Add a Disconnector class.

lib/src/disconnector.dart

Index: lib/src/disconnector.dart
diff --git a/lib/src/disconnector.dart b/lib/src/disconnector.dart
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+++ b/lib/src/disconnector.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 '../stream_channel.dart';
+
+/// Allows the caller to force a channel to disconnect.
+///
+/// When [disconnect] is called, the channel (or channels) transformed by this
+/// transformer will act as though the remote end had disconnected—the stream
+/// will emit a done event, and the sink will ignore future inputs. The inner
+/// sink will also be closed to notify the remote end of the disconnection.
+///
+/// If a channel is transformed after the [disconnect] has been called, it will
+/// be disconnected immediately.
+class Disconnector<T> implements StreamChannelTransformer<T, T> {
+  /// Whether [disconnect] has been called.
+  bool get isDisconnected => _isDisconnected;
+  var _isDisconnected = false;
+
+  /// The sinks for transformed channels.
+  ///
+  /// Note that we assume that transformed channels provide the stream channel
+  /// guarantees. This allows us to only track sinks, because we know closing
+  /// the underlying sink will cause the stream to emit a done event.
+  final _sinks = <_DisconnectorSink<T>>[];
+
+  /// Disconnects all channels that have been transformed.
+  void disconnect() {
+    _isDisconnected = true;
+    for (var sink in _sinks) {
+      sink._disconnect();
+    }
+    _sinks.clear();
+  }
+
+  StreamChannel<T> bind(StreamChannel<T> channel) {
+    return channel.changeSink((innerSink) {
+      var sink = new _DisconnectorSink(innerSink);
+
+      if (_isDisconnected) {
+        sink._disconnect();
+      } else {
+        _sinks.add(sink);
+      }
+
+      return sink;
+    });
+  }
+}
+
+/// A sink wrapper that can force a disconnection.
+class _DisconnectorSink<T> implements StreamSink<T> {
+  /// The inner sink.
+  final StreamSink<T> _inner;
+
+  Future get done => _inner.done;
+
+  /// Whether [Disconnector.disconnect] has been called.
+  var _isDisconnected = false;

[tjblasi, 2016/02/09 16:45:37]

Out of curiosity, what is your rule for typing properties vs. using `var`?

[nweiz, 2016/02/09 19:10:53]

In general, I don't type a field if its type would be identical to the
initializer's type. The only exception is public fields, which I always
explicitly type so that the dartdoc looks right.

That's the theory, anyway. Sometimes I accidentally get it wrong.

+
+  /// Whether the user has called [close].
+  var _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;
+
+  _DisconnectorSink(this._inner);
+
+  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 (_isDisconnected) 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 (_isDisconnected) 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 (_isDisconnected) 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;
+    return _inner.close();
+  }
+
+  /// Disconnects this sink.
+  ///
+  /// This closes the underlying sink and stops forwarding events.
+  void _disconnect() {
+    _isDisconnected = true;
+    _inner.close();
+
+    if (!_inAddStream) return;
+    _addStreamCompleter.complete(_addStreamSubscription.cancel());
+    _addStreamCompleter = null;
+    _addStreamSubscription = null;
+  }
+}