A media track can be an audio or video stream object, or a text object (subtitle or caption). > [!NOTE] > **Note:** The [Styled](https://developers.google.com/cast/docs/styled_receiver) and [Default Media](https://developers.google.com/cast/docs/web_receiver#default_media_web_receiver) Receivers allow you to use only the text tracks with the API. To work with the audio and video tracks, you must develop a [Custom Receiver](https://developers.google.com/cast/docs/web_receiver/basic). A [`GCKMediaTrack`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_track) object represents a track. It consists of a unique numeric identifier and other attributes such as a content ID and title. A `GCKMediaTrack` instance can be created as follows: Swift ```swift let captionsTrack = GCKMediaTrack.init(identifier: 1, contentIdentifier: "https://some-url/caption_en.vtt", contentType: "text/vtt", type: GCKMediaTrackType.text, textSubtype: GCKMediaTextTrackSubtype.captions, name: "English Captions", languageCode: "en", customData: nil) ``` Objective-C ```objective-c GCKMediaTrack *captionsTrack = [[GCKMediaTrack alloc] initWithIdentifier:1 contentIdentifier:@"https://some-url/caption_en.vtt" contentType:@"text/vtt" type:GCKMediaTrackTypeText textSubtype:GCKMediaTextTrackSubtypeCaptions name:@"English Captions" languageCode:@"en" customData:nil]; ``` A media item can have multiple tracks; for example, it can have multiple subtitles (each for a different language) or multiple alternative audio streams (for different languages). [`GCKMediaInformation`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_information) is the class that represents a media item. To associate a collection of [`GCKMediaTrack`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_track) objects with a media item, your app should update its `mediaTracks` property. Your app needs to make this association before it loads the media to the receiver, as in the following code: Swift ```swift let tracks = [captionsTrack] let url = URL.init(string: "https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4") guard let mediaURL = url else { print("invalid mediaURL") return } let mediaInfoBuilder = GCKMediaInformationBuilder.init(contentURL: mediaURL) mediaInfoBuilder.streamType = GCKMediaStreamType.none; mediaInfoBuilder.contentType = "video/mp4" mediaInfoBuilder.metadata = metadata; mediaInfoBuilder.mediaTracks = tracks; mediaInformation = mediaInfoBuilder.build() ``` Objective-C ```objective-c NSArray *tracks = @[captionsTrack]; GCKMediaInformationBuilder *mediaInfoBuilder = [[GCKMediaInformationBuilder alloc] initWithContentURL: [NSURL URLWithString:@"https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4"]]; mediaInfoBuilder.streamType = GCKMediaStreamTypeNone; mediaInfoBuilder.contentType = @"video/mp4"; mediaInfoBuilder.metadata = metadata; mediaInfoBuilder.mediaTracks = tracks; self.mediaInformation = [mediaInfoBuilder build]; ``` Activate one or more tracks that were associated with the media item (after the media is loaded) by calling `-[setActiveTrackIDs:]` on [`GCKRemoteMediaClient`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_remote_media_client) and passing the IDs of the tracks to be activated. For example, the following code activates the captions track created above. Swift ```swift sessionManager.currentSession?.remoteMediaClient?.setActiveTrackIDs([1]) ``` Objective-C ```objective-c [self.sessionManager.currentSession.remoteMediaClient setActiveTrackIDs:@[@1]]; ``` To deactivate a track on the current media item, call `-[setActiveTrackIDs:]` on [`GCKRemoteMediaClient`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_remote_media_client) with an empty array or nil. The following code disables the captions track. Swift ```swift sessionManager.currentSession?.remoteMediaClient?.setActiveTrackIDs([]) ``` Objective-C ```objective-c [self.sessionManager.currentSession.remoteMediaClient setActiveTrackIDs:@[]]; ``` ## Style text tracks The [`GCKMediaTextTrackStyle`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_text_track_style) class encapsulates the style information of a text track. A track style can be applied to the currently playing media item by calling [`-[GCKRemoteMediaClient setTextTrackStyle]`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_control_channel). The track style created in the code below turns text red (FF) at 50% opacity (80) and sets a serif font. Swift ```swift let textTrackStyle = GCKMediaTextTrackStyle.createDefault() textTrackStyle.foregroundColor = GCKColor.init(cssString: "#FF000080") textTrackStyle.fontFamily = "serif" styleChangeRequest = sessionManager.currentSession?.remoteMediaClient?.setTextTrackStyle(textTrackStyle) styleChangeRequest?.delegate = self ``` Objective-C ```objective-c GCKMediaTextTrackStyle *textTrackStyle = [GCKMediaTextTrackStyle createDefault]; [textTrackStyle setForegroundColor:[[GCKColor alloc] initWithCSSString:@"#FF000080"]]; [textTrackStyle setFontFamily:@"serif"]; self.styleChangeRequest = [self.sessionManager.currentSession.remoteMediaClient setTextTrackStyle:textTrackStyle]; self.styleChangeRequest.delegate = self; ``` You can use the returned [`GCKRequest`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_request) object for tracking this request. Swift ```swift // MARK: - GCKRequestDelegate func requestDidComplete(_ request: GCKRequest) { if request == styleChangeRequest { print("Style update completed.") styleChangeRequest = nil } } ``` Objective-C ```objective-c #pragma mark - GCKRequestDelegate - (void)requestDidComplete:(GCKRequest *)request { if (request == self.styleChangeRequest) { NSLog(@"Style update completed."); self.styleChangeRequest = nil; } } ``` See [Status updates](https://developers.google.com/cast/docs/ios_sender_advanced#receive_status_updates) below for more information. Apps should allow users to update the style for text tracks, either using the settings provided by the system or by the app itself. There is a default style provided, which can be retrieved via the static method [`+[GCKMediaTextTrackStyle createDefault]`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_text_track_style). The following text track style elements can be changed: - Foreground (text) color and opacity - Background color and opacity - Edge type - Edge Color - Font Scale - Font Family - Font Style ## Receive status updates When multiple senders are connected to the same receiver, it is important for each sender to be aware of the changes on the receiver even if those changes were initiated from other senders. > [!NOTE] > **Note:** This is important for all apps, not only those that explicitly support multiple senders, because some Cast devices have control inputs (remotes, buttons) that behave as virtual senders, affecting the status on the receiver. To ensure your sender receives status updates from the receiver, your app should register a [`GCKRemoteMediaClientListener`](https://developers.google.com/cast/docs/reference/ios/protocol_g_c_k_remote_media_client_listener-p). If the [`GCKMediaTextTrackStyle`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_text_track_style) of the current media changes, then all of the connected senders will be notified through both the `-[remoteMediaClient:didUpdateMediaMetadata:]` and the `-[remoteMediaClient:didUpdateMediaStatus:]` callbacks. In this case, the Receiver SDK does not verify whether the new style is different from the previous one and notifies all the connected senders regardless. If, however, the list of active tracks is updated, only the `-[remoteMediaClient:didUpdateMediaStatus:]` in connected senders will be notified. > [!NOTE] > **Note:** The list of tracks associated with the currently loaded media cannot be changed. If needed, you have to update the tracks information on the [`GCKMediaInformation`](https://developers.google.com/cast/docs/reference/ios/interface_g_c_k_media_information) object and reload the media. ## Satisfy CORS requirements For adaptive media streaming, Google Cast requires the presence of CORS headers, but even simple mp4 media streams require CORS if they include Tracks. If you want to enable Tracks for any media, you must enable CORS for both your track streams and your media streams. So, if you do not have CORS headers available for your simple mp4 media on your server, and you then add a simple subtitle track, you will not be able to stream your media unless you update your server to include the appropriate CORS header. In addition, you need to allow at least the following headers: Content-Type, Accept-Encoding, and Range. Note that the last two headers are additional headers that you may not have needed previously.