Matter Casting APIs

Matter Casting consists of three parts:

  • The mobile app: For most content providers, this would be your consumer-facing mobile app. By making your mobile app a Matter “Casting Client”, you enable the user to discover casting targets, cast content, and control casting sessions. The example Matter tv-casting-app for Android / iOS and Linux builds on top of the Matter SDK to demonstrate how a TV Casting mobile app works.
  • The TV content app: For most content providers, this would be your consumer-facing app on a Smart TV. By enhancing your TV app to act as a Matter “Content app”, you enable Matter Casting Clients to cast content. The example Matter content-app for Android builds on top of the Matter SDK to demonstrate how a TV Content app works.
  • The TV platform app: The TV platform app implements the Casting Video
    Player device type and provides common capabilities around media playback on the TV such as play/pause, keypad navigation, input and output control, content search, and an implementation of an app platform as described in the media chapter of the device library specification. This is generally implemented by the TV manufacturer. The example Matter tv-app for Android builds on top of the Matter SDK to demonstrate how a TV platform app works.

This document describes how enable your Android and iOS apps to act as a Matter “Casting Client”. This documentation is also designed to work with the example example Matter tv-casting-app samples so you can see the experience end to end.

Introduction

A Casting Client (e.g. a mobile phone app) is expected to be a Matter Commissionable Node and a CastingPlayer (i.e. a TV) is expected to be a Matter Commissioner. In the context of the Matter Video Player architecture, a CastingPlayer would map to Casting “Video” Player. The CastingPlayer is expected to be hosting one or more Endpoints (some of which can represent Content Apps in the Matter Video Player architecture) that support one or more Matter Media Clusters.

The steps to start a casting session are:

  1. Initialize the CastingClient using the Matter SDK.
  2. Discover CastingPlayer devices using Matter Commissioner discovery.
  3. Connect to the CastingPlayer to discover available endpoints. By connecting, the CastingClient will send a User Directed Commissioning (UDC) request to the CastingPlayer device in order to make a Matter commissioning request. The CastingPlayer will then obtain the appropriate user consent to allow a connection from this CastingClient and obtain the setup code needed to commission the CastingClient. The setup code will typically come from a corresponding TV content app or be input by the user.
  4. Select an available Endpoint hosted by the CastingPlayer.

Next, you're ready to:

  1. Issue commands to the Endpoint.
  2. Read endpoint attributes like playback state.
  3. Subscribe to playback events.

In order to illustrate these steps, refer to the figure below workflow of casting video player

Build and Setup

The Casting Client is expected to consume the Matter TV Casting library built for its respective platform which implements the APIs described in this document. Refer to the tv-casting-app READMEs for Linux, Android and iOS to understand how to build and consume each platform‘s specific libraries. The libraries MUST be built with the client’s specific values for CHIP_DEVICE_CONFIG_DEVICE_VENDOR_ID and CHIP_DEVICE_CONFIG_DEVICE_PRODUCT_ID updated in the CHIPProjectAppConfig.h file. Other values like the CHIP_DEVICE_CONFIG_DEVICE_NAME may be updated as well to correspond to the client being built.

Initialize the Casting Client

{Complete Initialization examples: Linux | Android | iOS}

A Casting Client must first initialize the Matter SDK and define the following DataProvider objects for the the Matter Casting library to use throughout the client's lifecycle:

  1. Rotating Device Identifier - “This unique per-device identifier SHALL consist of a randomly-generated 128-bit or longer octet string.” Refer to the Matter specification for more details. Instantiate a DataProvider object as described below to provide this identifier.

    On Linux, define a RotatingDeviceIdUniqueIdProvider to provide the Casting Client's RotatingDeviceIdUniqueId, by implementing a matter:casting::support::MutableByteSpanDataProvider:

    class RotatingDeviceIdUniqueIdProvider : public MutableByteSpanDataProvider {
private:
    chip::MutableByteSpan rotatingDeviceIdUniqueIdSpan;
    uint8_t rotatingDeviceIdUniqueId[chip::DeviceLayer::ConfigurationManager::kRotatingDeviceIDUniqueIDLength];

public:
    RotatingDeviceIdUniqueIdProvider() {
        // generate a random Unique ID for this example app for demonstration
        for (size_t i = 0; i < sizeof(rotatingDeviceIdUniqueId); i++) {
            rotatingDeviceIdUniqueId[i] = chip::Crypto::GetRandU8();
        }
        rotatingDeviceIdUniqueIdSpan = chip::MutableByteSpan(rotatingDeviceIdUniqueId);
    }

    chip::MutableByteSpan * Get() { return &rotatingDeviceIdUniqueIdSpan; }
};

    On Android, define a rotatingDeviceIdUniqueIdProvider to provide the Casting Client's RotatingDeviceIdUniqueId, by implementing a com.matter.casting.support.DataSource:

    private static final DataProvider<byte[]> rotatingDeviceIdUniqueIdProvider =
  new DataProvider<byte[]>() {
    private static final String ROTATING_DEVICE_ID_UNIQUE_ID =
        "EXAMPLE_IDENTIFIER"; // dummy value for demonstration only

    @Override
    public byte[] get() {
      return ROTATING_DEVICE_ID_UNIQUE_ID.getBytes();
    }
  };

    On iOS, define the func castingAppDidReceiveRequestForRotatingDeviceIdUniqueId in a class, MCAppParametersDataSource, that implements the MCDataSource:

    class MCAppParametersDataSource : NSObject, MCDataSource
{
    func castingAppDidReceiveRequestForRotatingDeviceIdUniqueId(_ sender: Any) -> Data {
        // dummy value, with at least 16 bytes (ConfigurationManager::kMinRotatingDeviceIDUniqueIDLength), for demonstration only
        return "0123456789ABCDEF".data(using: .utf8)!
    }
    ...
}

  2. Commissioning Data - This object contains the passcode, discriminator, etc. which identify the app and are provided to the CastingPlayer during the commissioning process. “A Passcode SHALL be included as a 27-bit unsigned integer, which serves as proof of possession during commissioning.” “A Discriminator SHALL be included as a 12-bit unsigned integer, which SHALL match the value which a device advertises during commissioning.” Refer to the Matter specification's “Onboarding Payload” section for more details on commissioning data.

    On Linux, define a function InitCommissionableDataProvider to initialize a LinuxCommissionableDataProvider that can provide the required values to the CastingApp.

    CHIP_ERROR InitCommissionableDataProvider(LinuxCommissionableDataProvider & provider, LinuxDeviceOptions & options) {
    chip::Optional<uint32_t> setupPasscode;

    if (options.payload.setUpPINCode != 0) {
        setupPasscode.SetValue(options.payload.setUpPINCode);
    } else if (!options.spake2pVerifier.HasValue()) {
        // default to TestOnlyCommissionableDataProvider for demonstration
        uint32_t defaultTestPasscode = 0;
        chip::DeviceLayer::TestOnlyCommissionableDataProvider TestOnlyCommissionableDataProvider;
        VerifyOrDie(TestOnlyCommissionableDataProvider.GetSetupPasscode(defaultTestPasscode) == CHIP_NO_ERROR);
        setupPasscode.SetValue(defaultTestPasscode);
        options.payload.setUpPINCode = defaultTestPasscode;
    }
    uint32_t spake2pIterationCount = chip::Crypto::kSpake2p_Min_PBKDF_Iterations;
    if (options.spake2pIterations != 0) {
        spake2pIterationCount = options.spake2pIterations;
    }
    return provider.Init(options.spake2pVerifier, options.spake2pSalt, spake2pIterationCount, setupPasscode,
                     options.payload.discriminator.GetLongValue());
}

    On Android, define a commissioningDataProvider that can provide the required values to the CastingApp.

    private static final DataProvider<CommissionableData> commissionableDataProvider =
  new DataProvider<CommissionableData>() {
    @Override
    public CommissionableData get() {
      // dummy values for demonstration only
      return new CommissionableData(20202021, 3874);
    }
  };

    On iOS, add a func commissioningDataProvider to the MCAppParametersDataSource class defined above, that can provide the required values to the MCCastingApp.

    func castingAppDidReceiveRequestForCommissionableData(_ sender: Any) -> MCCommissionableData {
    // dummy values for demonstration only
    return MCCommissionableData(
        passcode: 20202021,
        discriminator: 3874,
        spake2pIterationCount: 1000,
        spake2pVerifier: nil,
        spake2pSalt: nil)
}

  3. Device Attestation Credentials - This object contains the DeviceAttestationCertificate, ProductAttestationIntermediateCertificate, etc. and implements a way to sign messages when called upon by the Matter TV Casting Library as part of the Matter Device Attestation process during commissioning.

    On Linux, implement a define a dacProvider to provide the Casting Client's Device Attestation Credentials, by implementing a chip::Credentials::DeviceAttestationCredentialsProvider. For this example, we will use the chip::Credentials::Examples::ExampleDACProvider

    On Android, define a dacProvider to provide the Casting Client's Device Attestation Credentials, by implementing a com.matter.casting.support.DACProvider:

    private static final DACProvider dacProvider = new DACProviderStub();
private final static DataProvider<DeviceAttestationCredentials> dacProvider = new DataProvider<DeviceAttestationCredentials>() {
    private static final String kDevelopmentDAC_Cert_FFF1_8001 = "MIIB5z...<snipped>...CXE1M=";     // dummy values for demonstration only
    private static final String kDevelopmentDAC_PrivateKey_FFF1_8001 = "qrYAror...<snipped>...StE+/8=";
    private static final String KPAI_FFF1_8000_Cert_Array = "MIIByzC...<snipped>...pwP4kQ==";

    @Override
    public DeviceAttestationCredentials get() {
        DeviceAttestationCredentials deviceAttestationCredentials = new DeviceAttestationCredentials() {
        @Override
        public byte[] SignWithDeviceAttestationKey(byte[] message) {
            try {
	        byte[] privateKeyBytes = Base64.decode(kDevelopmentDAC_PrivateKey_FFF1_8001, Base64.DEFAULT);
	        AlgorithmParameters algorithmParameters = AlgorithmParameters.getInstance("EC");
	        algorithmParameters.init(new ECGenParameterSpec("secp256r1"));
	        ECParameterSpec parameterSpec = algorithmParameters.getParameterSpec(ECParameterSpec.class);
	        ECPrivateKeySpec ecPrivateKeySpec = new ECPrivateKeySpec(new BigInteger(1, privateKeyBytes), parameterSpec);

	        KeyFactory keyFactory = KeyFactory.getInstance("EC");
	        PrivateKey privateKey = keyFactory.generatePrivate(ecPrivateKeySpec);

	        Signature signature = Signature.getInstance("SHA256withECDSA");
	        signature.initSign(privateKey);
	        signature.update(message);

	        return signature.sign();
            } catch (Exception e) {
	        return null;
            }
        }
        };

        deviceAttestationCredentials.setDeviceAttestationCert(
        Base64.decode(kDevelopmentDAC_Cert_FFF1_8001, Base64.DEFAULT));
        deviceAttestationCredentials.setProductAttestationIntermediateCert(
        Base64.decode(KPAI_FFF1_8000_Cert_Array, Base64.DEFAULT));
        return deviceAttestationCredentials;
    }
};

    On iOS, add functions castingAppDidReceiveRequestForDeviceAttestationCredentials and didReceiveRequestToSignCertificateRequest to the MCAppParametersDataSource class defined above, that can return MCDeviceAttestationCredentials and sign messages for the Casting Client, respectively.

    // dummy DAC values for demonstration only
let kDevelopmentDAC_Cert_FFF1_8001: Data = Data(base64Encoded: "MIIB..<snipped>..CXE1M=")!;
let kDevelopmentDAC_PrivateKey_FFF1_8001: Data = Data(base64Encoded: "qrYA<snipped>tE+/8=")!;
let kDevelopmentDAC_PublicKey_FFF1_8001: Data = Data(base64Encoded: "BEY6<snipped>I=")!;
let KPAI_FFF1_8000_Cert_Array: Data = Data(base64Encoded: "MIIB<snipped>4kQ==")!;
let kCertificationDeclaration: Data = Data(base64Encoded: "MII<snipped>fA==")!;

func castingAppDidReceiveRequestForDeviceAttestationCredentials(_ sender: Any) -> MCDeviceAttestationCredentials {
    return MCDeviceAttestationCredentials(
        certificationDeclaration: kCertificationDeclaration,
        firmwareInformation: Data(),
        deviceAttestationCert: kDevelopmentDAC_Cert_FFF1_8001,
        productAttestationIntermediateCert: KPAI_FFF1_8000_Cert_Array)
}

func castingApp(_ sender: Any, didReceiveRequestToSignCertificateRequest csrData: Data, outRawSignature: AutoreleasingUnsafeMutablePointer<NSData>) -> MatterError {
    Log.info("castingApp didReceiveRequestToSignCertificateRequest")

    // get the private SecKey
    var privateKeyData = Data()
    privateKeyData.append(kDevelopmentDAC_PublicKey_FFF1_8001);
    privateKeyData.append(kDevelopmentDAC_PrivateKey_FFF1_8001);
    let privateSecKey: SecKey = SecKeyCreateWithData(privateKeyData as NSData,
                                [
                                    kSecAttrKeyType: kSecAttrKeyTypeECSECPrimeRandom,
                                    kSecAttrKeyClass: kSecAttrKeyClassPrivate,
                                    kSecAttrKeySizeInBits: 256
                                ] as NSDictionary, nil)!

    // sign csrData to get asn1SignatureData
    var error: Unmanaged<CFError>?
    let asn1SignatureData: CFData? = SecKeyCreateSignature(privateSecKey, .ecdsaSignatureMessageX962SHA256, csrData as CFData, &error)
    if(error != nil)
    {
        Log.error("Failed to sign message. Error: \(String(describing: error))")
        return MATTER_ERROR_INVALID_ARGUMENT
    }
    else if (asn1SignatureData == nil)
    {
        Log.error("Failed to sign message. asn1SignatureData is nil")
        return MATTER_ERROR_INVALID_ARGUMENT
    }

    // convert ASN.1 DER signature to SEC1 raw format
    return MCCryptoUtils.ecdsaAsn1SignatureToRaw(withFeLengthBytes: 32,
                                                asn1Signature: asn1SignatureData!,
                                                     outRawSignature: &outRawSignature.pointee)
}

Once you have created the DataProvider objects above, you are ready to initialize the Casting App as described below. Note: When you initialize the Casting client, make sure your code initializes it only once, before it starts a Matter casting session.

On Linux, create an AppParameters object using the RotatingDeviceIdUniqueIdProvider, LinuxCommissionableDataProvider, CommonCaseDeviceServerInitParamsProvider, ExampleDACProvider and DefaultDACVerifier, and call CastingApp::GetInstance()->Initialize with it. Then, call Start on the CastingApp.

LinuxCommissionableDataProvider gCommissionableDataProvider;
int main(int argc, char * argv[]) {
    // Create AppParameters that need to be passed to CastingApp.Initialize()
    AppParameters appParameters;
    RotatingDeviceIdUniqueIdProvider rotatingDeviceIdUniqueIdProvider;
    CommonCaseDeviceServerInitParamsProvider serverInitParamsProvider;
    CHIP_ERROR err = CHIP_NO_ERROR;
    err            = InitCommissionableDataProvider(gCommissionableDataProvider, LinuxDeviceOptions::GetInstance());
    VerifyOrReturnValue(
        err == CHIP_NO_ERROR, 0,
        ChipLogError(AppServer, "Initialization of CommissionableDataProvider failed %" CHIP_ERROR_FORMAT, err.Format()));
    err = appParameters.Create(&rotatingDeviceIdUniqueIdProvider, &gCommissionableDataProvider,
                               chip::Credentials::Examples::GetExampleDACProvider(),
                               GetDefaultDACVerifier(chip::Credentials::GetTestAttestationTrustStore()), &serverInitParamsProvider);
    VerifyOrReturnValue(err == CHIP_NO_ERROR, 0,
                        ChipLogError(AppServer, "Creation of AppParameters failed %" CHIP_ERROR_FORMAT, err.Format()));

    // Initialize the CastingApp
    err = CastingApp::GetInstance()->Initialize(appParameters);
    VerifyOrReturnValue(err == CHIP_NO_ERROR, 0,
                        ChipLogError(AppServer, "Initialization of CastingApp failed %" CHIP_ERROR_FORMAT, err.Format()));

    // Initialize Linux KeyValueStoreMgr
    chip::DeviceLayer::PersistedStorage::KeyValueStoreMgrImpl().Init(CHIP_CONFIG_KVS_PATH);

    // Start the CastingApp
    err = CastingApp::GetInstance()->Start();
    VerifyOrReturnValue(err == CHIP_NO_ERROR, 0,
                        ChipLogError(AppServer, "CastingApp::Start failed %" CHIP_ERROR_FORMAT, err.Format()));
    ...
}

On Android, create an AppParameters object using the rotatingDeviceIdUniqueIdProvider, commissioningDataProvider, dacProvider and DataProvider<ConfigurationManager>, and call CastingApp.getInstance().initialize with it. Then, call start on the CastingApp

public static MatterError initAndStart(Context applicationContext) {
    // Create an AppParameters object to pass in global casting parameters to the SDK
    final AppParameters appParameters =
        new AppParameters(
            applicationContext,
            new DataProvider<ConfigurationManager>() {
              @Override
              public ConfigurationManager get() {
                return new PreferencesConfigurationManager(
                    applicationContext, "chip.platform.ConfigurationManager");
              }
            },
            rotatingDeviceIdUniqueIdProvider,
            commissionableDataProvider,
            dacProvider);

    // Initialize the SDK using the appParameters and check if it returns successfully
    MatterError err = CastingApp.getInstance().initialize(appParameters);
    if (err.hasError()) {
      Log.e(TAG, "Failed to initialize Matter CastingApp");
      return err;
    }

    // Start the CastingApp
    err = CastingApp.getInstance().start();
    if (err.hasError()) {
      Log.e(TAG, "Failed to start Matter CastingApp");
      return err;
    }
    return err;
}

On iOS, call MCCastingApp.initialize with an object of the MCAppParametersDataSource.

func initialize() -> MatterError {
    if let castingApp = MCCastingApp.getSharedInstance() {
        return castingApp.initialize(with: MCAppParametersDataSource())
    } else {
        return MATTER_ERROR_INCORRECT_STATE
    }
}

After initialization, on iOS, call start and stop on the MCCastingApp shared instance when the App sends the UIApplication.didBecomeActiveNotification and UIApplication.willResignActiveNotification

struct TvCastingApp: App {
    let Log = Logger(subsystem: "com.matter.casting", category: "TvCastingApp")
    @State
    var firstAppActivation: Bool = true

    var body: some Scene {
        WindowGroup {
            ContentView()
                .onAppear(perform: {
                    let err: Error? = MCInitializationExample().initialize()
                    if err != nil
                    {
                        self.Log.error("MCCastingApp initialization failed \(err)")
                        return
                    }
                })
                .onReceive(NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification)) { _ in
                    self.Log.info("TvCastingApp: UIApplication.didBecomeActiveNotification")
                    if let castingApp = MCCastingApp.getSharedInstance()
                    {
                        castingApp.start(completionBlock: { (err : Error?) -> () in
                            if err != nil
                            {
                                self.Log.error("MCCastingApp start failed \(err)")
                            }
                        })
                    }
                }
                .onReceive(NotificationCenter.default.publisher(for: UIApplication.willResignActiveNotification)) { _ in
                    self.Log.info("TvCastingApp: UIApplication.willResignActiveNotification")
                    if let castingApp = MCCastingApp.getSharedInstance()
                    {
                        castingApp.stop(completionBlock: { (err : Error?) -> () in
                            if err != nil
                            {
                                self.Log.error("MCCastingApp stop failed \(err)")
                            }
                        })
                    }
                }
            }   // WindowGroup
    }   // body
}   // App

Note about on-device cache: The Casting App maintains an on-device cache containing information about the Casting Players it has connected with so far. This cached information allows the Casting App to connect with Casting Players (that it had previously connected with) faster and using fewer resources, by potentially skipping the longer commissioning process and instead, simply re-establishing the CASE session. This cache can be cleared by calling the ClearCache API on the CastingApp, say when the user signs out of the app. See API and its documentation for Linux, Android and iOS.

Discover Casting Players

{Complete Discovery examples: Linux | Android | iOS}

The Casting Client discovers CastingPlayers using Matter Commissioner discovery over DNS-SD by listening for CastingPlayer events as they are discovered, updated, or lost from the network.

On Linux, define a DiscoveryDelegateImpl that implements the matter::casting::core::DiscoveryDelegate.

class DiscoveryDelegateImpl : public DiscoveryDelegate {
private:
    int commissionersCount = 0;

public:
    void HandleOnAdded(matter::casting::memory::Strong<CastingPlayer> player) override {
        if (commissionersCount == 0) {
            ChipLogProgress(AppServer, "Select discovered CastingPlayer to request commissioning");
            ChipLogProgress(AppServer, "Example: cast request 0");
        }
        ++commissionersCount;
        ChipLogProgress(AppServer, "Discovered CastingPlayer #%d", commissionersCount);
        player->LogDetail();
    }

    void HandleOnUpdated(matter::casting::memory::Strong<CastingPlayer> player) override {
        ChipLogProgress(AppServer, "Updated CastingPlayer with ID: %s", player->GetId());
    }
};

On Android, implement the CastingPlayerDiscovery.CastingPlayerChangeListener.

private static final CastingPlayerDiscovery.CastingPlayerChangeListener castingPlayerChangeListener =
    new CastingPlayerDiscovery.CastingPlayerChangeListener() {
        private final String TAG = CastingPlayerDiscovery.CastingPlayerChangeListener.class.getSimpleName();

        @Override
        public void onAdded(CastingPlayer castingPlayer) {
            Log.i(TAG, "onAdded() Discovered CastingPlayer deviceId: " + castingPlayer.getDeviceId());
            // Display CastingPlayer info on the screen
            new Handler(Looper.getMainLooper()).post(() -> {
                arrayAdapter.add(castingPlayer);
            });
        }

        @Override
        public void onChanged(CastingPlayer castingPlayer) {
            Log.i(TAG, "onChanged() Discovered changes to CastingPlayer with deviceId: " + castingPlayer.getDeviceId());
            // Update the CastingPlayer on the screen
            new Handler(Looper.getMainLooper()).post(() -> {
                final Optional<CastingPlayer> playerInList = castingPlayerList.stream().filter(node -> castingPlayer.equals(node)).findFirst();
                if (playerInList.isPresent()) {
                    Log.d(TAG, "onChanged() Updating existing CastingPlayer entry " + playerInList.get().getDeviceId() + " in castingPlayerList list");
                    arrayAdapter.remove(playerInList.get());
                }
                arrayAdapter.add(castingPlayer);
            });
        }

        @Override
        public void onRemoved(CastingPlayer castingPlayer) {
            Log.i(TAG, "onRemoved() Removed CastingPlayer with deviceId: " + castingPlayer.getDeviceId());
            // Remove CastingPlayer from the screen
            new Handler(Looper.getMainLooper()).post(() -> {
                final Optional<CastingPlayer> playerInList = castingPlayerList.stream().filter(node -> castingPlayer.equals(node)).findFirst();
                if (playerInList.isPresent()) {
                    Log.d(TAG, "onRemoved() Removing existing CastingPlayer entry " + playerInList.get().getDeviceId() + " in castingPlayerList list");
                    arrayAdapter.remove(playerInList.get());
                }
            });
        }
};

On iOS, implement a func addDiscoveredCastingPlayers, func removeDiscoveredCastingPlayers and func updateDiscoveredCastingPlayers which listen to notifications as Casting Players are added, removed, or updated.

@objc
func didAddDiscoveredCastingPlayers(notification: Notification)
{
    Log.info("didAddDiscoveredCastingPlayers() called")
    guard let userInfo = notification.userInfo,
        let castingPlayer     = userInfo["castingPlayer"] as? MCCastingPlayer else {
        self.Log.error("didAddDiscoveredCastingPlayers called with no MCCastingPlayer")
        return
    }

    self.Log.info("didAddDiscoveredCastingPlayers notified of a MCCastingPlayer with ID: \(castingPlayer.identifier())")
    DispatchQueue.main.async
    {
        self.displayedCastingPlayers.append(castingPlayer)
    }
}

@objc
func didRemoveDiscoveredCastingPlayers(notification: Notification)
{
    Log.info("didRemoveDiscoveredCastingPlayers() called")
    guard let userInfo = notification.userInfo,
        let castingPlayer     = userInfo["castingPlayer"] as? MCCastingPlayer else {
        self.Log.error("didRemoveDiscoveredCastingPlayers called with no MCCastingPlayer")
        return
    }

    self.Log.info("didRemoveDiscoveredCastingPlayers notified of a MCCastingPlayer with ID: \(castingPlayer.identifier())")
    DispatchQueue.main.async
    {
        self.displayedCastingPlayers.removeAll(where: {$0 == castingPlayer})
    }
}

@objc
func didUpdateDiscoveredCastingPlayers(notification: Notification)
{
    Log.info("didUpdateDiscoveredCastingPlayers() called")
    guard let userInfo = notification.userInfo,
        let castingPlayer     = userInfo["castingPlayer"] as? MCCastingPlayer else {
        self.Log.error("didUpdateDiscoveredCastingPlayers called with no MCCastingPlayer")
        return
    }

    self.Log.info("didUpdateDiscoveredCastingPlayers notified of a MCCastingPlayer with ID: \(castingPlayer.identifier())")
    if let index = displayedCastingPlayers.firstIndex(where: { castingPlayer.identifier() == $0.identifier() })
    {
        DispatchQueue.main.async
        {
            self.displayedCastingPlayers[index] = castingPlayer
        }
    }
}

Finally, register these listeners and start discovery.

On Linux, register an instance of the DiscoveryDelegateImpl with matter::casting::core::CastingPlayerDiscovery by calling SetDelegate on its singleton instance. Then, call StartDiscovery by optionally specifying the kTargetPlayerDeviceType to filter results by.

const uint64_t kTargetPlayerDeviceType = 35; // 35 represents device type of Matter Video Player
...
...
DiscoveryDelegateImpl delegate;
CastingPlayerDiscovery::GetInstance()->SetDelegate(&delegate);
VerifyOrReturnValue(err == CHIP_NO_ERROR, 0,
                    ChipLogError(AppServer, "CastingPlayerDiscovery::SetDelegate failed %" CHIP_ERROR_FORMAT, err.Format()));

err = CastingPlayerDiscovery::GetInstance()->StartDiscovery(kTargetPlayerDeviceType);
VerifyOrReturnValue(err == CHIP_NO_ERROR, 0,
                    ChipLogError(AppServer, "CastingPlayerDiscovery::StartDiscovery failed %" CHIP_ERROR_FORMAT, err.Format()));
chip::DeviceLayer::PlatformMgr().RunEventLoop();
...

On Android, add the implemented castingPlayerChangeListener as a listener to the singleton instance of MatterCastingPlayerDiscovery to listen to changes in the discovered Casting Players and call startDiscovery.

MatterError err = MatterCastingPlayerDiscovery.getInstance().addCastingPlayerChangeListener(castingPlayerChangeListener);
if (err.hasError()) {
    Log.e(TAG, "startDiscovery() addCastingPlayerChangeListener() called, err Add: " + err);
    return false;
}

// Start discovery
Log.i(TAG, "startDiscovery() calling CastingPlayerDiscovery.startDiscovery()");
err = MatterCastingPlayerDiscovery.getInstance().startDiscovery(DISCOVERY_TARGET_DEVICE_TYPE);
if (err.hasError()) {
    Log.e(TAG, "Error in startDiscovery(): " + err);
    return false;
}

On iOS, register the listeners by calling addObserver on the NotificationCenter with the appropriate selector, and then call start on the sharedInstance of MCCastingPlayerDiscovery.

func startDiscovery() {
    NotificationCenter.default.addObserver(self, selector: #selector(self.didAddDiscoveredCastingPlayers), name: NSNotification.Name.didAddCastingPlayers, object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(self.didRemoveDiscoveredCastingPlayers), name: NSNotification.Name.didRemoveCastingPlayers, object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(self.didUpdateDiscoveredCastingPlayers), name: NSNotification.Name.didUpdateCastingPlayers, object: nil)

    MCCastingPlayerDiscovery.sharedInstance().start()
    ...
}

Note: You will need to connect with a Casting Player as described below to se the list of Endpoints that they support. Refer to the Connection section for details on how to discover available endpoints supported by a Casting Player.

Connect to a Casting Player

{Complete Connection examples: Linux | Android | iOS}

Each CastingPlayer object created during Discovery contains information such as deviceName, vendorId, productId, etc. which can help the user pick the right CastingPlayer. A Casting Client can attempt to connect to the selectedCastingPlayer using Matter User Directed Commissioning (UDC). The Matter TV Casting library locally caches information required to reconnect to a CastingPlayer, once the Casting client has been commissioned by it. After that, the Casting client is able to skip the full UDC process by establishing CASE with the CastingPlayer directly. Once connected, the CastingPlayer object will contain the list of available Endpoints on that CastingPlayer. Optionally, the following arguments may also be passed in. The optional commissioningWindowTimeoutSec indicates how long to keep the commissioning window open, if commissioning is required. And DesiredEndpointFilter specifies the attributes, such as Vendor ID and Product ID of the Endpoint, the Casting client desires to interact with after connecting. This forces the Matter TV Casting library to go through the full UDC process in search of the desired Endpoint, in cases where it is not available in the Casting client's cache.

On Linux, the Casting Client can connect to a CastingPlayer by successfully calling VerifyOrEstablishConnection on it.

// VendorId of the Endpoint on the CastingPlayer that the CastingApp desires to interact with after connection
const uint16_t kDesiredEndpointVendorId = 65521;

void ConnectionHandler(CHIP_ERROR err, matter::casting::core::CastingPlayer * castingPlayer)
{
    if(err == CHIP_NO_ERROR)
    {
        ChipLogProgress(AppServer, "ConnectionHandler: Successfully connected to CastingPlayer(ID: %s)", castingPlayer->GetId());
        ...
    }
}

...
// targetCastingPlayer is a discovered CastingPlayer
matter::casting::core::EndpointFilter desiredEndpointFilter;
desiredEndpointFilter.vendorId = kDesiredEndpointVendorId;
targetCastingPlayer->VerifyOrEstablishConnection(ConnectionHandler,
                                                    matter::casting::core::kCommissioningWindowTimeoutSec,
                                                    desiredEndpointFilter);
...

On Android, the Casting Client may call verifyOrEstablishConnection on the CastingPlayer object it wants to connect to.

private static final long MIN_CONNECTION_TIMEOUT_SEC = 3 * 60;

EndpointFilter desiredEndpointFilter = new EndpointFilter();
desiredEndpointFilter.vendorId = DESIRED_ENDPOINT_VENDOR_ID;

MatterError err = targetCastingPlayer.verifyOrEstablishConnection(
                      MIN_CONNECTION_TIMEOUT_SEC,
                      desiredEndpointFilter,
                      new MatterCallback<Void>() {
                        @Override
                        public void handle(Void v) {
                          Log.i(
                              TAG,
                              "Connected to CastingPlayer with deviceId: "
                                  + targetCastingPlayer.getDeviceId());
                          getActivity()
                              .runOnUiThread(
                                  () -> {
                                    connectionFragmentStatusTextView.setText(
                                        "Connected to Casting Player with device name: "
                                            + targetCastingPlayer.getDeviceName()
                                            + "\n\n");
                                    connectionFragmentNextButton.setEnabled(true);
                                  });
                        }
                      },
                      new MatterCallback<MatterError>() {
                        @Override
                        public void handle(MatterError err) {
                          Log.e(TAG, "CastingPLayer connection failed: " + err);
                          getActivity()
                              .runOnUiThread(
                                  () -> {
                                    connectionFragmentStatusTextView.setText(
                                        "Casting Player connection failed due to: " + err + "\n\n");
                                  });
                        }
                      });

if (err.hasError())
{
    getActivity()
        .runOnUiThread(
            () -> {
                connectionFragmentStatusTextView.setText(
                    "Casting Player connection failed due to: " + err + "\n\n");
            });
}

On iOS, the Casting Client may call verifyOrEstablishConnection on the MCCastingPlayer object it wants to connect to and handle any NSErrors that may happen in the process.

// VendorId of the MCEndpoint on the MCCastingPlayer that the MCCastingApp desires to interact with after connection
let kDesiredEndpointVendorId: UInt16 = 65521;

@Published var connectionSuccess: Bool?;

@Published var connectionStatus: String?;

func connect(selectedCastingPlayer: MCCastingPlayer?) {
    let desiredEndpointFilter: MCEndpointFilter = MCEndpointFilter()
    desiredEndpointFilter.vendorId = kDesiredEndpointVendorId
    selectedCastingPlayer?.verifyOrEstablishConnection(completionBlock: { err in
        self.Log.error("MCConnectionExampleViewModel connect() completed with \(err)")
        if(err == nil)
        {
            self.connectionSuccess = true
            self.connectionStatus = "Connected!"
        }
        else
        {
            self.connectionSuccess = false
            self.connectionStatus = "Connection failed with \(String(describing: err))"
        }
    }, desiredEndpointFilter: desiredEndpointFilter)
}

Select an Endpoint on the Casting Player

{Complete Endpoint selection examples: Linux | Android | iOS}

On a successful connection with a CastingPlayer, a Casting Client may select one of the Endpoints to interact with based on its attributes (e.g. Vendor ID, Product ID, list of supported Clusters, etc).

On Linux, for example, it may select an Endpoint with a particular VendorID.

// VendorId of the Endpoint on the CastingPlayer that the CastingApp desires to interact with after connection
const uint16_t kDesiredEndpointVendorId = 65521;

std::vector<matter::casting::memory::Strong<matter::casting::core::Endpoint>> endpoints = castingPlayer->GetEndpoints();
// Find the desired Endpoint and auto-trigger some Matter Casting demo interactions
auto it = std::find_if(endpoints.begin(), endpoints.end(),
                        [](const matter::casting::memory::Strong<matter::casting::core::Endpoint> & endpoint) {
                            return endpoint->GetVendorId() == 65521;
                        });
if (it != endpoints.end())
{
    // The desired endpoint is endpoints[index]
    unsigned index = (unsigned int) std::distance(endpoints.begin(), it);
    ...
}

On Android, it can select an Endpoint as shown below.

private static final Integer SAMPLE_ENDPOINT_VID = 65521;

private Endpoint selectFirstEndpointByVID()
{
    Endpoint endpoint = null;
    if(selectedCastingPlayer != null)
    {
        List<Endpoint> endpoints = selectedCastingPlayer.getEndpoints();
        if (endpoints == null)
        {
            Log.e(TAG, "No Endpoints found on CastingPlayer");
        }
        else
        {
            endpoint = endpoints
                    .stream()
                    .filter(e -> SAMPLE_ENDPOINT_VID.equals(e.getVendorId()))
                    .findFirst()
                    .get();
        }
    }
    return endpoint;
}

On iOS, it can select an MCEndpoint similarly and as shown below.

// VendorId of the MCEndpoint on the MCCastingPlayer that the MCCastingApp desires to interact with after connection
let sampleEndpointVid: Int = 65521
...
// select the MCEndpoint on the MCCastingPlayer to invoke the command on
if let endpoint: MCEndpoint = castingPlayer.endpoints().filter({ $0.vendorId().intValue == sampleEndpointVid}).first
{
...
}

Interacting with a Casting Endpoint

Once the Casting Client has selected an Endpoint, it is ready to issue commands to it, read current playback state, and subscribe to playback events.

Refer to the following platform specific files, to find the list of clusters, commands and attributes, with their request/response types available for use with the Matter TV Casting library.

For Linux, refer to the following files:

  1. For a list of supported clusters, commands and attributes: tv-casting-common/clusters/Clusters.h
  2. For the IDs and request / response types to use with these APIs: /zzz_generated/app-common/app-common/zap-generated/cluster-objects.h

For Android, refer to the following files:

  1. For a list of supported clusters, commands and attributes: /src/controller/java/generated/java/chip/devicecontroller/ChipClusters.java

On iOS, refer to the following files:

  1. For a list of supported clusters, commands and attribute: darwin/MatterTvCastingBridge/MatterTvCastingBridge/zap-generated/MCClusterObjects.h
  2. For the IDs and request / response types to use with the commands: darwin/MatterTvCastingBridge/MatterTvCastingBridge/zap-generated/MCCommandObjects.h and darwin/MatterTvCastingBridge/MatterTvCastingBridge/zap-generated/MCCommandPayloads.h
  3. For attribute read operations and subscriptions: darwin/MatterTvCastingBridge/MatterTvCastingBridge/zap-generated/MCAttributeObjects.h

Issuing Commands

{Complete Command invocation examples: Linux | Android | iOS}

The Casting Client can get a reference to an Endpoint on a CastingPlayer, check if it supports the required cluster/command, and send commands to it. It can then handle any command response / error the CastingPlayer sends back.

On Linux, for example, given an Endpoint, it can send a LaunchURL command (part of the Content Launcher cluster) by calling the Invoke API on a Command of type matter::casting::core::Command<chip::app::Clusters::ContentLauncher::Commands::LaunchURL::Type>

void InvokeContentLauncherLaunchURL(matter::casting::memory::Strong<matter::casting::core::Endpoint> endpoint)
{
    // get contentLauncherCluster from the endpoint
    matter::casting::memory::Strong<matter::casting::clusters::content_launcher::ContentLauncherCluster> contentLauncherCluster =
        endpoint->GetCluster<matter::casting::clusters::content_launcher::ContentLauncherCluster>();
    VerifyOrReturn(contentLauncherCluster != nullptr);

    // get the launchURLCommand from the contentLauncherCluster
    matter::casting::core::Command<chip::app::Clusters::ContentLauncher::Commands::LaunchURL::Type> * launchURLCommand =
        static_cast<matter::casting::core::Command<chip::app::Clusters::ContentLauncher::Commands::LaunchURL::Type> *>(
            contentLauncherCluster->GetCommand(chip::app::Clusters::ContentLauncher::Commands::LaunchURL::Id));
    VerifyOrReturn(launchURLCommand != nullptr, ChipLogError(AppServer, "LaunchURL command not found on ContentLauncherCluster"));

    // create the LaunchURL request
    chip::app::Clusters::ContentLauncher::Commands::LaunchURL::Type request;
    request.contentURL    = chip::CharSpan::fromCharString(kContentURL);
    request.displayString = chip::Optional<chip::CharSpan>(chip::CharSpan::fromCharString(kContentDisplayStr));
    request.brandingInformation =
        chip::MakeOptional(chip::app::Clusters::ContentLauncher::Structs::BrandingInformationStruct::Type());

    // call Invoke on launchURLCommand while passing in success/failure callbacks
    launchURLCommand->Invoke(
        request, nullptr,
        [](void * context, const chip::app::Clusters::ContentLauncher::Commands::LaunchURL::Type::ResponseType & response) {
            ChipLogProgress(AppServer, "LaunchURL Success with response.data: %.*s", static_cast<int>(response.data.Value().size()),
                            response.data.Value().data());
        },
        [](void * context, CHIP_ERROR error) {
            ChipLogError(AppServer, "LaunchURL Failure with err %" CHIP_ERROR_FORMAT, error.Format());
        },
        chip::MakeOptional(kTimedInvokeCommandTimeoutMs));      // time out after kTimedInvokeCommandTimeoutMs
}

On Android, given an Endpoint, it can send a LaunchURL command (part of the Content Launcher cluster) by calling the launchURL API on a ChipClusters.ContentLauncherCluster object.

// get ChipClusters.ContentLauncherCluster from the endpoint
ChipClusters.ContentLauncherCluster cluster =
    endpoint.getCluster(ChipClusters.ContentLauncherCluster.class);
if (cluster == null) {
    Log.e(TAG, "Could not get ContentLauncherCluster for endpoint with ID: " + endpoint.getId());
    return;
}

// call launchURL on the cluster object while passing in a
// ChipClusters.ContentLauncherCluster.LauncherResponseCallback and request parameters
cluster.launchURL(
    new ChipClusters.ContentLauncherCluster.LauncherResponseCallback() {
    @Override
    public void onSuccess(Integer status, Optional<String> data) {
        Log.d(TAG, "LaunchURL success. Status: " + status + ", Data: " + data);
        new Handler(Looper.getMainLooper())
            .post(
                () -> {
                TextView launcherResult = getView().findViewById(R.id.launcherResult);
                launcherResult.setText(
                    "LaunchURL result\nStatus: " + status + ", Data: " + data);
                });
    }

    @Override
    public void onError(Exception error) {
        Log.e(TAG, "LaunchURL failure " + error);
        new Handler(Looper.getMainLooper())
            .post(
                () -> {
                TextView launcherResult = getView().findViewById(R.id.launcherResult);
                launcherResult.setText("LaunchURL result\nError: " + error);
                });
    }
    },
    contentUrl,
    Optional.of(contentDisplayString),
    Optional.empty());

On iOS, given an MCEndpoint endpoint, it can send a LaunchURL command (part of the Content Launcher cluster) by calling the invoke API on a MCContentLauncherClusterLaunchURLCommand

// validate that the selected endpoint supports the ContentLauncher cluster
if(!endpoint.hasCluster(MCEndpointClusterTypeContentLauncher))
{
    self.Log.error("No ContentLauncher cluster supporting endpoint found")
    DispatchQueue.main.async
    {
        self.status = "No ContentLauncher cluster supporting endpoint found"
    }
    return
}

// get ContentLauncher cluster from the endpoint
let contentLaunchercluster: MCContentLauncherCluster = endpoint.cluster(for: MCEndpointClusterTypeContentLauncher) as! MCContentLauncherCluster

// get the launchURLCommand from the contentLauncherCluster
let launchURLCommand: MCContentLauncherClusterLaunchURLCommand? = contentLaunchercluster.launchURLCommand()
if(launchURLCommand == nil)
{
    self.Log.error("LaunchURL not supported on cluster")
    DispatchQueue.main.async
    {
        self.status = "LaunchURL not supported on cluster"
    }
    return
}

// create the LaunchURL request
let request: MCContentLauncherClusterLaunchURLParams = MCContentLauncherClusterLaunchURLParams()
request.contentURL = contentUrl
request.displayString = displayString

// call invoke on launchURLCommand while passing in a completion block
launchURLCommand!.invoke(request, context: nil, completion: { context, err, response in
    DispatchQueue.main.async
    {
        if(err == nil)
        {
            self.Log.info("LaunchURLCommand invoke completion success with \(String(describing: response))")
            self.status = "Success. Response data: \(String(describing: response?.data))"
        }
        else
        {
            self.Log.error("LaunchURLCommand invoke completion failure with \(String(describing: err))")
            self.status = "Failure: \(String(describing: err))"
        }
    }
},
timedInvokeTimeoutMs: 5000) // time out after 5000ms

Read Operations

{Complete Attribute Read examples: Linux | Android | iOS}

The CastingClient may read an Attribute from the Endpoint on the CastingPlayer. It should ensure that the desired cluster / attribute are available for reading on the endpoint before trying to read it.

On Linux, for example, given an endpoint, it can read the VendorID (part of the Application Basic cluster) by calling the Read API on an Attribute of type matter::casting::core::Attribute<chip::app::Clusters::ApplicationBasic::Attributes::VendorID::TypeInfo>

void ReadApplicationBasicVendorID(matter::casting::memory::Strong<matter::casting::core::Endpoint> endpoint)
{
    // get applicationBasicCluster from the endpoint
    matter::casting::memory::Strong<matter::casting::clusters::application_basic::ApplicationBasicCluster> applicationBasicCluster =
        endpoint->GetCluster<matter::casting::clusters::application_basic::ApplicationBasicCluster>();
    VerifyOrReturn(applicationBasicCluster != nullptr);

    // get the vendorIDAttribute from the applicationBasicCluster
    matter::casting::core::Attribute<chip::app::Clusters::ApplicationBasic::Attributes::VendorID::TypeInfo> * vendorIDAttribute =
        static_cast<matter::casting::core::Attribute<chip::app::Clusters::ApplicationBasic::Attributes::VendorID::TypeInfo> *>(
            applicationBasicCluster->GetAttribute(chip::app::Clusters::ApplicationBasic::Attributes::VendorID::Id));
    VerifyOrReturn(vendorIDAttribute != nullptr,
                   ChipLogError(AppServer, "VendorID attribute not found on ApplicationBasicCluster"));

    // call Read on vendorIDAttribute while passing in success/failure callbacks
    vendorIDAttribute->Read(
        nullptr,
        [](void * context,
           chip::Optional<chip::app::Clusters::ApplicationBasic::Attributes::VendorID::TypeInfo::DecodableArgType> before,
           chip::app::Clusters::ApplicationBasic::Attributes::VendorID::TypeInfo::DecodableArgType after) {
            if (before.HasValue())
            {
                ChipLogProgress(AppServer, "Read VendorID value: %d [Before reading value: %d]", after, before.Value());
            }
            else
            {
                ChipLogProgress(AppServer, "Read VendorID value: %d", after);
            }
        },
        [](void * context, CHIP_ERROR error) {
            ChipLogError(AppServer, "VendorID Read failure with err %" CHIP_ERROR_FORMAT, error.Format());
        });
}

On Android, given an Endpoint, the VendorID can be read, by calling readVendorIDAttribute on the ChipClusters.ApplicationBasicCluster object.

// get ChipClusters.ApplicationBasic from the endpoint
ChipClusters.ApplicationBasicCluster cluster = endpoint.getCluster(ChipClusters.ApplicationBasicCluster.class);
if (cluster == null) {
    Log.e(TAG, "Could not get ApplicationBasicCluster for endpoint with ID: " + endpoint.getId());
    return;
}

// call readVendorIDAttribute on the cluster object while passing in a
// ChipClusters.IntegerAttributeCallback
cluster.readVendorIDAttribute(new ChipClusters.IntegerAttributeCallback() {
    @Override
    public void onSuccess(int value) {
        Log.d(TAG, "ReadVendorID success. Value: " + value);
        new Handler(Looper.getMainLooper())
                .post(
                        () -> {
                            TextView vendorIdResult = getView().findViewById(R.id.vendorIdResult);
                            vendorIdResult.setText(
                                    "Read VendorID result\nValue: " + value );
                        });
    }

    @Override
    public void onError(Exception error) {
        Log.e(TAG, "ReadVendorID failure " + error);
        new Handler(Looper.getMainLooper())
                .post(
                        () -> {
                            TextView vendorIdResult = getView().findViewById(R.id.vendorIdResult);
                            vendorIdResult.setText("Read VendorID result\nError: " + error);
                        });
    }
});

On iOS, given a MCEndpoint, the VendorID can be read similarly, by calling the read API on the MCApplicationBasicClusterVendorIDAttribute

// validate that the selected endpoint supports the ApplicationBasic cluster
if(!endpoint.hasCluster(MCEndpointClusterTypeApplicationBasic))
{
    self.Log.error("No ApplicationBasic cluster supporting endpoint found")
    DispatchQueue.main.async
    {
        self.status = "No ApplicationBasic cluster supporting endpoint found"
    }
    return
}

// get ApplicationBasic cluster from the endpoint
let applicationBasiccluster: MCApplicationBasicCluster = endpoint.cluster(for: MCEndpointClusterTypeApplicationBasic) as! MCApplicationBasicCluster

// get the vendorIDAttribute from the applicationBasiccluster
let vendorIDAttribute: MCApplicationBasicClusterVendorIDAttribute? = applicationBasiccluster.vendorIDAttribute()
if(vendorIDAttribute == nil)
{
    self.Log.error("VendorID attribute not supported on cluster")
    DispatchQueue.main.async
    {
        self.status = "VendorID attribute not supported on cluster"
    }
    return
}

// call read on vendorIDAttribute and pass in a completion block
vendorIDAttribute!.read(nil) { context, before, after, err in
    DispatchQueue.main.async
    {
        if(err != nil)
        {
            self.Log.error("Error when reading VendorID value \(String(describing: err))")
            self.status = "Error when reading VendorID value \(String(describing: err))"
            return
        }

        if(before != nil)
        {
            self.Log.info("Read VendorID value: \(String(describing: after)) Before: \(String(describing: before))")
            self.status = "Read VendorID value: \(String(describing: after)) Before: \(String(describing: before))"
        }
        else
        {
            self.Log.info("Read VendorID value: \(String(describing: after))")
            self.status = "Read VendorID value: \(String(describing: after))"
        }
    }
}

Subscriptions

{Complete Attribute subscription examples: Linux | Android | |iOS}

A Casting Client may subscribe to an attribute on an Endpoint of the CastingPlayer to get data reports when the attributes change.

On Linux, for example, given an endpoint, it can subscribe to the CurrentState (part of the Media Playback Basic cluster) by calling the Subscribe API on an Attribute of type matter::casting::core::Attribute<chip::app::Clusters::MediaPlayback::Attributes::CurrentState::TypeInfo>

void SubscribeToMediaPlaybackCurrentState(matter::casting::memory::Strong<matter::casting::core::Endpoint> endpoint)
{
    // get mediaPlaybackCluster from the endpoint
    matter::casting::memory::Strong<matter::casting::clusters::media_playback::MediaPlaybackCluster> mediaPlaybackCluster =
        endpoint->GetCluster<matter::casting::clusters::media_playback::MediaPlaybackCluster>();
    VerifyOrReturn(mediaPlaybackCluster != nullptr);

    // get the currentStateAttribute from the mediaPlaybackCluster
    matter::casting::core::Attribute<chip::app::Clusters::MediaPlayback::Attributes::CurrentState::TypeInfo> *
        currentStateAttribute =
            static_cast<matter::casting::core::Attribute<chip::app::Clusters::MediaPlayback::Attributes::CurrentState::TypeInfo> *>(
                mediaPlaybackCluster->GetAttribute(chip::app::Clusters::MediaPlayback::Attributes::CurrentState::Id));
    VerifyOrReturn(currentStateAttribute != nullptr,
                   ChipLogError(AppServer, "CurrentState attribute not found on MediaPlaybackCluster"));

    // call Subscribe on currentStateAttribute while passing in success/failure callbacks
    currentStateAttribute->Subscribe(
        nullptr,
        [](void * context,
           chip::Optional<chip::app::Clusters::MediaPlayback::Attributes::CurrentState::TypeInfo::DecodableArgType> before,
           chip::app::Clusters::MediaPlayback::Attributes::CurrentState::TypeInfo::DecodableArgType after) {
            if (before.HasValue())
            {
                ChipLogProgress(AppServer, "Read CurrentState value: %d [Before reading value: %d]", static_cast<int>(after),
                                static_cast<int>(before.Value()));
            }
            else
            {
                ChipLogProgress(AppServer, "Read CurrentState value: %d", static_cast<int>(after));
            }
        },
        [](void * context, CHIP_ERROR error) {
            ChipLogError(AppServer, "CurrentState Read failure with err %" CHIP_ERROR_FORMAT, error.Format());
        },
        kMinIntervalFloorSeconds, kMaxIntervalCeilingSeconds);
}

On Android, given an Endpoint, CurrentState can be subscribe to by calling subscribeCurrentStateAttribute on a ChipClusters.MediaPlaybackCluster object.

                    // get ChipClusters.MediaPlaybackCluster from the endpoint
                    ChipClusters.MediaPlaybackCluster cluster =
                            endpoint.getCluster(ChipClusters.MediaPlaybackCluster.class);
                    if (cluster == null) {
                        Log.e(
                                TAG,
                                "Could not get ApplicationBasicCluster for endpoint with ID: " + endpoint.getId());
                        return;
                    }

                    // call subscribeCurrentStateAttribute on the cluster object while passing in a
                    // ChipClusters.IntegerAttributeCallback and [0, 1] for min and max interval params
                    cluster.subscribeCurrentStateAttribute(new ChipClusters.IntegerAttributeCallback() {
                        @Override
                        public void onSuccess(int value) {
                            Log.d(TAG, "Read success on subscription. Value: " + value + " @ " + new Date());
                            new Handler(Looper.getMainLooper())
                                    .post(
                                            () -> {
                                                TextView currentStateResult = getView().findViewById(R.id.currentStateResult);
                                                currentStateResult.setText(
                                                        "Current State result\nValue: " + value );
                                            });
                        }

                        @Override
                        public void onError(Exception error) {
                            Log.e(TAG, "Read failure on subscription: " + error);
                            new Handler(Looper.getMainLooper())
                                    .post(
                                            () -> {
                                                TextView currentStateResult = getView().findViewById(R.id.currentStateResult);
                                                currentStateResult.setText("Current State result\nError: " + error);
                                            });
                        }
                    }, 0, 1);

On iOS, given a MCEndpoint, CurrentState can be subscribed to by calling the subscribe API on the it can subscribe to the CurrentState (part of the Media Playback Basic cluster) by calling the Subscribe API on the MCMediaPlaybackClusterCurrentStateAttribute

// validate that the selected endpoint supports the MediaPlayback cluster
if(!endpoint.hasCluster(MCEndpointClusterTypeMediaPlayback))
{
    self.Log.error("No MediaPlayback cluster supporting endpoint found")
    DispatchQueue.main.async
    {
        self.status = "No MediaPlayback cluster supporting endpoint found"
    }
    return
}

// get MediaPlayback cluster from the endpoint
let mediaPlaybackCluster: MCMediaPlaybackCluster = endpoint.cluster(for: MCEndpointClusterTypeMediaPlayback) as! MCMediaPlaybackCluster

// get the currentStateAttribute from the mediaPlaybackCluster
let currentStateAttribute: MCMediaPlaybackClusterCurrentStateAttribute? = mediaPlaybackCluster.currentStateAttribute()
if(currentStateAttribute == nil)
{
    self.Log.error("CurrentState attribute not supported on cluster")
    DispatchQueue.main.async
    {
        self.status = "CurrentState attribute not supported on cluster"
    }
    return
}

// call read on currentStateAttribute and pass in a completion block
currentStateAttribute!.subscribe(nil, completion: { context, before, after, err in

    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "HH:mm:ss"
    let currentTime = dateFormatter.string(from: Date())
    DispatchQueue.main.async
    {
        if(err != nil)
        {
            self.Log.error("Error when reading CurrentState value \(String(describing: err)) at \(currentTime)")
            self.status = "Error when reading CurrentState value \(String(describing: err)) at \(currentTime)"
            return
        }
        if(before != nil)
        {
            self.Log.info("Read CurrentState value: \(String(describing: after)) Before: \(String(describing: before)) at \(currentTime)")
            self.status = "Read CurrentState value: \(String(describing: after)) Before: \(String(describing: before)) at \(currentTime)"
        }
        else
        {
            self.Log.info("Read CurrentState value: \(String(describing: after)) at \(currentTime)")
            self.status = "Read CurrentState value: \(String(describing: after)) at \(currentTime)"
        }
    }
}, minInterval: 0, maxInterval: 1)

The Casting client can Shutdown all running Subscriptions by calling the ShutdownAllSubscriptions API on the CastingApp on Linux/Android and MCCastingApp on iOS. See API and its documentation for Linux, Android and iOS.