What is a Future in Dart, and how does it represent a value that will not actually be available until some point later in time?

A Future represents a value or an error that will become available at some point in the future, rather than immediately, and it is Dart's core mechanism for representing the result of an asynchronous operation, such as fetching data from a network server or reading a file from disk, both of which naturally take some amount of time to actually complete, and instead of your code blocking and waiting idly for that entire duration, a Future lets your program continue doing other useful work while that time consuming operation happens in the background.

Future<String> fetchUserName() {
  return Future.delayed(Duration(seconds: 2), () => 'Alex');
}