example.dart

class Data {
  /// This class holds all the data from the API
  final String a;
  final int b;

  Data.fromJson(Map<String, dynamic> data) : a = data['a'], b = data['b'];
}

class UIProperty {
  /// This class is used inside a widget in order to display a value with a label and later be able to update it
  final String key;
  final String label;
  
  // Would like to type it as String | int, but doesn't seem like that is possible
  dynamic value;

  UIProperty({@required this.key, @required this.label, this.value});
}

Future<Data> getData() {
  /// Mock API
  return Future(() => Data.fromJson({'a': 'a', 'b': 2}));
}

void createPropertyList() {
  /// This is probably run inside a bloc or a service
  List<UIProperty> list = [
    UIProperty(key: 'a', label: 'Label A'),
    UIProperty(key: 'b', label: 'Label B'),
  ];

  getData().then((data) {
    // This is pretty shitty code
    list[0].value = data.a;
    list[1].value = data.b;
  });
}