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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 class that multiplexes multiple virtual channels across a single
/// underlying transport layer.
/// This should be connected to another [MultiChannel] on the other end of the
/// underlying channel. It starts with a single default virtual channel,
/// accessible via [stream] and [sink]. Additional virtual channels can be
/// created with [virtualChannel].
/// When a virtual channel is created by one endpoint, the other must connect to
/// it before messages may be sent through it. The first endpoint passes its
/// [] to the second, which then creates a channel from that id
/// also using [virtualChannel]. For example:
/// ```dart
/// // First endpoint
/// var virtual = multiChannel.virtualChannel();
/// multiChannel.sink.add({
/// "channel":
/// });
/// // Second endpoint
/// {
/// var virtual = multiChannel.virtualChannel(message["channel"]);
/// // ...
/// });
/// ```
/// Sending errors across a [MultiChannel] is not supported. Any errors from the
/// underlying stream will be reported only via the default
/// [].
/// Each virtual channel may be closed individually. When all of them are
/// closed, the underlying [StreamSink] is closed automatically.
abstract class MultiChannel implements StreamChannel {
/// The default input stream.
/// This connects to the remote [sink].
Stream get stream;
/// The default output stream.
/// This connects to the remote [stream]. If this is closed, the remote
/// [stream] will close, but other virtual channels will remain open and new
/// virtual channels may be opened.
StreamSink get sink;
/// Creates a new [MultiChannel] that sends and receives messages over
/// [inner].
/// The inner channel must take JSON-like objects.
factory MultiChannel(StreamChannel inner) => new _MultiChannel(inner);
/// Creates a new virtual channel.
/// If [id] is not passed, this creates a virtual channel from scratch. Before
/// it's used, its [] must be sent to the remote endpoint
/// where [virtualChannel] should be called with that id.
/// If [id] is passed, this creates a virtual channel corresponding to the
/// channel with that id on the remote channel.
/// Throws an [ArgumentError] if a virtual channel already exists for [id].
/// Throws a [StateError] if the underlying channel is closed.
VirtualChannel virtualChannel([id]);
/// The implementation of [MultiChannel].
/// This is private so that [VirtualChannel] can inherit from [MultiChannel]
/// without having to implement all the private members.
class _MultiChannel extends StreamChannelMixin implements MultiChannel {
/// The inner channel over which all communication is conducted.
/// This will be `null` if the underlying communication channel is closed.
StreamChannel _inner;
/// The subscription to [].
StreamSubscription _innerStreamSubscription;
Stream get stream =>;
StreamSink get sink => _mainController.foreign.sink;
/// The controller for this channel.
final _mainController = new StreamChannelController(sync: true);
/// A map from virtual channel ids to [StreamChannelController]s that should
/// be used to communicate over those channels.
final _controllers = <int, StreamChannelController>{};
/// The next id to use for a local virtual channel.
/// Ids are used to identify virtual channels. Each message is tagged with an
/// id; the receiving [MultiChannel] uses this id to look up which
/// [VirtualChannel] the message should be dispatched to.
/// The id scheme for virtual channels is somewhat complicated. This is
/// necessary to ensure that there are no conflicts even when both endpoints
/// have virtual channels with the same id; since both endpoints can send and
/// receive messages across each virtual channel, a naïve scheme would make it
/// impossible to tell whether a message was from a channel that originated in
/// the remote endpoint or a reply on a channel that originated in the local
/// endpoint.
/// The trick is that each endpoint only uses odd ids for its own channels.
/// When sending a message over a channel that was created by the remote
/// endpoint, the channel's id plus one is used. This way each [MultiChannel]
/// knows that if an incoming message has an odd id, it's using the local id
/// scheme, but if it has an even id, it's using the remote id scheme.
var _nextId = 1;
_MultiChannel(this._inner) {
// The default connection is a special case which has id 0 on both ends.
// This allows it to begin connected without having to send over an id.
_controllers[0] = _mainController;
(message) => _inner.sink.add([0, message]),
onDone: () => _closeChannel(0, 0));
_innerStreamSubscription = {
var id = message[0];
var controller = _controllers[id];
// A controller might not exist if the channel was closed before an
// incoming message was processed.
if (controller == null) return;
if (message.length > 1) {
// A message without data indicates that the channel has been closed. We
// can only close the sink here without doing any more cleanup, because
// the sink closing will cause the stream to emit a done event which will
// trigger more cleanup.
onDone: _closeInnerChannel,
onError: _mainController.local.sink.addError);
VirtualChannel virtualChannel([id]) {
var inputId;
var outputId;
if (id != null) {
// Since the user is passing in an id, we're connected to a remote
// VirtualChannel. This means messages they send over this channel will
// have the original odd id, but our replies will have an even id.
inputId = id;
outputId = (id as int) + 1;
} else {
// Since we're generating an id, we originated this VirtualChannel. This
// means messages we send over this channel will have the original odd id,
// but the remote channel's replies will have an even id.
inputId = _nextId + 1;
outputId = _nextId;
_nextId += 2;
// If the inner channel has already closed, create new virtual channels in a
// closed state.
if (_inner == null) {
return new VirtualChannel._(
this, inputId, new Stream.empty(), new NullStreamSink());
if (_controllers.containsKey(inputId)) {
throw new ArgumentError("A virtual channel with id $id already exists.");
var controller = new StreamChannelController(sync: true);
_controllers[inputId] = controller;
(message) => _inner.sink.add([outputId, message]),
onDone: () => _closeChannel(inputId, outputId));
return new VirtualChannel._(
this, outputId,, controller.foreign.sink);
/// Closes the virtual channel for which incoming messages have [inputId] and
/// outgoing messages have [outputId].
void _closeChannel(int inputId, int outputId) {
var controller = _controllers.remove(inputId);
if (_inner == null) return;
// A message without data indicates that the virtual channel has been
// closed.
if (_controllers.isEmpty) _closeInnerChannel();
/// Closes the underlying communication channel.
void _closeInnerChannel() {
_inner = null;
// Convert this to a list because the close is dispatched synchronously, and
// that could conceivably remove a controller from [_controllers].
for (var controller in new List.from(_controllers.values)) {
/// A virtual channel created by [MultiChannel].
/// This implements [MultiChannel] for convenience.
/// [VirtualChannel.virtualChannel] is semantically identical to the parent's
/// [MultiChannel.virtualChannel].
class VirtualChannel extends StreamChannelMixin implements MultiChannel {
/// The [MultiChannel] that created this.
final MultiChannel _parent;
/// The identifier for this channel.
/// This can be sent across the [MultiChannel] to provide the remote endpoint
/// a means to connect to this channel. Nothing about this is guaranteed
/// except that it will be JSON-serializable.
final id;
final Stream stream;
final StreamSink sink;
VirtualChannel._(this._parent,,, this.sink);
VirtualChannel virtualChannel([id]) => _parent.virtualChannel(id);