Add StreamChannel and MultiChannel.

lib/stream_channel.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+
+export 'src/multi_channel.dart';
+
+/// An abstract class representing a two-way communication channel.
+///
+/// Subclasses are strongly encouraged to mix in or extend [StreamChannelMixin]
+/// to get default implementations of the various instance methods. Adding new
+/// methods to this interface will not be considered a breaking change if
+/// implementations are also added to [StreamChannelMixin].
+abstract class StreamChannel<T> {
+  /// The stream that emits values from the other endpoint.
+  Stream<T> get stream;
+
+  /// The sink for sending values to the other endpoint.
+  StreamSink<T> get sink;
+
+  /// Creates a new [StreamChannel] that communicates over [stream] and [sink].
+  factory StreamChannel(Stream<T> stream, StreamSink<T> sink) =>
+      new _StreamChannel<T>(stream, sink);
+
+  /// Connects [this] to [other], so that any values emitted by either are sent
+  /// directly to the other.
+  void pipe(StreamChannel<T> other);
+}
+
+/// An implementation of [StreamChannel] that simply takes a stream and a sink
+/// as parameters.
+///
+/// This is distinct from [StreamChannel] so that it can use
+/// [StreamChannelMixin].
+class _StreamChannel<T> extends StreamChannelMixin<T> {
+  final Stream<T> stream;
+  final StreamSink<T> sink;
+
+  _StreamChannel(this.stream, this.sink);
+}
+
+/// A mixin that implements the instance methods of [StreamChannel] in terms of
+/// [stream] and [sink].
+abstract class StreamChannelMixin<T> implements StreamChannel<T> {
+  void pipe(StreamChannel<T> other) {
+    stream.pipe(other.sink);
+    other.stream.pipe(sink);
+  }
+}