Add a StreamSinkTransformer class.

lib/src/stream_sink_transformer/handler_transformer.dart

Index: lib/src/stream_sink_transformer/handler_transformer.dart
diff --git a/lib/src/stream_sink_transformer/handler_transformer.dart b/lib/src/stream_sink_transformer/handler_transformer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e5d8e3c62fd40740b289c0f0d756b10b554e0426
--- /dev/null
+++ b/lib/src/stream_sink_transformer/handler_transformer.dart
@@ -0,0 +1,107 @@
+// 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.
+
+library async.stream_sink_transformer.handler_transformer;
+
+import 'dart:async';
+
+import '../stream_sink_transformer.dart';
+import '../delegate/stream_sink.dart';
+
+/// The type of the callback for handling data events.
+typedef void HandleData<S, T>(S data, EventSink<T> sink);
+
+/// The type of the callback for handling error events.
+typedef void HandleError<T>(
+    Object error, StackTrace stackTrace, EventSink<T> sink);
+
+/// The type of the callback for handling done events.
+typedef void HandleDone<T>(EventSink<T> sink);
+
+/// A [StreamSinkTransformer] that delegates events to the given handlers.
+class HandlerTransformer<S, T> implements StreamSinkTransformer<S, T> {
+  /// The handler for data events.
+  final HandleData<S, T> _handleData;
+
+  /// The handler for error events.
+  final HandleError<T> _handleError;
+
+  /// The handler for done events.
+  final HandleDone<T> _handleDone;
+
+  HandlerTransformer(
+      this._handleData, this._handleError, this._handleDone);
+
+  StreamSink<S> bind(StreamSink<T> sink) => new _HandlerSink<S, T>(this, sink);
+}
+
+/// A sink created by [HandlerTransformer].
+class _HandlerSink<S, T> implements StreamSink<S> {
+  /// The transformer that created this sink.
+  final HandlerTransformer<S, T> _transformer;
+
+  /// The original sink that's being transformed.
+  final StreamSink<T> _inner;
+
+  /// The wrapper for [_inner] whose [StreamSink.close] method can't emit
+  /// errors.
+  final StreamSink<T> _safeCloseInner;
+
+  Future get done => _inner.done;
+
+  _HandlerSink(this._transformer, StreamSink<T> inner)
+      : _inner = inner,
+        _safeCloseInner = new _SafeCloseSink<T>(inner);
+
+  void add(S event) {
+    if (_transformer._handleData == null) {
+      // [event] is an S and [_inner.add] takes a T. This style of conversion
+      // will throw an error in checked mode if [_inner] is actually a
+      // [StreamSink<T>], but will work if [_inner] isn't reified and won't add
+      // an extra check in unchecked mode.
+      _inner.add(event as dynamic);
+    } else {
+      _transformer._handleData(event, _safeCloseInner);
+    }
+  }
+
+  void addError(error, [StackTrace stackTrace]) {
+    if (_transformer._handleError == null) {
+      _inner.addError(error, stackTrace);
+    } else {
+      _transformer._handleError(error, stackTrace, _safeCloseInner);
+    }
+  }
+
+  Future addStream(Stream<S> stream) {
+    return _inner.addStream(stream.transform(
+        new StreamTransformer<S, T>.fromHandlers(
+            handleData: _transformer._handleData,
+            handleError: _transformer._handleError,
+            handleDone: _closeSink)));
+  }
+
+  Future close() {
+    if (_transformer._handleDone == null) return _inner.close();
+
+    _transformer._handleDone(_safeCloseInner);
+    return _inner.done;
+  }
+}
+
+/// A wrapper for [StreamSink]s that swallows any errors returned by [close].
+///
+/// [HandlerTransformer] passes this to its handlers to ensure that when they
+/// call [close], they don't leave any dangling [Future]s behind that might emit
+/// unhandleable errors.
+class _SafeCloseSink<T> extends DelegatingStreamSink<T> {
+  _SafeCloseSink(StreamSink<T> inner) : super(inner);
+
+  Future close() => super.close().catchError((_) {});
+}
+
+/// A function to pass as a [StreamTransformer]'s `handleDone` callback.
+void _closeSink(EventSink sink) {
+  sink.close();
+}