Dart read from console line by line asynchronously.

readline.dart

import 'dart:async';
import 'dart:convert';
import 'dart:io' as io;

/// Reads a single line from [stdin] asynchronously.
Future<String> readLine() async {
  final c = Completer<String>(); // completer
  final l = io.stdin // stdin
      .transform(utf8.decoder) // decode
      .transform(const LineSplitter()) // split line
      .asBroadcastStream() // make it stream
      .listen((line) => !c.isCompleted ? c.complete(line) : 0); // listen

  final o = await c.future; // get output from future
  l.cancel(); // cancel stream after future is completed
  return o;
}

For those who have stumbled upon this from google, package:async saves the day:

var _stdinLines = StreamQueue(LineSplitter().bind(Utf8Decoder().bind(stdin)));

Future<String> readLine([String? query]) async {
 if (query != null) stdout.write(query);
 return _stdinLines.next;
}

What works for me at Dart SDK version: 2.19.4 (stable):

class ReadLineAsync {
  static final stream = stdin
      .asBroadcastStream()
      .transform(utf8.decoder)
      .transform(const LineSplitter());

  static Future<String> readLineAsync() async {
    final completer = Completer<String>();
    late StreamSubscription<String> subscription;
    subscription = stream.listen((line) async {
      await subscription.cancel();
      if (!completer.isCompleted) completer.complete(line);
    });

    return completer.future;
  }
}

In the original example subsequent invocations of the readLine() function will cause Bad state: Stream has already been listened to. error to be thrown. It seems it is not possible to cancel a subscription to stdin. This code also throws the same error (on both Linux and Windows):

import 'dart:io';

Future<void> main(List<String> arguments) async {
  final s1 = stdin.listen((event) {});
  await s1.cancel();
  final s2 = stdin.listen((event) {});
  await s2.cancel();
}