Why does an asynchronous function not return the specified type immediately?

An asynchronous function in Flutter/Dart does not immediately return the specified type because it performs an operation that may take time (e.g., a network request, file reading). Instead of blocking the execution thread until the operation completes, an asynchronous function immediately returns a Future.

A Future is an object representing a potential value or error that will be available in the future. It acts as a promise that the operation will be completed, and once it is, the result will be either the value of the specified type or an error.

Using Future allows the main execution thread to continue processing other tasks without waiting for the asynchronous operation to finish. When the operation completes, the Future notifies about it, and registered code (e.g., via .then() or await) can handle the result.

Example:

Future<String> fetchData() async {
  // Simulate a long operation
  await Future.delayed(Duration(seconds: 2));
  return "Data received!";
}

void main() {
  print("Start");
  fetchData().then((data) {
    print(data); // Executes after the Future completes
  });
  print("End"); // Executes immediately, without waiting for fetchData
}

The output of this code will be:

Start
End
Data received!

This demonstrates non-blocking behavior. If fetchData returned a String immediately, the program would block for 2 seconds before printing "End". Future allows avoiding such blocking.