Add StreamChannelTransformer.

lib/src/stream_channel_transformer.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';
+import 'dart:convert';
+
+import 'package:async/async.dart';
+
+import '../stream_channel.dart';
+
+/// A [StreamChannelTransformer] transforms the events being passed to and
+/// emitted by a [StreamChannel].
+///
+/// This works on the same principle as [StreamTransformer] and
+/// [StreamSinkTransformer]. Each transformer defines a [bind] method that takes
+/// in the original [StreamChannel] and returns the transformed version.
+///
+/// Transformers must be able to have `bind` called multiple times.
+class StreamChannelTransformer<S, T> {
+  /// The transformer to use on the channel's stream.
+  final StreamTransformer _streamTransformer;
+
+  /// The transformer to use on the channel's sink.
+  final StreamSinkTransformer _sinkTransformer;
+
+  /// Creates a [StreamChannelTransformer] from existing stream and sink
+  /// transformers.
+  const StreamChannelTransformer(
+      this._streamTransformer, this._sinkTransformer);
+
+  /// Creates a [StreamChannelTransformer] from a codec's encoder and decoder.
+  ///
+  /// All input to the inner channel's sink is encoded using [Codec.encoder],
+  /// and all output from its stream is decoded using [Codec.decoder].
+  StreamChannelTransformer.fromCodec(Codec<S, T> codec)
+      : this(
+          codec.decoder,
+          new StreamSinkTransformer.fromStreamTransformer(codec.encoder));
+
+  /// Transforms the events sent to and emitted by [channel].
+  ///
+  /// Creates a new channel. When events are passed to the returned channel's
+  /// sink, the transformer will transform them and pass the transformed
+  /// versions to `channel.sink`. When events are emitted from the
+  /// `channel.straem`, the transformer will transform them and pass the
+  /// transformed versions to the returned channel's stream.
+  StreamChannel<S> bind(StreamChannel<T> channel) =>
+      new StreamChannel<S>(
+          channel.stream.transform(_streamTransformer),
+          _sinkTransformer.bind(channel.sink));
+}