Using the DataTrack API with iOS v3.x

The DataTrack API lets you create a DataTrack channel which can be used to send low latency messages to zero or more receivers subscribed to the data. DataTracks have the following properties.

• DataTracks are unidirectional.
  • TVILocalDataTrack is used to send messages to a room.
  • Implement TVIRemoteDataTrackDelegate to receive messages.
• DataTracks have built-in mechanisms to support reliable transmission. Check out the section on Configuring DataTrack reliability.
• Recommended maximum payload size of data sent over the DataTrack is 16KiB.
• string or byte data can be sent over the DataTrack.

In the next section we will show you how to use the DataTrack API with the iOS SDK.

The TVILocalDataTrack is a Track that represents data that can be published to a Room by the TVILocalParticipant.

var localDataTrack = LocalDataTrack()

Next, we want to connect to a Room with the TVILocalDataTrack we created earlier.

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let connectOptions = ConnectOptions(token: accessToken){ (builder) in
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    builder.roomName = "my-room"
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    if let localDataTrack = self.localDataTrack {
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        builder.dataTracks = [localDataTrack]
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    }
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}
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var room = TwilioVideoSDK.connect(options: connectOptions, delegate: self)

In the previous example, TVILocalDataTrack was published when connecting to the Room. You can also publish TVILocalDataTrack after connecting to a Room:

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if let localParticipant = room.localParticipant,
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    let localDataTrack = self.localDataTrack {
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    localParticipant.publishDataTrack(localDataTrack)
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}

The DataTrack API supports sending string as well as byte data. You can use sendString or sendData to send data to the Room. DataTracks behave similarly to audio and video Tracks in the sense that, Participants will only receive data that was sent after:

• The TVILocalDataTrack was successfully published to the Room, and
• The Participant subscribed to the DataTrack.

For example, if Alice starts sending a stream of consecutive natural numbers (one number per second), and Bob joins the Room and subscribes to Alice's DataTrack after 5 seconds while Charlie joins the Room and subscribes to Alice's DataTrack after 10 seconds, then Bob will receive all the numbers starting from 6, and Charlie will receive all the numbers starting from 11.

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// MARK: LocalParticipantDelegate
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extension MyClass : LocalParticipantDelegate {
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    func localParticipantDidPublishDataTrack(participant: LocalParticipant, dataTrackPublication: LocalDataTrackPublication) {
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        // The data track has been published and is ready for use
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        guard let localDataTrack = dataTrackPublication.localTrack else {
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            return
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        }
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        let message = "Hello DataTrack!"
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        localDataTrack.send(message)
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        // Assume bytes and length are defined elsewhere
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        var messageBuffer = Data(bytes: bytes, length: length)
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        localDataTrack.send(messageBuffer)
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    }
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}

The TVIRemoteParticipant class provides a delegate protocol named TVIRemoteParticipantDelegate. You can implement this protocol to learn about published and unpublished DataTrack events.

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// MARK: RemoteParticipantDelegate
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extension MyClass : RemoteParticipantDelegate {
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    // Participant has published a data track.
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    func remoteParticipantDidPublishDataTrack(participant: RemoteParticipant,
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                                              publication: RemoteDataTrackPublication) {
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    }
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    // Participant has unpublished a data track.
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    func remoteParticipantDidUnpublishDataTrack(participant: RemoteParticipant,
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                                                publication: RemoteDataTrackPublication) {
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    }
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    // Data track has been subscribed to and messages can be observed.
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    func didSubscribeToDataTrack(dataTrack: RemoteDataTrack, 
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                                 publication: RemoteDataTrackPublication, 
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                                 participant: RemoteParticipant) {
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        // Respond to incoming messages.
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        dataTrack.delegate = self
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    }
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    // Data track has been unsubsubscribed from and messages cannot be observed.
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    func didUnsubscribeFromDataTrack(dataTrack: RemoteDataTrack,
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                                     publication: RemoteDataTrackPublication, 
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                                     participant: RemoteParticipant) {
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    }
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}

You can implement TVIRemoteDataTrackDelegate to receive incoming messages on a DataTrack.

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// MARK: RemoteDataTrackDelegate
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extension ViewController : RemoteDataTrackDelegate {
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    func remoteDataTrackDidReceiveString(remoteDataTrack: RemoteDataTrack, message: String) {
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    }
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    func remoteDataTrackDidReceiveData(remoteDataTrack: RemoteDataTrack, message: Data) {
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    }
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}

For a fully working example and to learn more about Data Tracks, take a look at the iOS DataTrack Example.

DataTracks are intended for low-latency communication between Participants. Importantly, to optimize for lowest latency possible, delivery of DataTrack messages is not guaranteed. You can think of them more like UDP messages, rather than TCP.

You can configure the retry parameters for your DataTrack with the following options:

• maxPacketLifeTime sets the time in milliseconds during which the DataTrack will transmit or retransmit a message until that message is acknowledged.
• maxRetransmits sets the maximum number of retransmit attempts that will be made.

DataTrack connections are established between Participants via the media server. Under the hood, there is one connection between a local Participant to the Media server and a second connection from the Media server to the remote Participant. Twilio's media server configures the same maxPacketLifeTime value on each remote Participant's connection. Therefore you should set the maxPacketLifetime to half the acceptable max lifetime for each message you send.