DART: How to write timeconsuming function returning a Future

Question

I have to write a timeconsuming function that will return a Future, if it´s ready. Is the approach below correct, or does my timeconsuming algorithm in line 9 block the program til it´s ready. In this case, what would I have to do, to give control back to the eventloop or what else could be a solution?

Future<int> timeconsumingFunctionReturningFuture(int i) {
  var completer = new Completer();
 
  if (i==0) { 
    completer.completeError(88);
    return completer.future;
  } else {
    int rc;
    // Line9: rc = timeconsuming algorithm, to calculate rc
    completer.complete(rc);
    return completer.future;
  }
}

Answer 1

Your code probably wouldn't work as expected as your algorythm may block the returning of the completer. Try it this way:

Future<int> timeconsumingFunctionReturningFuture(int i) {
  var completer = new Completer();

  if (i==0) { 
    completer.completeError(88);
  } else {
    Timer.run(() {
      int rc;
      // Line9: rc = timeconsuming algorithm, to calculate rc
      completer.complete(rc);
    });
  }
  return completer.future;
}

This way your timeconsuming algorithm runs asynchronously and the future is returned immediately.

I have not tried it myself but this shorter version should also work (without creating a completer)

return new Future.delayed(Duration.ZERO, () {
    // timeconsuming algorithm
  });

Answer 2

Günter Zöchbauer's solution is correct, but you can simplify the code:

Future<int> timeconsumingFunctionReturningFuture(int i) {
  if (i == 0) return new Future.error(88);
  return new Future(() {
    int rc;
    // Line9: rc = timeconsuming algorithm, to calculate rc
    return rc;
  });
}

Alternatively you can even put the error-check into the Future:

Future<int> timeconsumingFunctionReturningFuture(int i) {
  return new Future(() {
    if (i == 0) throw 88;
    int rc;
    // Line9: rc = timeconsuming algorithm, to calculate rc
    return rc;
  });
}

Also note: if passing 0 for i is an error (programmer error), then throwing synchronously is usually ok. It would then behave the same way as passing in a string in checked mode.