Tracking specific points in world space

Retrieve the position and orientation of anchors your app stores in ARKit.

Overview

Use world anchors along with an ARKit session’s WorldTrackingProvider to track points of interest in the world over time, as a person moves while wearing the device, and across device usage sessions. For example, someone might place a 3D object in a specific position on their desk and expect it to come back the next time they use the device.

ARKit keeps track of a unique identifier for each world anchor your app creates and automatically places those anchors back in the space when the person returns to your app in the same location. A world tracking provider also provides the position of the device the person is wearing.

Start an ARKit session with world tracking

Use an ARKitSession configured for world tracking to start receiving updates on the world anchors your app places. The following shows updates to world anchors your app previously registered using the addAnchor(_:) method:

let session = ARKitSession()
let worldInfo = WorldTrackingProvider()

Task {
    try await session.run([worldInfo])
    
    for await update in worldInfo.anchorUpdates {
        switch update.event {
        case .added, .updated:
            // Update the app's understanding of this world anchor.
            print("Anchor position updated.")
        case .removed:
            // Remove content related to this anchor.
            print("Anchor position now unknown.")
    }
}

Create and add world anchors

You can create world anchors for any point of interest in your app’s world coordinate system once you’ve started a world tracking ARKit session. For example, you might track that a person placed an item at a particular offset from a desk in their space:

let anchor = WorldAnchor(originFromAnchorTransform: deskPlane.originFromAnchorTransform + offset)
try await worldInfo.addAnchor(anchor)

Once you add a world anchor to your app’s tracking provider using the addAnchor(_:) method, the anchorUpdates sequence in the current session and future runs of your app provides updates to the current position of that new world anchor.

Persist world anchors across sessions

The only information ARKit persists about the world anchors in your app is their UUID — a WorldAnchor instance’s id property — and pose in a particular space. It’s your app’s responsibility to persist additional information, such as the meaning of each anchor. For example, you might save local data about a custom 3D lamp model that a person placed on their desk.

As a person moves from town-to-town or room-to-room, your app won’t receive all of the world anchor updates from each place someone used your app. Instead, the anchorUpdates sequence only provides world anchors for nearby objects.

Track the device position in the world

Use the Compositor Services framework and the WorldTrackingProvider class’s queryDeviceAnchor(atTimestamp:) method to get low-latency information about the current and future-predicted pose of the person’s device in world space. For more information, see Drawing fully immersive content using Metal.