Media Remoting: Data/Control plumbing between renderer and Media Router.

chrome/browser/media/cast_remoting_connector.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/media/cast_remoting_connector.h"
+
+#include <stdio.h>
+
+#include "base/bind.h"
+#include "base/bind_helpers.h"
+#include "base/callback.h"
+#include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/strings/string_piece.h"
+#include "base/strings/stringprintf.h"
+#include "chrome/browser/media/cast_remoting_sender.h"
+#include "chrome/browser/media/router/media_router.h"
+#include "chrome/browser/media/router/media_router_factory.h"
+#include "chrome/browser/media/router/media_source_helper.h"
+#include "chrome/browser/media/router/route_message.h"
+#include "chrome/browser/media/router/route_message_observer.h"
+#include "content/public/browser/browser_thread.h"
+#include "mojo/public/cpp/bindings/strong_binding.h"
+
+DEFINE_WEB_CONTENTS_USER_DATA_KEY(CastRemotingConnector);
+
+using content::BrowserThread;
+using media::mojom::RemotingStartFailReason;
+using media::mojom::RemotingStopReason;
+
+namespace {
+
+// Simple command messages sent from/to the connector to/from the Media Router
+// Cast Provider to start/stop media remoting to a Cast device.
+//
+// Field separator (for tokenizing parts of messages).
+constexpr char kMessageFieldSeparator = ':';
+// Message sent by CastRemotingConnector to Cast provider to start remoting.
+// Example:
+//   "START_CAST_REMOTING:session=1f"
+constexpr char kStartRemotingMessageFormat[] =
+    "START_CAST_REMOTING:session=%x";
+// Message sent by CastRemotingConnector to Cast provider to start the remoting
+// RTP stream(s). Example:
+//   "START_CAST_REMOTING_STREAMS:session=1f:audio=N:video=Y"
+constexpr char kStartStreamsMessageFormat[] =
+    "START_CAST_REMOTING_STREAMS:session=%x:audio=%c:video=%c";
+// Start acknowledgement message sent by Cast provider to CastRemotingConnector
+// once remoting RTP streams have been set up. Examples:
+//   "STARTED_CAST_REMOTING_STREAMS:session=1f:audio_stream_id=2e:"
+//                                                         "video_stream_id=3d"
+//   "STARTED_CAST_REMOTING_STREAMS:session=1f:video_stream_id=b33f"
+constexpr char kStartedStreamsMessageFormatPartial[] =
+    "STARTED_CAST_REMOTING_STREAMS:session=%x";
+constexpr char kStartedStreamsMessageAudioIdSpecifier[] = ":audio_stream_id=";
+constexpr char kStartedStreamsMessageVideoIdSpecifier[] = ":video_stream_id=";
+// Stop message sent by CastRemotingConnector to Cast provider. Example:
+//   "STOP_CAST_REMOTING:session=1f"
+constexpr char kStopRemotingMessageFormat[] =
+    "STOP_CAST_REMOTING:session=%x";
+// Stop acknowledgement message sent by Cast provider to CastRemotingConnector
+// once remoting is available again after the last session ended. Example:
+//   "STOPPED_CAST_REMOTING:session=1f"
+constexpr char kStoppedMessageFormat[] =
+    "STOPPED_CAST_REMOTING:session=%x";
+// Failure message sent by Cast provider to CastRemotingConnector any time there
+// was a fatal error (e.g., the Cast provider failed to set up the RTP streams,
+// or there was some unexpected external event). Example:
+//   "FAILED_CAST_REMOTING:session=1f"
+constexpr char kFailedMessageFormat[] = "FAILED_CAST_REMOTING:session=%x";
+
+// Returns true if the given |message| matches the given |format| and the
+// session ID in the |message| is equal to the |expected_session_id|.
+bool IsMessageForSession(const std::string& message, const char* format,
+                         unsigned int expected_session_id) {
+  unsigned int session_id;
+  if (sscanf(message.c_str(), format, &session_id) == 1)
+    return session_id == expected_session_id;
+  return false;
+}
+
+// Scans |message| for |specifier| and extracts the remoting stream ID that
+// follows the specifier. Returns a negative value on error.
+int32_t GetStreamIdFromStartedMessage(const std::string& message,

[dcheng, 2016/09/20 08:20:12]

Nit: StringPiece here will net slightly more efficient code (the substr() on
line 96 won't have to create an actual new string object)

[miu, 2016/09/21 03:15:50]

Done.

+                                      const char* specifier,
+                                      size_t specifier_length) {
+  auto start = message.find(specifier, 0, specifier_length);
+  if (start == std::string::npos)
+    return -1;
+  start += specifier_length;
+  if (start + 1 >= message.size())
+    return -1; // Must be at least one hex digit following the specifier.
+  int parsed_value;
+  if (!base::HexStringToInt(
+          message.substr(start, message.find(kMessageFieldSeparator, start)),
+          &parsed_value) ||
+      parsed_value < 0 ||
+      parsed_value > std::numeric_limits<int32_t>::max()) {
+    return -1; // Non-hex digits, or outside valid range.
+  }
+  return static_cast<int32_t>(parsed_value);
+}
+
+}  // namespace
+
+class CastRemotingConnector::FrameRemoterFactory
+    : public media::mojom::RemoterFactory {
+ public:
+  // |render_frame_host| represents the source render frame.
+  explicit FrameRemoterFactory(content::RenderFrameHost* render_frame_host)
+      : host_(render_frame_host) {
+    DCHECK(host_);
+  }
+  ~FrameRemoterFactory() final {}
+
+  void Create(media::mojom::RemotingSourcePtr source,
+              media::mojom::RemoterRequest request) final {
+    CastRemotingConnector::Get(content::WebContents::FromRenderFrameHost(host_))
+        ->CreateBridge(std::move(source), std::move(request));
+  }
+
+ private:
+  content::RenderFrameHost* const host_;
+
+  DISALLOW_COPY_AND_ASSIGN(FrameRemoterFactory);
+};
+
+class CastRemotingConnector::RemotingBridge : public media::mojom::Remoter {
+ public:
+  // Constructs a "bridge" to delegate calls between the given |source| and
+  // |connector|. |connector| must outlive this instance.
+  RemotingBridge(media::mojom::RemotingSourcePtr source,
+                 CastRemotingConnector* connector)
+      : source_(std::move(source)), connector_(connector) {
+    DCHECK(connector_);
+    source_.set_connection_error_handler(base::Bind(
+        &RemotingBridge::Stop, base::Unretained(this),
+        RemotingStopReason::SOURCE_GONE));
+    connector_->RegisterBridge(this);
+  }
+
+  ~RemotingBridge() final {
+    connector_->DeregisterBridge(this, RemotingStopReason::SOURCE_GONE);
+  }
+
+  // The CastRemotingConnector calls these to call back to the RemotingSource.
+  void OnSinkAvailable() { source_->OnSinkAvailable(); }
+  void OnSinkGone() { source_->OnSinkGone(); }
+  void OnStarted() { source_->OnStarted(); }
+  void OnStartFailed(RemotingStartFailReason reason) {
+    source_->OnStartFailed(reason);
+  }
+  void OnMessageFromSink(const std::vector<uint8_t>& message) {
+    source_->OnMessageFromSink(message);
+  }
+  void OnStopped(RemotingStopReason reason) { source_->OnStopped(reason); }
+
+  // media::mojom::Remoter implementation. The source calls these to start/stop
+  // media remoting and send messages to the sink. These simply delegate to the
+  // CastRemotingConnector, which mediates to establish only one remoting
+  // session among possibly multiple requests. The connector will respond to
+  // this request by calling one of: OnStarted() or OnStartFailed().
+  void Start() final {
+    connector_->StartRemoting(this);
+  }
+  void StartDataStreams(
+      mojo::ScopedDataPipeConsumerHandle audio_pipe,
+      mojo::ScopedDataPipeConsumerHandle video_pipe,
+      media::mojom::RemotingDataStreamSenderRequest audio_sender_request,
+      media::mojom::RemotingDataStreamSenderRequest video_sender_request)
+      final {
+    connector_->StartRemotingDataStreams(
+        this, std::move(audio_pipe), std::move(video_pipe),
+        std::move(audio_sender_request), std::move(video_sender_request));
+  }
+  void Stop(RemotingStopReason reason) final {
+    connector_->StopRemoting(this, reason);
+  }
+  void SendMessageToSink(const std::vector<uint8_t>& message) final {
+    connector_->SendMessageToSink(this, message);
+  }
+
+ private:
+  media::mojom::RemotingSourcePtr source_;
+  CastRemotingConnector* const connector_;
+
+  DISALLOW_COPY_AND_ASSIGN(RemotingBridge);
+};
+
+class CastRemotingConnector::MessageObserver
+    : public media_router::RouteMessageObserver {
+ public:
+  MessageObserver(media_router::MediaRouter* router,
+                  const media_router::MediaRoute::Id& route_id,
+                  CastRemotingConnector* connector)
+      : RouteMessageObserver(router, route_id), connector_(connector) {}
+  ~MessageObserver() final {}
+
+ private:
+  void OnMessagesReceived(
+      const std::vector<media_router::RouteMessage>& messages) final {
+    connector_->ProcessMessagesFromRoute(messages);
+  }
+
+  CastRemotingConnector* const connector_;
+};
+
+// static
+CastRemotingConnector* CastRemotingConnector::Get(
+    content::WebContents* contents) {
+  CastRemotingConnector* connector = FromWebContents(contents);
+  if (connector)
+    return connector;
+  connector = new CastRemotingConnector(contents);
+  // The following transfers ownership of |connector| to WebContents.
+  contents->SetUserData(UserDataKey(), connector);
+  return connector;
+}
+
+// static
+void CastRemotingConnector::CreateRemoterFactory(
+    content::RenderFrameHost* render_frame_host,
+    media::mojom::RemoterFactoryRequest request) {
+  mojo::MakeStrongBinding(
+      base::MakeUnique<FrameRemoterFactory>(render_frame_host),
+      std::move(request));
+}
+
+CastRemotingConnector::CastRemotingConnector(content::WebContents* contents)
+    : CastRemotingConnector(
+          media_router::MediaRouterFactory::GetApiForBrowserContext(
+              contents->GetBrowserContext()),
+          media_router::MediaSourceForTabContentRemoting(contents).id()) {}
+
+CastRemotingConnector::CastRemotingConnector(
+    media_router::MediaRouter* router,
+    const media_router::MediaSource::Id& route_source_id)
+    : media_router::MediaRoutesObserver(router, route_source_id),
+      session_counter_(0),
+      active_bridge_(nullptr),
+      weak_factory_(this) {}
+
+CastRemotingConnector::~CastRemotingConnector() {
+  // Remoting should not be active at this point, and this instance is expected
+  // to outlive all bridges. See comment in CreateBridge().
+  DCHECK(!active_bridge_);
+  DCHECK(bridges_.empty());
+}
+
+void CastRemotingConnector::CreateBridge(media::mojom::RemotingSourcePtr source,
+                                         media::mojom::RemoterRequest request) {
+  // Create a new RemotingBridge, which will become owned by the message pipe
+  // associated with |request|. |this| CastRemotingConnector should be valid
+  // for the full lifetime of the bridge because it can be deduced that the
+  // connector will always outlive the mojo message pipe: A single WebContents
+  // will destroy the render frame tree (which destroys all associated mojo
+  // message pipes) before CastRemotingConnector. To ensure this assumption is
+  // not broken by future design changes in external modules, a DCHECK() has
+  // been placed in the CastRemotingConnector destructor as a sanity-check.
+  mojo::MakeStrongBinding(
+      base::MakeUnique<RemotingBridge>(std::move(source), this),
+      std::move(request));
+}
+
+void CastRemotingConnector::RegisterBridge(RemotingBridge* bridge) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  DCHECK(bridges_.find(bridge) == bridges_.end());
+
+  bridges_.insert(bridge);
+  if (message_observer_ && !active_bridge_)
+    bridge->OnSinkAvailable();
+}
+
+void CastRemotingConnector::DeregisterBridge(RemotingBridge* bridge,
+                                             RemotingStopReason reason) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  DCHECK(bridges_.find(bridge) != bridges_.end());
+
+  if (bridge == active_bridge_)
+    StopRemoting(bridge, reason);
+  bridges_.erase(bridge);
+}
+
+void CastRemotingConnector::StartRemoting(RemotingBridge* bridge) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  DCHECK(bridges_.find(bridge) != bridges_.end());
+
+  // Refuse to start if there is no remoting route available, or if remoting is
+  // already active.
+  if (!message_observer_) {
+    bridge->OnStartFailed(RemotingStartFailReason::ROUTE_TERMINATED);
+    return;
+  }
+  if (active_bridge_) {
+    bridge->OnStartFailed(RemotingStartFailReason::CANNOT_START_MULTIPLE);
+    return;
+  }
+
+  // Notify all other sources that the sink is no longer available for remoting.
+  // A race condition is possible, where one of the other sources will try to
+  // start remoting before receiving this notification; but that attempt will
+  // just fail later on.
+  for (RemotingBridge* notifyee : bridges_) {
+    if (notifyee == bridge)
+      continue;
+    notifyee->OnSinkGone();
+  }
+
+  active_bridge_ = bridge;
+
+  // Send a start message to the Cast Provider.
+  ++session_counter_;  // New remoting session ID.
+  SendMessageToProvider(
+      base::StringPrintf(kStartRemotingMessageFormat, session_counter_));
+
+  bridge->OnStarted();
+}
+
+void CastRemotingConnector::StartRemotingDataStreams(
+    RemotingBridge* bridge,
+    mojo::ScopedDataPipeConsumerHandle audio_pipe,
+    mojo::ScopedDataPipeConsumerHandle video_pipe,
+    media::mojom::RemotingDataStreamSenderRequest audio_sender_request,
+    media::mojom::RemotingDataStreamSenderRequest video_sender_request) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  // Refuse to start if there is no remoting route available, or if remoting is
+  // not active for this |bridge|.
+  if (!message_observer_ || active_bridge_ != bridge)
+    return;
+  // Also, if neither audio nor video pipe was provided, or if a request for a
+  // RemotingDataStreamSender was not provided for a data pipe, error-out early.
+  if ((!audio_pipe.is_valid() && !video_pipe.is_valid()) ||
+      (audio_pipe.is_valid() && !audio_sender_request.is_pending()) ||
+      (video_pipe.is_valid() && !video_sender_request.is_pending())) {
+    StopRemoting(active_bridge_, RemotingStopReason::DATA_SEND_FAILED);
+    return;
+  }
+
+  // Hold on to the data pipe handles and interface requests until one/both
+  // CastRemotingSenders are created and ready for use.
+  pending_audio_pipe_ = std::move(audio_pipe);
+  pending_video_pipe_ = std::move(video_pipe);
+  pending_audio_sender_request_ = std::move(audio_sender_request);
+  pending_video_sender_request_ = std::move(video_sender_request);
+
+  // Send a "start streams" message to the Cast Provider. The provider is
+  // responsible for creating and setting up a remoting Cast Streaming session
+  // that will result in new CastRemotingSender instances being created here in
+  // the browser process.
+  SendMessageToProvider(base::StringPrintf(
+      kStartStreamsMessageFormat, session_counter_,
+      pending_audio_sender_request_.is_pending() ? 'Y' : 'N',
+      pending_video_sender_request_.is_pending() ? 'Y' : 'N'));
+}
+
+void CastRemotingConnector::StopRemoting(RemotingBridge* bridge,
+                                         RemotingStopReason reason) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  if (active_bridge_ != bridge)
+    return;
+
+  active_bridge_ = nullptr;
+
+  // Explicitly close the data pipes (and related requests) just in case the
+  // "start streams" operation was interrupted.
+  pending_audio_pipe_.reset();
+  pending_video_pipe_.reset();
+  pending_audio_sender_request_.PassMessagePipe().reset();

[dcheng, 2016/09/20 08:20:12]

Would it make sense to call ResetWithReason here?

[miu, 2016/09/21 03:15:50]

This is brand-new, so I hadn't considered it. Having looked at it, I think it is
not needed by our use case:

1. We already make Mojo calls back to the renderer to indicate the reasons for
shutting down (media.mojom.RemotingStopReason).

2. From a structured data POV, I kind of like how my Mojo interface defines an
enumeration of reasons for shutting down. The ResetWithReason() just provides a
uint32_t, but no decent way to check that each end of the pipe is "speaking the
same language" w.r.t. the values. (BTW--I added a comment on
http://crbug.com/634502 to discuss this.)

That said, I did make a small change here: There seems to be a new assign
nullptr assignment operator, which I feel makes the code look cleaner.

+  pending_video_sender_request_.PassMessagePipe().reset();
+
+  // Cancel all outstanding callbacks related to the remoting session.
+  weak_factory_.InvalidateWeakPtrs();
+
+  // Prevent the source from trying to start again until the Cast Provider has
+  // indicated the stop operation has completed.
+  bridge->OnSinkGone();
+  // Note: At this point, all sources should think the sink is gone.
+
+  SendMessageToProvider(
+      base::StringPrintf(kStopRemotingMessageFormat, session_counter_));
+  // Note: Once the Cast Provider sends back an acknowledgement message, all
+  // sources will be notified that the remoting sink is available again.
+
+  bridge->OnStopped(reason);
+}
+
+void CastRemotingConnector::SendMessageToSink(
+    RemotingBridge* bridge, const std::vector<uint8_t>& message) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  // During an active remoting session, simply pass all binary messages through
+  // to the sink.
+  if (!message_observer_ || active_bridge_ != bridge)
+    return;
+  media_router::MediaRoutesObserver::router()->SendRouteBinaryMessage(
+      message_observer_->route_id(),
+      base::MakeUnique<std::vector<uint8_t>>(message),
+      base::Bind(&CastRemotingConnector::HandleSendMessageResult,
+                 weak_factory_.GetWeakPtr()));
+}
+
+void CastRemotingConnector::SendMessageToProvider(const std::string& message) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  if (!message_observer_)
+    return;
+
+  if (active_bridge_) {
+    media_router::MediaRoutesObserver::router()->SendRouteMessage(
+        message_observer_->route_id(), message,
+        base::Bind(&CastRemotingConnector::HandleSendMessageResult,
+                   weak_factory_.GetWeakPtr()));
+  } else {
+    struct Helper {
+      static void IgnoreSendMessageResult(bool ignored) {}
+    };
+    media_router::MediaRoutesObserver::router()->SendRouteMessage(
+        message_observer_->route_id(), message,
+        base::Bind(&Helper::IgnoreSendMessageResult));
+  }
+}
+
+void CastRemotingConnector::ProcessMessagesFromRoute(
+    const std::vector<media_router::RouteMessage>& messages) {

[dcheng, 2016/09/20 08:20:12]

For fuzzing, I think this is one of the interfaces we'll want to fuzz: we'll
want to feed in RouteMessages with random content (I /think/, but I don't
remember off the top of my head, if we validate that the type and message
contents match somewhere upstream when we turn the mojom into a C++.
Incidentally a StructTrait would be really good for this =)

[miu, 2016/09/21 03:15:50]

Acknowledged.

+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  for (const media_router::RouteMessage& message : messages) {
+    switch (message.type) {
+      case media_router::RouteMessage::TEXT:
+        // This is a notification message from the Cast Provider, about the
+        // execution state of the media remoting session between Chrome and the
+        // remote device.
+        DCHECK(message.text);

[dcheng, 2016/09/20 08:20:12]

Not for this CL, but we should consider making the mojom definition for this a
union (then we can avoid the enum doesn't match the data type problem)

[miu, 2016/09/21 03:15:50]

Agreed.

+
+        // If this is a "start streams" acknowledgement message, the
+        // CastRemotingSenders should now be available to begin consuming from
+        // the data pipes.
+        if (active_bridge_ &&
+            IsMessageForSession(*message.text,
+                                kStartedStreamsMessageFormatPartial,
+                                session_counter_)) {
+          if (pending_audio_sender_request_.is_pending()) {
+            cast::CastRemotingSender::FindAndBind(
+                GetStreamIdFromStartedMessage(
+                    *message.text, kStartedStreamsMessageAudioIdSpecifier,
+                    sizeof(kStartedStreamsMessageAudioIdSpecifier) - 1),

[dcheng, 2016/09/20 08:20:12]

Nit: use strlen here and below, our compilers should all be evaluating this at
compile-time in release builds.

Or just use StringPiece in GetStreamIdFromStartedMessage for both args.

[miu, 2016/09/21 03:15:50]

Done.

+                std::move(pending_audio_pipe_),
+                std::move(pending_audio_sender_request_),
+                base::Bind(&CastRemotingConnector::OnDataSendFailed,
+                           weak_factory_.GetWeakPtr()));
+          }
+          if (pending_video_sender_request_.is_pending()) {
+            cast::CastRemotingSender::FindAndBind(
+                GetStreamIdFromStartedMessage(
+                    *message.text, kStartedStreamsMessageVideoIdSpecifier,
+                    sizeof(kStartedStreamsMessageVideoIdSpecifier) - 1),
+                std::move(pending_video_pipe_),
+                std::move(pending_video_sender_request_),
+                base::Bind(&CastRemotingConnector::OnDataSendFailed,
+                           weak_factory_.GetWeakPtr()));
+          }
+          break;
+        }
+
+        // If this is a failure message, call StopRemoting().
+        if (active_bridge_ &&
+            IsMessageForSession(*message.text, kFailedMessageFormat,
+                                session_counter_)) {
+          StopRemoting(active_bridge_, RemotingStopReason::UNEXPECTED_FAILURE);
+          break;
+        }
+
+        // If this is a stop acknowledgement message, indicating that the last
+        // session was stopped, notify all sources that the sink is once again
+        // available.
+        if (IsMessageForSession(*message.text, kStoppedMessageFormat,
+                                session_counter_)) {
+          if (active_bridge_) {
+            // Hmm...The Cast Provider was in a state that disagrees with this
+            // connector. Attempt to resolve this by shutting everything down to
+            // effectively reset to a known state.
+            LOG(WARNING) << "BUG: Cast Provider sent 'stopped' message during "
+                            "an active remoting session.";
+            StopRemoting(active_bridge_,
+                         RemotingStopReason::UNEXPECTED_FAILURE);
+          }
+          for (RemotingBridge* notifyee : bridges_)
+            notifyee->OnSinkAvailable();
+          break;
+        }
+
+        LOG(WARNING) << "BUG: Unexpected message from Cast Provider: "
+                     << *message.text;
+        break;
+
+      case media_router::RouteMessage::BINARY:  // This is for the source.
+        DCHECK(message.binary);
+
+        // All binary messages are passed through to the source during an active
+        // remoting session.
+        if (active_bridge_)
+          active_bridge_->OnMessageFromSink(*message.binary);
+        break;
+    }
+  }
+}
+
+void CastRemotingConnector::HandleSendMessageResult(bool success) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  // A single message send failure is treated as fatal to an active remoting
+  // session.
+  if (!success && active_bridge_)
+    StopRemoting(active_bridge_, RemotingStopReason::MESSAGE_SEND_FAILED);
+}
+
+void CastRemotingConnector::OnDataSendFailed() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  // A single data send failure is treated as fatal to an active remoting
+  // session.
+  if (active_bridge_)
+    StopRemoting(active_bridge_, RemotingStopReason::DATA_SEND_FAILED);
+}
+
+void CastRemotingConnector::OnRoutesUpdated(
+    const std::vector<media_router::MediaRoute>& routes,
+    const std::vector<media_router::MediaRoute::Id>& joinable_route_ids) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  // If a remoting route has already been identified, check that it still
+  // exists. Otherwise, shut down messaging and any active remoting, and notify
+  // the sources that remoting is no longer available.
+  if (message_observer_) {
+    for (const media_router::MediaRoute& route : routes) {
+      if (message_observer_->route_id() == route.media_route_id())
+        return;  // Remoting route still exists. Take no further action.
+    }
+    message_observer_.reset();
+    if (active_bridge_)
+      StopRemoting(active_bridge_, RemotingStopReason::ROUTE_TERMINATED);
+    for (RemotingBridge* notifyee : bridges_)
+      notifyee->OnSinkGone();
+  }
+
+  // There shouldn't be an active RemotingBridge at this point, since there is
+  // currently no known remoting route.
+  DCHECK(!active_bridge_);
+
+  // Scan |routes| for a new remoting route. If one is found, begin processing
+  // messages on the route, and notify the sources that remoting is now
+  // available.
+  if (!routes.empty()) {
+    const media_router::MediaRoute& route = routes.front();
+    message_observer_.reset(new MessageObserver(
+        media_router::MediaRoutesObserver::router(), route.media_route_id(),
+        this));
+    // TODO(miu): In the future, scan the route ID for sink capabilities
+    // properties and pass these to the source in the OnSinkAvailable()
+    // notification.
+    for (RemotingBridge* notifyee : bridges_)
+      notifyee->OnSinkAvailable();
+  }
+}