Integrate Cast SDK into Your Web Sender App

This developer guide describes how to add Google Cast support to your Web Sender app using the Cast SDK.

Terminology

The mobile device or browser is the sender, which controls the playback; the Google Cast device is the receiver, which displays the content on the screen for playback.

The Web Sender SDK consists of two parts: the Framework API (cast.framework) and the Base API (chrome.cast) In general, you make calls on the simpler, higher-level Framework API, which are then processed by the lower-level Base API.

The sender framework refers to the Framework API, module and associated resources that provide a wrapper around lower-level functionality. The sender app or Google Cast Chrome app refers to a web (HTML/JavaScript) app running inside a Chrome browser on a sender device. A Web Receiver app refers to an HTML/JavaScript app running on Chromecast or a Google Cast device.

The sender framework uses an asynchronous callback design to inform the sender app of events and to transition between various states of the Cast app life cycle.

Load the library

For your app to implement the features of Google Cast, it needs to know the location of the Google Cast Web Sender SDK, as shown below. Add the loadCastFramework URL query parameter to load the Web Sender Framework API as well. All pages of your app must refer to the library as follows:

<script src="https://www.gstatic.com/cv/js/sender/v1/cast_sender.js?loadCastFramework=1"></script>

Framework

The Web Sender SDK uses the cast.framework.* namespace. The namespace represents the following:

  • Methods or functions that invoke operations on the API
  • Event listeners for listener functions in the API

The framework consists of these main components:

  • The CastContext is a singleton object that provides information about the current Cast state, and triggers events for Cast state and Cast session state changes.
  • The CastSession object manages the session -- it provides state information and triggers events, such as changes to device volume, mute state, and app metadata.
  • The Cast button element, which is a simple HTML custom element that extends the HTML button. If the provided Cast button is not sufficient, you can use the Cast state to implement a Cast button.
  • The RemotePlayerController provides the data binding to simplify implementation of the remote player.

Review the Google Cast Web Sender API Reference for a complete description of the namespace.

Cast button

The Cast button component in your app is handled entirely by the framework. This includes visibility management, as well as click event handling.

<google-cast-launcher></google-cast-launcher>

Alternatively, you can create the button programmatically:

document.createElement("google-cast-launcher");

You can apply any additional styling, such as size or positioning, to the element as necessary. Use the --connected-color attribute to choose the color for the connected Web Receiver state, and --disconnected-color for the disconnected state.

Initialization

After loading the framework API, the app will call the handler window.__onGCastApiAvailable. You should ensure that the app sets this handler on the window before loading the sender library.

Within this handler, you initialize the Cast interaction by calling the setOptions(options) method of CastContext.

For example:

<script>
window['__onGCastApiAvailable'] = function(isAvailable) {
  if (isAvailable) {
    initializeCastApi();
  }
};
</script>

Then, you initialize the API as follows:

initializeCastApi = function() {
  cast.framework.CastContext.getInstance().setOptions({
    receiverApplicationId: applicationId,
    autoJoinPolicy: chrome.cast.AutoJoinPolicy.ORIGIN_SCOPED
  });
};

First the app retrieves the singleton instance of the CastContext object provided by the framework. It then uses setOptions(options) using a CastOptions object to set the applicationID.

If you are using the Default Media Receiver, which doesn't require registration, you use a constant predefined by the Web Sender SDK, as shown below, instead of the applicationID:

cast.framework.CastContext.getInstance().setOptions({
  receiverApplicationId: chrome.cast.media.DEFAULT_MEDIA_RECEIVER_APP_ID
});

Media control

Once the CastContext has been initialized, the app can retrieve the current CastSession at any time using getCurrentSession().

var castSession = cast.framework.CastContext.getInstance().getCurrentSession();

The CastSession can be used to load media to the connected Cast device using loadMedia(loadRequest). First, create a MediaInfo, using the contentId and contentType as well as any other information related to the content. Then create a LoadRequest from it, setting all of the relevant information for the request. Finally, call loadMedia(loadRequest) on your CastSession.

var mediaInfo = new chrome.cast.media.MediaInfo(currentMediaURL, contentType);
var request = new chrome.cast.media.LoadRequest(mediaInfo);
castSession.loadMedia(request).then(
  function() { console.log('Load succeed'); },
  function(errorCode) { console.log('Error code: ' + errorCode); });

The loadMedia method will return a Promise that can be used to perform any necessary operations for a successful result. If the Promise is rejected, the function argument will be a chrome.cast.ErrorCode.

You can access player state variables in RemotePlayer. All interactions with the RemotePlayer, including media event callbacks and commands, are handled with the RemotePlayerController.

var player = new cast.framework.RemotePlayer();
var playerController = new cast.framework.RemotePlayerController(player);

The RemotePlayerController gives the app full media control of PLAY, PAUSE, STOP and SEEK for the loaded media.

  • PLAY/PAUSE: playerController.playOrPause();
  • STOP: playerController.stop();
  • SEEK: playerController.seek();

The RemotePlayer and RemotePlayerController can be used with data binding frameworks, such as Polymer or Angular, to implement a remote player.

Here is a code snippet for Angular:

<button id="playPauseButton" class="playerButton"
  ng-disabled="!player.canPause"
  ng-click="controller.playOrPause()">
    {{player.isPaused ? 'Play' : 'Pause'}}
</button>
<script>
var player = new cast.framework.RemotePlayer();
var controller = new cast.framework.RemotePlayerController(player);
// Listen to any player update, and trigger angular data binding
update.controller.addEventListener(
  cast.framework.RemotePlayerEventType.ANY_CHANGE,
  function(event) {
    if (!$scope.$$phase) $scope.$apply();
  });
</script>

Media status

During media playback, various events occur which can be captured by setting listeners for various cast.framework.RemotePlayerEventType events on the RemotePlayerControllerobject.

To get the media status information, use the cast.framework.RemotePlayerEventType.MEDIA_INFO_CHANGED event, which triggers when playback changes and when the CastSession.getMediaSession().media changes.

playerController.addEventListener(
  cast.framework.RemotePlayerEventType.MEDIA_INFO_CHANGED, function() {
    // Use the current session to get an up to date media status.
    let session = cast.framework.CastContext.getInstance().getCurrentSession();

    if (!session) {
        return;
    }

    // Contains information about the playing media including currentTime.
    let mediaStatus = session.getMediaSession();
    if (!mediaStatus) {
        return;
    }

    // mediaStatus also contains the mediaInfo containing metadata and other
    // information about the in progress content.
    let mediaInfo = mediaStatus.media;
  });

When events such as pause, play, resume or seek occur, the app will need to act on them and synchronize between itself and the Web Receiver app on the Cast device. See Status updates for more information.

How session management works

The Cast SDK introduces the concept of a Cast session, the establishment of which combines the steps of connecting to a device, launching (or joining) a Web Receiver app, connecting to that app, and initializing a media control channel. See the Web Receiver Application life cycle guide for more information about Cast sessions and the Web Receiver life cycle.

Sessions are managed by the class CastContext, which your app can retrieve via cast.framework.CastContext.getInstance(). Individual sessions are represented by subclasses of the class Session. For example, CastSession represents sessions with Cast devices. Your app can access the currently active Cast session via CastContext.getCurrentSession().

To monitor the session state, add a listener to the CastContext for the CastContextEventType.SESSION_STATE_CHANGED event type.

var context = cast.framework.CastContext.getInstance();
context.addEventListener(
  cast.framework.CastContextEventType.SESSION_STATE_CHANGED,
  function(event) {
    switch (event.sessionState) {
      case cast.framework.SessionState.SESSION_STARTED:
      case cast.framework.SessionState.SESSION_RESUMED:
        break;
      case cast.framework.SessionState.SESSION_ENDED:
        console.log('CastContext: CastSession disconnected');
        // Update locally as necessary
        break;
    }
  })

For disconnection, such as when the user clicks the "stop casting" button from the Cast dialog, you can add a listener for the RemotePlayerEventType.IS_CONNECTED_CHANGED event type in your listener. In your listener check if RemotePlayer is disconnected. If so, update the local player state as necessary. For example:

playerController.addEventListener(
  cast.framework.RemotePlayerEventType.IS_CONNECTED_CHANGED, function() {
    if (!player.isConnected) {
      console.log('RemotePlayerController: Player disconnected');
      // Update local player to disconnected state
    }
  });

While the user can directly control Cast termination via the framework Cast button, the sender itself can stop casting using the current CastSession object.

function stopCasting() {
  var castSession = cast.framework.CastContext.getInstance().getCurrentSession();
  // End the session and pass 'true' to indicate
  // that Web Receiver app should be stopped.
  castSession.endSession(true);
}

Stream transfer

Preserving session state is the basis of stream transfer, where users can move existing audio and video streams across devices using voice commands, Google Home App, or smart displays. Media stops playing on one device (the source) and continues on another (the destination). Any Cast device with the latest firmware can serve as sources or destinations in a stream transfer.

To get the new destination device during the stream transfer, call CastSession#getCastDevice() when the cast.framework.SessionState.SESSION_RESUMED event is called.

See Stream transfer on Web Receiver for more information.

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