Add Core Features to Your Web Receiver

This page contains code snippets and descriptions of the features available for customizing a Web Receiver app.

Creating a customized Web Receiver app

The main structure of a customized Web Receiver app includes required elements (shown in bold) along with optional features to customize the app for your particular use case.

  1. A cast-media-player element that represents the built-in player UI provided with Web Receiver.
  2. Custom CSS-like styling for the cast-media-player element to style various UI elements such as the background-image, splash-image, and font-family.
  3. A script element to load the Web Receiver framework.
  4. JavaScript code to customize Web Receiver app by intercepting messages and handling events.
  5. Queue for autoplay.
  6. Options to configure playback.
  7. Options to set the Web Receiver context.
  8. Options to set commands which are supported by the Web Receiver app.
  9. A JavaScript call to start the Web Receiver application.

See the CastReceiver GitHub sample for a Web Receiver application that illustrates this full structure.

Application configuration and options

The CastReceiverContext is the outermost class exposed to the developer, and it manages loading of underlying libraries and handles the initialization of the Web Receiver SDK.

If the Web Receiver API detects that a sender is disconnected it will raise the SENDER_DISCONNECTED event. If the Web Receiver has not been able to communicate with the sender for what we described as maxInactivity seconds, it will also raise the SENDER_DISCONNECTED event. During development it is a good idea to set maxInactivity to a high value so that the Web Receiver app does not close when debugging the app with the Chrome Remote Debugger:

const context = cast.framework.CastReceiverContext.getInstance();
const options = new cast.framework.CastReceiverOptions();
options.maxInactivity = 3600; //Development only
context.start(options);

However, for a published Web Receiver application it is better to not set maxInactivity and instead rely on the default value. Note that the Web Receiver options are set only once in the application.

The other configuration is the cast.framework.PlaybackConfig. This can be set as follows:

const playbackConfig = new cast.framework.PlaybackConfig();
playbackConfig.manifestRequestHandler = requestInfo => {
  requestInfo.withCredentials = true;
};
context.start({playbackConfig: playbackConfig});

This configuration affects each content playback and essentially provides override behavior. For a list of behaviors that developers can override, see the definition of cast.framework.PlaybackConfig. To change the configuration in between contents, one can use the PlayerManager to get its current playbackConfig, modify or add an override and reset the playbackConfig like this:

const playerManager =
    cast.framework.CastReceiverContext.getInstance().getPlayerManager();
const playbackConfig = (Object.assign(
            new cast.framework.PlaybackConfig(), playerManager.getPlaybackConfig()));
playbackConfig.autoResumeNumberOfSegments = 1;
playerManager.setPlaybackConfig(playbackConfig);

Note that if PlaybackConfig was not overridden, getPlaybackConfig() returns a null object. And any property on PlaybackConfig that is undefined will use the default values.

Event listener

The Web Receiver SDK allows your Web Receiver app to handle player events. The event listener takes a cast.framework.events.EventType parameter (or an array of these parameters) that specifies the event(s) that should trigger the listener. Preconfigured arrays of cast.framework.events.EventType that are useful for debugging can be found in cast.framework.events.category. The event parameter provides additional information about the event.

For example, if you want to know when a mediaStatus change is being broadcasted, you can use the following logic to handle the event:

const playerManager =
    cast.framework.CastReceiverContext.getInstance().getPlayerManager();
playerManager.addEventListener(
    cast.framework.events.EventType.MEDIA_STATUS, (event) => {
      // Write your own event handling code, for example
      // using the event.mediaStatus value
});

Note: The Web Receiver framework automatically tracks when a sender connects or disconnects from it and doesn't require an explicit SENDER_DISCONNECTED event listener in your own Web Receiver logic (as in Web Receiver v2).

Message interception

Web Receiver SDK allows your Web Receiver app to intercept messages and execute custom code on those messages. The message interceptor takes a cast.framework.messages.MessageType parameter that specifies what type of message should be intercepted.

Note: The interceptor should return the modified request or a Promise that resolves with the modified request value. Returning null will prevent calling the default message handler.

For example, if you want to change the load request data, you can use the following logic to intercept and modify it.

Tip: Also see Loading media using contentId, contentUrl and entity.

const context = cast.framework.CastReceiverContext.getInstance();
const playerManager = context.getPlayerManager();

playerManager.setMessageInterceptor(
    cast.framework.messages.MessageType.LOAD, loadRequestData => {
      const error = new cast.framework.messages.ErrorData(
                      cast.framework.messages.ErrorType.LOAD_FAILED);
      if (!loadRequestData.media) {
        error.reason = cast.framework.messages.ErrorReason.INVALID_PARAM;
        return error;
      }

      if (!loadRequestData.media.entity) {
        return loadRequestData;
      }

      return thirdparty.fetchAssetAndAuth(loadRequestData.media.entity,
                                          loadRequestData.credentials)
        .then(asset => {
          if (!asset) {
            throw cast.framework.messages.ErrorReason.INVALID_REQUEST;
          }

          loadRequestData.media.contentUrl = asset.url;
          loadRequestData.media.metadata = asset.metadata;
          loadRequestData.media.tracks = asset.tracks;
          return loadRequestData;
        }).catch(reason => {
          error.reason = reason; // cast.framework.messages.ErrorReason
          return error;
        });
    });

context.start();

Error handling

When errors happen in message interceptor, your Web Receiver app should return an appropriate cast.framework.messages.ErrorType and cast.framework.messages.ErrorReason.

playerManager.setMessageInterceptor(
    cast.framework.messages.MessageType.LOAD, loadRequestData => {
      const error = new cast.framework.messages.ErrorData(
                      cast.framework.messages.ErrorType.LOAD_CANCELLED);
      if (!loadRequestData.media) {
        error.reason = cast.framework.messages.ErrorReason.INVALID_PARAM;
        return error;
      }

      ...

      return fetchAssetAndAuth(loadRequestData.media.entity,
                               loadRequestData.credentials)
        .then(asset => {
          ...
          return loadRequestData;
        }).catch(reason => {
          error.reason = reason; // cast.framework.messages.ErrorReason
          return error;
        });
    });

Message interception vs event listener

Some key differences between message interception and event listener are as follows:

  • An event listener does not allow you to modify the request data.
  • An event listener is best used to trigger analytics or a custom function.

Note: An event listener can also be used to listen to an umbrella of events using the CORE, DEBUG, or FINE enum.

playerManager.addEventListener(cast.framework.events.category.CORE,
    event => {
        console.log(event);
    });
  • Message interception allows you to listen to a message, intercept it, and modify the request data itself.
  • Message interception is best used to handle custom logic with regards to request data.

Tip: Loading media using contentId, contentUrl and entity

We suggest you use entity in your implementation for both your sender and receiver apps. The entity property is a deep link URL that can be either a playlist or a specific media content.

The contentUrl is designed for a playable URL.

The contentId has been deprecated. It is typically the URL of the media, and can be used as a real ID or key parameter for custom lookup. We suggest you to use entity to store the real ID or key parameters, and use contentUrl for the URL of the media.

playerManager.setMessageInterceptor(
    cast.framework.messages.MessageType.LOAD, loadRequestData => {
      ...

      if (!loadRequestData.media.entity) {
        // Copy the value from contentId for legacy reasons if needed
        loadRequestData.media.entity = loadRequestData.media.contentId;
      }

      return thirdparty.fetchAssetAndAuth(loadRequestData.media.entity,
                                          loadRequestData.credentials)
        .then(asset => {
          loadRequestData.media.contentUrl = asset.url;
          ...
          return loadRequestData;
        });
    });

Device capabilities

The getDeviceCapabilities method provides device information on the connected Cast device and the video or audio device attached to it. The getDeviceCapabilities method provides support information for Google Assistant, Bluetooth, and the connected display and audio devices.

This method returns an object which you can query by passing in one of the specified enums to get the device capability for that enum. The enums are defined in cast.framework.system.DeviceCapabilities.

This example checks if the Web Receiver device is capable of playing HDR and DolbyVision (DV) with the IS_HDR_SUPPORTED and IS_DV_SUPPORTED keys, respectively.

const context = cast.framework.CastReceiverContext.getInstance();
context.addEventListener(cast.framework.system.EventType.READY, () => {
  const deviceCapabilities = context.getDeviceCapabilities();
  if (deviceCapabilities &&
      deviceCapabilities[cast.framework.system.DeviceCapabilities.IS_HDR_SUPPORTED]) {
    // Write your own event handling code, for example
    // using the deviceCapabilities[cast.framework.system.DeviceCapabilities.IS_HDR_SUPPORTED] value
  }
  if (deviceCapabilities &&
      deviceCapabilities[cast.framework.system.DeviceCapabilities.IS_DV_SUPPORTED]) {
    // Write your own event handling code, for example
    // using the deviceCapabilities[cast.framework.system.DeviceCapabilities.IS_DV_SUPPORTED] value
  }
});
context.start();

Supported media commands

The UI controls provided on the iOS and Android sender expanded controllers and touch devices are driven by the MediaStatus.supportedMediaCommands. This value can be changed on the Web Receiver by:

  1. Using PlayerManager.setSupportedMediaCommands to set the specific Commands
  2. Adding a new command using addSupportedMediaCommands
  3. Removing an existing command using removeSupportedMediaCommands.
playerManager.setSupportedMediaCommands(cast.framework.messages.Command.SEEK |
  cast.framework.messages.Command.PAUSE);

For more information about supported media commands and touch devices see Accessing UI controls guide.

Voice commands

The following media commands are currently supported in the Web Receiver SDK for Assistant-enabled devices. The default implementations of these commands are found in cast.framework.PlayerManager.

Command Description
Play Play or resume playback from paused state.
Pause Pause currently playing content.
Previous Skip to the previous media item in your media queue.
Next Skip to the next media item in your media queue.
Stop Stop the currently playing media.
Repeat None Disable repeating of media items in the queue once the last item in the queue is done playing.
Repeat Single Repeat the currently playing media indefinitely.
Repeat All Repeat all items in the queue once the last item in the queue is played.
Repeat All and Shuffle Once the last item in the queue is done playing, shuffle the queue and repeat all items in the queue.
Shuffle Shuffle media items in your media queue.
Closed Captions ON / OFF Enable / Disable Closed Captioning for your media. Enable / Disable is also available by language.
Seek to absolute time Jumps to the specified absolute time.
Seek to time relative to current time Jumps forward or backward by the specified time period relative to the current playback time.
Play Again Restart the currently playing media or play the last played media item if nothing is currently playing.
Set playback rate Vary media playback rate. This should be handled by default. You can use the SET_PLAYBACK_RATE message interceptor to override incoming rate requests.

Supported media commands with voice

To prevent a voice command from triggering a media command on an Assistant- enabled device, you must first set the supported media commands that you plan on supporting. Then you must enforce those commands by enabling the CastReceiverOptions.enforceSupportedCommands property. The UI on Cast SDK senders and touch-enabled devices will change to reflect these configurations. If the flag is not enabled the incoming voice commands will execute.

For example, if you allow PAUSE from your sender applications and touch-enabled devices, you must also configure your receiver to reflect those settings. When configured, any incoming voice commands will be dropped if not included in the list of supported commands.

In the example below we are supplying the CastReceiverOptions when starting the CastReceiverContext. We've added support for the PAUSE command and enforced the player to support only that command. Now if a voice command requests another operation such as SEEK it will be denied. The user will be notified that the command is not supported yet.

const context = cast.framework.CastReceiverContext.getInstance();

context.start({
  enforceSupportedCommands: true,
  supportedCommands: cast.framework.messages.Command.PAUSE
});

To further customize the behavior you can apply separate logic for each command that you want to restrict. Remove the enforceSupportedCommands flag and for each command that you want to restrict you can intercept the incoming message. Here we intercept the request provided by the SDK so that SEEK commands issued to Assistant-enabled devices do not trigger a seek in your Web Receiver application.

For media commands your application does not support, return an appropriate error reason, such as NOT_SUPPORTED.

playerManager.setMessageInterceptor(cast.framework.messages.MessageType.SEEK,
  seekData => {
    // Block seeking if the SEEK supported media command is disabled
    if (!(playerManager.getSupportedMediaCommands() & cast.framework.messages.Command.SEEK)) {
      let e = new cast.framework.messages.ErrorData(cast.framework.messages.ErrorType
      .INVALID_REQUEST);
      e.reason = cast.framework.messages.ErrorReason.NOT_SUPPORTED;
      return e;
    }

    return seekData;
  });

Ducking from voice activity

If the Cast platform ducks your application's sound due to Assistant activity such as listening to user speech or talking back, a FocusState message of NOT_IN_FOCUS is sent to the Web Receiver application when the activity starts. Another message with IN_FOCUS is sent when the activity ends. Depending on your application and the media being played, you might want to pause media when the FocusState is NOT_IN_FOCUS.

For example, it's a good user experience to pause audiobook playback if the Assistant is responding to a user query.

Voice-specified caption language

When a user does not explicitly state the language for the captions, the language used for captions is the same language in which the command was spoken. In these scenarios, the isSuggestedLanguage parameter of the incoming message indicates whether the associated language was suggested or explicitly requested by user.

For example, isSuggestedLanguage is set to true for the command "OK Google, turn captions on," because the language was inferred by the language the command was spoken in. If the language is explicitly requested, such as in "OK Google, turn on English captions," isSuggestedLanguage is set to false.

Metadata and voice casting

While voice commands are handled by the Web Receiver by default, you should ensure the metadata for your content is complete and accurate. This ensures that voice commands are handled properly by the Assistant and that the metadata surfaces properly across new types of interfaces such as the Google Home app and smart displays like the Google Home Hub.

Preserving session state

The Web Receiver SDK provides a default implementation for Web Receiver apps to preserve session states by taking a snapshot of current media status, converting the status into a load request, and resuming the session with the load request.

The load request generated by the Web Receiver can be overridden in the SESSION_STATE message interceptor if necessary. If you want to add custom data into the load request, we suggest putting them in loadRequestData.customData.

playerManager.setMessageInterceptor(
    cast.framework.messages.MessageType.SESSION_STATE,
    function (sessionState) {
        // Override sessionState.loadRequestData if needed.
        const newCredentials = updateCredentials_(sessionState.loadRequestData.credentials);
        sessionState.loadRequestData.credentials = newCredentials;

        // Add custom data if needed.
        sessionState.loadRequestData.customData = {
            'membership': 'PREMIUM'
        };

        return sessionState;
    });

The custom data can be retrieved from loadRequestData.customData in the RESUME_SESSION message interceptor.

let cred_ = null;
let membership_ = null;

playerManager.setMessageInterceptor(
    cast.framework.messages.MessageType.RESUME_SESSION,
    function (resumeSessionRequest) {
        let sessionState = resumeSessionRequest.sessionState;

        // Modify sessionState.loadRequestData if needed.
        cred_ = sessionState.loadRequestData.credentials;

        // Retrieve custom data.
        membership_ = sessionState.loadRequestData.customData.membership;

        return resumeSessionRequest;
    });

Stream transfer

Preserving session state is the basis of stream transfer, a CAF feature 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.

The event flow for stream transfer is:

  1. On the source device:
    1. Media stops playing.
    2. The Web Receiver application receives a command to save the current media state.
    3. The Web Receiver application is shut down.
  2. On the destination device:
    1. The Web Receiver application is loaded.
    2. The Web Receiver application receives a command to restore the saved media state.
    3. Media resumes playing.

Elements of media state include:

  • Specific position or timestamp of the song, video, or media item.
  • Its place in a broader queue (such as a playlist or artist radio).
  • The authenticated user.
  • Playback state (for example, playing or paused).

Enabling stream transfer

To implement stream transfer for your Web Receiver:

  1. Update supportedMediaCommands with the STREAM_TRANSFER command: 
    playerManager.addSupportedMediaCommands(
cast.framework.messages.Command.STREAM_TRANSFER, true);
  2. Optionally override the SESSION_STATE and RESUME_SESSION message interceptors as described in Preserving session state. Only override these if custom data needs to be stored as part of the session snapshot. Otherwise, the default implementation for preserving session states will support stream transfer.

Custom UI data binding

If you want to use your own custom UI element instead of cast-media-player, you can do that; instead of adding cast-media-player element to your HTML, instead use the PlayerDataBinder class to bind the UI to the player state. The binder also supports sending events for data changes, if the app does not support data binding.

const context = cast.framework.CastReceiverContext.getInstance();
const player = context.getPlayerManager();

const playerData = {};
const playerDataBinder = new cast.framework.ui.PlayerDataBinder(playerData);

// Update ui according to player state
playerDataBinder.addEventListener(
    cast.framework.ui.PlayerDataEventType.STATE_CHANGED,
    e => {
      switch (e.value) {
        case cast.framework.ui.State.LAUNCHING:
        case cast.framework.ui.State.IDLE:
          // Write your own event handling code
          break;
        case cast.framework.ui.State.LOADING:
          // Write your own event handling code
          break;
        case cast.framework.ui.State.BUFFERING:
          // Write your own event handling code
          break;
        case cast.framework.ui.State.PAUSED:
          // Write your own event handling code
          break;
        case cast.framework.ui.State.PLAYING:
          // Write your own event handling code
          break;
      }
    });
context.start();

You should add at least one MediaElement to the HTML so that the Web Receiver can use it. If multiple MediaElement objects are available, you should tag the MediaElement that you want the Web Receiver to use. You do this by adding castMediaElement in the video's class list, as shown below; otherwise, the Web Receiver will choose the first MediaElement.

<video class="castMediaElement"></video>

Note, Web Receiver currently discourages passing a custom MediaElement as you lose many benefits of using Web Receiver SDK.

Content preload

The Web Receiver supports preloading of media items after the current playback item in the queue. The preload operation pre-downloads several segments of the upcoming items. The specification is done on the preloadTime value in the QueueItem object (default to 20 seconds if not provided). The time is expressed in seconds, relative to the end of the currently playing item . Only positive values are valid. For example, if the value is 10 seconds, this item will be preloaded 10 seconds before the previous item has finished. If the time to preload is higher than the time left on the currentItem, the preload will just happen as soon as possible. So if a very large value of preload is specified on the queueItem, one could achieve the effect of whenever we are playing the current item we are already preloading the next item. However, we leave the setting and choice of this to developer as this value can affect bandwidth and streaming performance of the current playing item.

Note that preloading will work for HLS and Smooth streaming content by default. For DASH content, preloading works if useLegacyDashSupport is specified in CastReceiverOptions, since Media Player Library (MPL) supports preload while Shaka does not yet. For regular MP4 video and audio files such as MP3, those will not be preloaded, as Cast devices support one media element only and cannot be used to preload while an existing content item is still playing.

Custom messages

Message exchange is the key interaction method for Web Receiver applications.

A sender issues messages to a Web Receiver using the sender APIs for the platform the sender is running (Android, iOS, Chrome). The event object (which is the manifestation of a message) that is passed to the event listeners has a data element (event.data) where the data takes on the properties of the specific event type.

A Web Receiver application may choose to listen for messages on a specified namespace. By virtue of doing so, the Web Receiver application is said to support that namespace protocol. It is then up to any connected senders wishing to communicate on that namespace to use the appropriate protocol.

All namespaces are defined by a string and must begin with "urn:x-cast:" followed by any string. For example, "urn:x-cast:com.example.cast.mynamespace".

Note: The media message namespace "urn:x-cast:com.google.cast.media" is reserved. Media messages sent using the Cast APIs on both the sender and Web Receiver use the media namespace protocol by convention. See Media and players for more about media messages.

Here is a code snippet for the Web Receiver to listen to custom messages from connected senders:

const context = cast.framework.CastReceiverContext.getInstance();

const CUSTOM_CHANNEL = 'urn:x-cast:com.example.cast.mynamespace';
context.addCustomMessageListener(CUSTOM_CHANNEL, function(customEvent) {
  // handle customEvent.
});

context.start();

Similarly, Web Receiver applications can keep senders informed about the state of the Web Receiver by sending messages to connected senders. A Web Receiver application can send messages using sendCustomMessage(namespace, senderId, message) on CastReceiverContext. A Web Receiver can send messages to an individual sender, either in response to a received message or due to an application state change. Beyond point-to-point messaging (with a limit of 64kb), a Web Receiver may also broadcast messages to all connected senders.

Cast for audio devices

See Google Cast for audio devices guide for support on audio only playback.

Android TV

This section discusses how the Google Web Receiver uses your inputs as playback, and Android TV compatibility.

Integrating your application with the remote control

The Google Web Receiver running on the Android TV device translates input from the device's control inputs (i.e. hand-held remote control) as media playback messages defined for the urn:x-cast:com.google.cast.media namespace, as described in Media Playback Messages. Your application must support these messages to control the application media playback in order to allow basic playback control from Android TV’s control inputs.

Guidelines for Android TV compatibility

Making your Cast application compatible with Android TV requires very little additional work. Here are some recommendations and common pitfalls to avoid in order to ensure your application is compatible with Android TV:

  • Be aware that the user-agent string contains both "Android" and "CrKey"; some sites may redirect to a mobile-only site because they detect the "Android" label. Don't assume that "Android" in the user-agent string always indicates a mobile user.
  • Android's media stack may use transparent GZIP for fetching data. Make sure your media data can respond to Accept-Encoding: gzip.
  • Android TV HTML5 media events may be triggered in different timings than Chromecast, this may reveal issues that were hidden on Chromecast.
  • When updating the media, use media related events fired by <audio>/<video> elements, like timeupdate, pause and waiting. Avoid using networking related events like progress, suspend and stalled, as these tend to be platform dependent. See Media events for more information about handling media events in your receiver.
  • When configuring your receiver site’s HTTPS certificates, be sure to include intermediate CA certificates. See the Qualsys SSL test page to verify: if the trusted certification path for your site includes a CA certificate labelled “extra download”, then it may not load on Android-based platforms.
  • While Chromecast displays the receiver page on a 720p graphics plane, other Cast platforms including Android TV may display the page up to 1080p. Ensure your receiver page scales gracefully at different resolutions.
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