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Multi-User Augmented Reality Experiences with Agora (Part 2 of 2) - Featured

Multi-User Augmented Reality Experiences with Agora (Part 2 of 2)

By Author: Max Cobb In Developer

CollaboratAR is an iOS example project created by Agora to show some ways you can use Agora SDKs to create an interactive experience in augmented reality. This post shows you how to connect the dots using Agora real-time messaging, making the experience shareable between multiple people from anywhere in the world.

To see an overview of how the example works, check out this post:

Multi-User Augmented Reality Experiences with Agora (Part 2 of 2) - screenshot 1

And the full source code:

Multi-User Augmented Reality Experiences with Agora (Part 2 of 2) - screenshot 2

What is Agora Real-time Messaging (RTM)?

Agora real-time Messaging is an SDK that enables you to send data between devices that are connected to the network. These data could contain an encoded struct, plain text, or even larger files, ranging from PDF to 3D files.

For example, one set of messages we will send across the network include our device location in the scene and details about the room we have created for others to be able to join it.

How Real-time Messaging is Used in CollaboratAR

Currently, six different types of messages will be sent across the network in CollaboratAR. They are stored in an enum to make it easy to tell them apart when sending and receiving:

enum RawRTMMessageDataType: String {
 
   // MARK: Globe Scene
 
   /// A session is available to display on the globe
   case channelAvailable
   /// Request all available sessions
   case getSessionData
 
   // MARK: Collaborative Session
 
   /// A model has been created or transform modified
   case singleCollabModel
   /// Multiple models are available to display in the scene
   case multiCollabModels
   /// A model has been deleted
   case removeCollabModel
   /// Transform update for a remote user
   case peerUpdate
}

Globe Scene 🌎

Sending

In the initial scene a globe is visible. If a remote user is currently in a channel or experience, we see them on the globe as a red circle with an orange centre.

Multi-User Augmented Reality Experiences with Agora (Part 2 of 2) - screenshot 3

First, let’s see how the presence of a channel is sent from the hosting device.

In our project, there are two methods called sendCollabChannel that have two different uses: one sends data to the channel, and the other sends data to a specific user. The channel-wide message is sent when a new experience is created. The message is sent to a specific user when a new user joins the globe lobby.

The data sent across starts in the form of a struct called ChannelData, which contains a few key data:

struct ChannelData: Codable {
   var channelName: String
   var channelID: String
   var position: SIMD3<Float>
   var systemImage: String?
}

The ChannelData struct conforms to Codable, which ensures we are able to encode all the data of our struct and provides methods to easily do so.

To send this ChannelData struct across the network, we need to create a message that can be sent across the Agora RTM network. The class for this message will be AgoraRtmRawMessage. The documentation for this class is available here:

Multi-User Augmented Reality Experiences with Agora (Part 2 of 2) - screenshot 4

Our data can be encoded with the Swift JSONEncoder like this:

// channelData: ChannelData
let jsonData: Data = try! JSONEncoder().encode(channelData)

Force unwrap here should be handled gracefully

The jsonData object is of type Data, which is ready to use in the creation of an AgoraRtmRawMessage instance. The second property when creating an AgoraRtmRawMessage is a description, which we will use to state the type of data in the message by the RawRTMMessageDataType’s rawValue, which is a String.

We are sending a message of type RawRTMMessageDataType.channelAvailable, so our AgoraRtmRawMessage creation will look like this:

let channelDataRTM = AgoraRtmRawMessage(
  rawData: jsonData,
  description: RawRTMMessageDataType.channelAvailable.rawValue
)

And to send that to our lobby channel, which is used for the globe scene, we can just do this:

// rtmChannel: AgoraRtmChannel
rtmChannel.send(channelDataRTM)

Receiving

Now that our message has been sent across the network, we need to use the Agora RTM Delegate methods to catch the incoming messages and interpret them as channels available to join.

As this message is sent across the channel, we need to use the AgoraRtmChannelDelegate to catch that, as well as the AgoraRtmDelegate for when messages are sent directly to users.

The same message may be received either channel-wide or directly to a user, so instead of handling them individually, we can catch both delegate callbacks and send the messages through one set of logic in another method, handleIncomingMessage, like this:

In the project on GitHub there is a big switch case that handles all the different messages, but we will initially discuss only the channelAvailable type we are interested in.

First, to determine the type of message, we can create an instance of RawRTMMessageDataType using the rawValue initialiser inside the handleIncomingMessage function:

let messageType = RawRTMMessageDataType(rawValue: message.text)

Then, if our messageType is .channelAvailable, we can decode the message back into a ChannelData type:

if messageType == .channelAvailable {
  let channelData = try! JSONDecoder().decode(
    ChannelData.self, from: rawMessage.rawData
  )
  // show channel data on globe...
}

In our example project, there is a method to take a ChannelData object and spawn a point on the globe, called spawnHitpoint. spawnHitpoint takes the data stored inside ChannelData to set the position of the target and the behaviour, when clicked.

The ChannelData object contains the unique channelID to join for both the real-time messaging channel and the audio channel. Read on to see how that is joined and what happens after joining the collaboration scene.

Globe Scene Conclusion

That’s all the data that needs to be sent across Agora RTM network in the initial globe scene. All each device needs to know is what available channels there are and where to place them on the globe. No audio channels are joined when in the lobby. That comes in the next section.

Collaboration Scene

A lot more data are sent around in the collaboration scene, including the positions of all the other users, any models they add, move, and delete, as well as the data interpreted from the audio channel they are all members of.

The first step is joining the scene. This is triggered by a call to a method called setState, which contains an enum of the following type:

enum CollaborationState {
  case globe
  case collab(data: ChannelData)
}

We will just cover the collab case, leaving globe empty in the snippets:

In the above snippet, rtcKit is our AgoraRtcEngineKit instance for joining the audio channel, and rtmKit is our AgoraRtmKit for joining the real-time messaging channels.

The audio channel ID is taken from collabData.channelID, the same as our real-time messaging channel. These can be different, but they are the same strings in this project example. The token handling for audio and RTM are not covered in this post or the example project. For testing, you can use a development project that does not require a token, create a temporary token, or generate the token from a token server.

Once we have joined the RTM channel, the designated host of the channel will send information about all the objects that are currently in the scene to the new joiner.

To do so, we use the AgoraRtmChannelDelegate method, memberJoined. From here, we check that the new member’s channelId is different from the lobby (the globe channel). And if we have been in the channel longer than anyone else, we gather all the data about entities in the scene and send that to the newest member.

The data shared will be an array of type [ModelData]. ModelData is defined here:

Sending All Models

All the entities we want to share conform to a custom protocol HasCollabModel. They are children of an entity we have a reference to, called collabBase.

guard let collabChildren = collabBase.children.compactMap{
  $0 as? HasCollabModel
} else { return }
let allModelData: [ModelData] = collabChildren.map { $0.modelData }

The struct ModelData conforms to Codable, which means that it and an array of ModelData can be encoded with JSON as easily as seen earlier with ChannelData, and then sent to our user the same way:

// member: AgoraRtmMember
let jsonData = try! JSONEncoder().encode(allModelData)
let rawMessge = AgoraRtmRawMessage(
  rawData: jsonData,
  description: RawRTMMessageDataType.multiCollabModels.rawValue
)
self.rtmKit.send(rawMessge, toPeer: member.userId)

Receiving All Models

On the other end, the device that just joined the channel needs to receive these models and place them in the scene.

We already have the RawRTMMessageDataType, as shown in the globe scene. We need to add a case to handleIncomingMessage to catch when our messageType is .multiCollabModels and then go on to add those models to the scene:

if messageType == .multiCollabModels {
  let modelDatas = try! JSONDecoder().decode(
    [ModelData].self, from: rawMessage.rawData
  )
  CollaborationExpEntity.shared.collab?.update(with: modelDatas)
}

Let’s take a look at what the update method does:

First, it checks if an entity with the same ID already exists in the scene. If it does, then the CollabModel entity transform is updated with a very short animation. Otherwise, a new CollabModel is created based on the data contained inside ModelData:

Updating User Positions

All the remote augmented reality users will have their location relative to the ground square sent to the channel so that all the other users know where they are in the world. The called method is CollaborationExpEntity.updatePositions. It is called on an interval of 0.3 seconds after you have joined a channel and set the ground square. The 0.3 seconds value is arbitrary and can be altered if a faster update is desired.

The type of object sent over in this is of a custom struct, PeerData:

The color and transform properties are created by using the other properties because Material.Color and Transform do not conform to Codable themselves. The above struct contains all the data needed to distinguish between the different remote users.

Sending User Position

In the updatePositions method, we first get the cameraTransform and then get that transform relative to the collabBase, which is the entity set when joining the collab session.

If we are running on an iOS device and not the simulator, we proceed to create a PeerData object, get the channel instance, and send our raw message with the description showing that it is of .peerUpdate type.

Receiving User Position

Very similarly to receiving all the model updates in the previous example, we catch the case when our messageType is .peerUpdate, decode it, and send it to a collaboration update function.

if messageType == .peerUpdate {
  let peerData = try! JSONDecoder().decode(
    PeerData.self, from: rawMessage.rawData
  )
  CollaborationExpEntity.shared.collab?.update(with: peerData)
}

Inside the update function, we either create a new PeerEntity, or update the transform of the existing one:

func update(with peerData: PeerData) {
  if let child = self.findEntity(
    named: peerData.rtmID
  ) as? PeerEntity {
    child.update(with: peerData)
  } else {
    self.createPeerEntity(with: peerData)
  }
}

Updating a Model

A similar update to .multiCollabModels is .singleCollabModel. This update is sent almost exactly the same way as .multiCollabModels, but it is a single model rather than an array.

We send this update instantly on creation of a model, and periodically if a model is selected and ready to be moved around the scene.

When joining the scene, a scheduled timer is initially fired to execute the function updatePositions(), which sends user location every 0.3 seconds (but could be more frequent if required). When a model is selected, updatePositions sees that selectedBox is not null, so it triggers a function (sendCollabData) that sends its position as an update to the channel:

func sendCollabData(for collabEnt: HasCollabModel) {
  let jsonData = try! JSONEncoder().encode(collabEnt.modelData)
  let rawMessge = AgoraRtmRawMessage(
    rawData: jsonData,
    description: RawRTMMessageDataType.singleCollabModel.rawValue
  )
  self.channels.collab?.send(rawMessge)
}

The message is then received by handleIncomingMessage on the other end and calls the same method as with .multiCollabModels:

if messageType == .singleCollabModel {
  let modelData = try! JSONDecoder().decode(
    ModelData.self, from: rawMessage.rawData
  )
  CollaborationExpEntity.shared.collab?.update(with: [modelData])
}

Deleting an Entity

When deleting an entity, the enum .removeCollabModel is used, and the individual model data is sent with the following message:

// model: CollabModel, collabChannel: AgoraRtmChannel
let jsonData = try! JSONEncoder().encode(model.modelData)
let rawMessge = AgoraRtmRawMessage(
  rawData: jsonData,
  description: RawRTMMessageDataType.removeCollabModel.rawValue
)
collabChannel.send(rawMessge)

On receiving the delete message, the model in question is located in the scene and is removed from its parent:

Testing

All of the above methods can be found in the full example project:

Multi-User Augmented Reality Experiences with Agora (Part 2 of 2) - screenshot 2

Some of the methods contain additional code to aid the rest of the experience. The above explanations focus mainly on using the Agora Real-time Messaging SDK to enable remote collaboration.

If anything is unclear, please feel free to send me a message on Twitter.

Other Resources

For more information about building applications using Agora Video and Audio Streaming SDKs, take a look at the Agora Video Call Quickstart Guide and Agora API Reference.

I also invite you to join the Agora Developer Slack community.

Conclusion

I hope you’ve enjoyed this post and the project that comes along with it. The idea of this project is to showcase how Agora Real-time Messaging SDK can be used to create interactive experiences with people around the world.


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