rxdart/lib/src/transformers/start_with.dart

import 'dart:async';

/// Prepends a value to the source Stream.
///
/// ### Example
///
///     new Stream.fromIterable([2])
///       .transform(new StartWithStreamTransformer(1))
///       .listen(print); // prints 1, 2
class StartWithStreamTransformer<T> extends StreamTransformerBase<T, T> {
  final StreamTransformer<T, T> transformer;

  StartWithStreamTransformer(T startValue)
      : transformer = _buildTransformer(startValue);

  @override
  Stream<T> bind(Stream<T> stream) => transformer.bind(stream);

  static StreamTransformer<T, T> _buildTransformer<T>(T startValue) {
    return StreamTransformer<T, T>((Stream<T> input, bool cancelOnError) {
      StreamController<T> controller;
      StreamSubscription<T> subscription;

      controller = StreamController<T>(
          sync: true,
          onListen: () {
            try {
              controller.add(startValue);
            } catch (e, s) {
              controller.addError(e, s);
            }

            subscription = input.listen(controller.add,
                onError: controller.addError,
                onDone: controller.close,
                cancelOnError: cancelOnError);
          },
          onPause: ([Future<dynamic> resumeSignal]) =>
              subscription.pause(resumeSignal),
          onResume: () => subscription.resume(),
          onCancel: () => subscription.cancel());

      return controller.stream.listen(null);
    });
  }
}