt("attributeName") == null ? 0 : (await SharedPreferences.getInstance()).getInt("attributeName");
  
  return value;
}

6. One-liner!

1-liner, just return the value :

static Future<int> getCount() async {
  return (await SharedPreferences.getInstance()).getInt("attributeName") == null ? 0 : (await SharedPreferences.getInstance()).getInt("attributeName");
}