Add StreamChannel.withGuarantees and StreamChannelController.

lib/src/guarantee_channel.dart

Index: lib/src/guarantee_channel.dart
diff --git a/lib/src/guarantee_channel.dart b/lib/src/guarantee_channel.dart
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index 0000000000000000000000000000000000000000..849cc2ffe74edc22e0ff79afe65749e0f5636af3
--- /dev/null
+++ b/lib/src/guarantee_channel.dart
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+// 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 'package:async/async.dart';
+
+import '../stream_channel.dart';
+
+/// A [StreamChannel] that enforces the stream channel guarantees.
+///
+/// This is exposed via [new StreamChannel.withGuarantees].
+class GuaranteeChannel<T> extends StreamChannelMixin<T> {
+  Stream<T> get stream => _streamController.stream;
+
+  StreamSink<T> get sink => _sink;
+  _GuaranteeSink<T> _sink;
+
+  /// The controller for [stream].
+  ///
+  /// This intermediate controller allows us to continue listening for a done
+  /// event even after the user has canceled their subscription, and to send our
+  /// own done event when the sink is closed.
+  StreamController<T> _streamController;
+
+  /// The subscription to the inner stream.
+  StreamSubscription<T> _subscription;
+
+  /// Whether the sink has closed, causing the underlying channel to disconnect.
+  bool _disconnected = false;
+
+  GuaranteeChannel(Stream<T> innerStream, StreamSink<T> innerSink) {
+    _sink = new _GuaranteeSink<T>(innerSink, this);
+
+    // Enforce the single-subscription guarantee by changing a broadcast stream
+    // to single-subscription.
+    if (innerStream.isBroadcast) {
+      innerStream = innerStream.transform(
+          const SingleSubscriptionTransformer());
+    }
+
+    _streamController = new StreamController<T>(onListen: () {
+      // If the sink has disconnected, we've already called
+      // [_streamController.close].
+      if (_disconnected) return;
+
+      _subscription = innerStream.listen(_streamController.add,
+          onError: _streamController.addError,
+          onDone: () {
+            _sink._onStreamDisconnected();
+            _streamController.close();
+          });
+    }, sync: true);
+  }
+
+  /// Called by [_GuaranteeSink] when the user closes it.
+  ///
+  /// The sink closing indicates that the connection is closed, so the stream
+  /// should stop emitting events.
+  void _onSinkDisconnected() {
+    _disconnected = true;
+    if (_subscription != null) _subscription.cancel();
+    _streamController.close();
+  }
+}
+
+/// The sink for [GuaranteeChannel].
+///
+/// This wraps the inner sink to ignore events and cancel any in-progress
+/// [addStream] calls when the underlying channel closes.
+class _GuaranteeSink<T> implements StreamSink<T> {
+  /// The inner sink being wrapped.
+  final StreamSink<T> _inner;
+
+  /// The [GuaranteeChannel] this belongs to.
+  final GuaranteeChannel<T> _channel;
+
+  Future get done => _inner.done;
+
+  /// Whether the stream has emitted a done event, causing the underlying
+  /// channel to disconnect.
+  bool _disconnected = false;
+
+  /// Whether the user has called [close].
+  bool _closed = false;
+
+  /// The subscription to the stream passed to [addStream], if a stream is
+  /// currently being added.
+  StreamSubscription<T> _addStreamSubscription;
+
+  /// The completer for the future returned by [addStream], if a stream is
+  /// currently being added.
+  Completer _addStreamCompleter;
+
+  /// Whether we're currently adding a stream with [addStream].
+  bool get _inAddStream => _addStreamSubscription != null;
+
+  _GuaranteeSink(this._inner, this._channel);
+
+  void add(T data) {
+    if (_closed) throw new StateError("Cannot add event after closing.");
+    if (_inAddStream) {
+      throw new StateError("Cannot add event while adding stream.");
+    }
+    if (_disconnected) return;
+
+    _inner.add(data);
+  }
+
+  void addError(error, [StackTrace stackTrace]) {
+    if (_closed) throw new StateError("Cannot add event after closing.");
+    if (_inAddStream) {
+      throw new StateError("Cannot add event while adding stream.");
+    }
+    if (_disconnected) return;
+
+    _inner.addError(error, stackTrace);
+  }
+
+  Future addStream(Stream<T> stream) {
+    if (_closed) throw new StateError("Cannot add stream after closing.");
+    if (_inAddStream) {
+      throw new StateError("Cannot add stream while adding stream.");
+    }
+    if (_disconnected) return new Future.value();
+
+    _addStreamCompleter = new Completer.sync();
+    _addStreamSubscription = stream.listen(
+        _inner.add,
+        onError: _inner.addError,
+        onDone: _addStreamCompleter.complete);
+    return _addStreamCompleter.future.then((_) {
+      _addStreamCompleter = null;
+      _addStreamSubscription = null;
+    });
+  }
+
+  Future close() {
+    if (_inAddStream) {
+      throw new StateError("Cannot close sink while adding stream.");
+    }
+
+    _closed = true;
+    if (_disconnected) return new Future.value();
+
+    _channel._onSinkDisconnected();
+    return _inner.close();
+  }
+
+  /// Called by [GuaranteeChannel] when the stream emits a done event.
+  ///
+  /// The stream being done indicates that the connection is closed, so the
+  /// sink should stop forwarding events.
+  void _onStreamDisconnected() {
+    _disconnected = true;
+    if (!_inAddStream) return;
+
+    _addStreamCompleter.complete(_addStreamSubscription.cancel());
+    _addStreamCompleter = null;
+    _addStreamSubscription = null;
+  }
+}