Skills
skills/dpearson2699/swift-ios-skills/dockkit

dockkit

Installation
SKILL.md

DockKit

Framework for integrating with motorized camera stands and gimbals that physically track subjects by rotating the iPhone. DockKit handles motor control, subject detection, and framing so camera apps get 360-degree pan and 90-degree tilt tracking with no additional code. Apps can override system tracking to supply custom observations, control motors directly, or adjust framing. iOS 17+, Swift 6.3.

Contents

Setup

Import DockKit:

import DockKit

DockKit requires a physical DockKit-compatible accessory and a real device. The Simulator cannot connect to dock hardware.

No special entitlements or Info.plist keys are required. The framework communicates with paired accessories automatically through the DockKit system daemon.

The app must use AVFoundation camera APIs. DockKit hooks into the camera pipeline to analyze frames for system tracking.

Discovering Accessories

Use DockAccessoryManager.shared to observe dock connections:

import DockKit

func observeAccessories() async throws {
    for await stateChange in try DockAccessoryManager.shared.accessoryStateChanges {
        switch stateChange.state {
        case .docked:
            guard let accessory = stateChange.accessory else { continue }
            // Accessory is connected and ready
            configureAccessory(accessory)
        case .undocked:
            // iPhone removed from dock
            handleUndocked()
        @unknown default:
            break
        }
    }
}

accessoryStateChanges is an AsyncSequence that emits DockAccessory.StateChange values. Each change includes:

  • state -- .docked or .undocked
  • accessory -- the DockAccessory instance (present when docked)
  • trackingButtonEnabled -- whether the physical tracking button is active

Accessory Identity

Each DockAccessory has an identifier with:

  • name -- human-readable accessory name
  • category -- .trackingStand (currently the only category)
  • uuid -- unique identifier for this accessory

Hardware details are available via firmwareVersion and hardwareModel.

System Tracking

System tracking is DockKit's default mode. When enabled, the system analyzes camera frames through built-in ML inference, detects faces and bodies, and drives the motors to keep subjects in frame. Any app using AVFoundation camera APIs benefits automatically.

Enable or Disable

// Enable system tracking (default)
try await DockAccessoryManager.shared.setSystemTrackingEnabled(true)

// Disable system tracking for custom control
try await DockAccessoryManager.shared.setSystemTrackingEnabled(false)

Check current state:

let isEnabled = DockAccessoryManager.shared.isSystemTrackingEnabled

System tracking state does not persist across app termination, reboots, or background/foreground transitions. Set it explicitly whenever the app needs a specific value.

Tap to Select Subject

Allow users to select a specific subject by tapping:

// Select the subject at a screen coordinate
try await accessory.selectSubject(at: CGPoint(x: 0.5, y: 0.5))

// Select specific subjects by identifier
try await accessory.selectSubjects([subjectUUID])

// Clear selection (return to automatic selection)
try await accessory.selectSubjects([])

Custom Tracking

Disable system tracking and provide your own observations when using custom ML models or the Vision framework.

Providing Observations

Construct DockAccessory.Observation values from your inference output and pass them to the accessory at 10-30 fps:

import DockKit
import AVFoundation

func processFrame(
    _ sampleBuffer: CMSampleBuffer,
    accessory: DockAccessory,
    device: AVCaptureDevice
) async throws {
    let cameraInfo = DockAccessory.CameraInformation(
        captureDevice: device.deviceType,
        cameraPosition: device.position,
        orientation: .corrected,
        cameraIntrinsics: nil,
        referenceDimensions: nil
    )

    // Create observation from your detection output
    let observation = DockAccessory.Observation(
        identifier: 0,
        type: .humanFace,
        rect: detectedFaceRect,       // CGRect in normalized coordinates
        faceYawAngle: nil
    )

    try await accessory.track(
        [observation],
        cameraInformation: cameraInfo
    )
}

Observation Types

Type Use
.humanFace Preserves system multi-person tracking and framing optimizations
.humanBody Full body tracking
.object Arbitrary objects (pets, hands, barcodes, etc.)

The rect uses normalized coordinates with a lower-left origin (same coordinate system as Vision framework -- no conversion needed).

Camera Information

DockAccessory.CameraInformation describes the active camera. Set orientation to .corrected when coordinates are already relative to the bottom-left corner of the screen. Optional cameraIntrinsics and referenceDimensions improve tracking accuracy.

Track variants also accept [AVMetadataObject] instead of observations, and an optional CVPixelBuffer for enhanced tracking.

Framing and Region of Interest

Framing Modes

Control how the system frames tracked subjects:

try await accessory.setFramingMode(.center)
Mode Behavior
.automatic System decides optimal framing
.center Keep subject centered (default)
.left Frame subject in left third
.right Frame subject in right third

Read the current mode:

let currentMode = accessory.framingMode

Use .left or .right when graphic overlays occupy part of the frame.

Region of Interest

Constrain tracking to a specific area of the video frame:

// Normalized coordinates, origin at upper-left
let squareRegion = CGRect(x: 0.25, y: 0.0, width: 0.5, height: 1.0)
try await accessory.setRegionOfInterest(squareRegion)

Read the current region:

let currentROI = accessory.regionOfInterest

Use region of interest when cropping to a non-standard aspect ratio (e.g., square video for conferencing) so subjects stay within the visible area.

Motor Control

Disable system tracking before controlling motors directly.

Angular Velocity

Set continuous rotation speed in radians per second:

import Spatial

// Pan right at 0.2 rad/s, tilt down at 0.1 rad/s
let velocity = Vector3D(x: 0.1, y: 0.2, z: 0.0)
try await accessory.setAngularVelocity(velocity)

// Stop all motion
try await accessory.setAngularVelocity(Vector3D())

Axes:

  • x -- pitch (tilt). Positive tilts down on iOS.
  • y -- yaw (pan). Positive pans right.
  • z -- roll (if supported by hardware).

Set Orientation

Move to a specific position over a duration:

let target = Vector3D(x: 0.0, y: 0.5, z: 0.0)  // Yaw 0.5 rad
let progress = try accessory.setOrientation(
    target,
    duration: .seconds(2),
    relative: false
)

Also accepts Rotation3D for quaternion-based orientation. Set relative: true to move relative to the current position. The returned Progress object tracks completion.

Motion State

Monitor the accessory's current position and velocity:

for await state in accessory.motionStates {
    let positions = state.angularPositions   // Vector3D
    let velocities = state.angularVelocities // Vector3D
    let time = state.timestamp
    if let error = state.error {
        // Motor error occurred
    }
}

Setting Limits

Restrict range of motion and maximum speed per axis:

let yawLimit = try DockAccessory.Limits.Limit(
    positionRange: -1.0 ..< 1.0,   // radians
    maximumSpeed: 0.5               // rad/s
)
let limits = DockAccessory.Limits(yaw: yawLimit, pitch: nil, roll: nil)
try accessory.setLimits(limits)

Animations

Built-in character animations that move the dock expressively:

// Disable system tracking before animating
try await DockAccessoryManager.shared.setSystemTrackingEnabled(false)

let progress = try await accessory.animate(motion: .kapow)

// Wait for completion
while !progress.isFinished && !progress.isCancelled {
    try await Task.sleep(for: .milliseconds(100))
}

// Restore system tracking
try await DockAccessoryManager.shared.setSystemTrackingEnabled(true)
Animation Effect
.yes Nodding motion
.no Shaking motion
.wakeup Startup-style motion
.kapow Dramatic pendulum swing

Animations start from the accessory's current position and execute asynchronously. Always restore tracking state after completion.

Tracking State and Subject Selection

iOS 18+ exposes ML-derived tracking signals through trackingStates. Each TrackingState contains a timestamp and a list of TrackedSubjectType values (.person or .object). Persons include speakingConfidence, lookingAtCameraConfidence, and saliencyRank (1 = most important, lower is more salient).

for await state in accessory.trackingStates {
    for subject in state.trackedSubjects {
        switch subject {
        case .person(let person):
            let speaking = person.speakingConfidence   // 0.0 - 1.0
            let saliency = person.saliencyRank
        case .object(let object):
            let saliency = object.saliencyRank
        }
    }
}

Use selectSubjects(_:) to lock tracking onto specific subjects by UUID. Pass an empty array to return to automatic selection. See references/dockkit-patterns.md for speaker-tracking and saliency filtering recipes.

Accessory Events

Physical buttons on the dock trigger events:

for await event in accessory.accessoryEvents {
    switch event {
    case .cameraShutter:
        // Start or stop recording / take photo
        break
    case .cameraFlip:
        // Switch front/back camera
        break
    case .cameraZoom(factor: let factor):
        // factor > 0 means zoom in, < 0 means zoom out
        break
    case .button(id: let id, pressed: let pressed):
        // Custom button with identifier
        break
    @unknown default:
        break
    }
}

For first-party apps (Camera, FaceTime), shutter/flip/zoom events work automatically. Third-party apps receive these events and implement behavior through AVFoundation.

Battery Monitoring

Monitor the dock's battery status (iOS 18+). A dock can report multiple batteries, each identified by name:

for await battery in accessory.batteryStates {
    let level = battery.batteryLevel       // 0.0 - 1.0
    let charging = battery.chargeState     // .charging, .notCharging, .notChargeable
    let low = battery.lowBattery
}

Common Mistakes

DON'T: Control motors without disabling system tracking

// WRONG -- system tracking fights manual commands
try await accessory.setAngularVelocity(velocity)

// CORRECT -- disable system tracking first
try await DockAccessoryManager.shared.setSystemTrackingEnabled(false)
try await accessory.setAngularVelocity(velocity)

DON'T: Assume tracking state persists across lifecycle events

// WRONG -- state may have reset after backgrounding
func applicationDidBecomeActive() {
    // Assume custom tracking is still active
}

// CORRECT -- re-set tracking state on foreground
func applicationDidBecomeActive() {
    Task {
        try await DockAccessoryManager.shared.setSystemTrackingEnabled(false)
    }
}

DON'T: Call track() outside the recommended rate

// WRONG -- calling once per second is too slow
try await accessory.track(observations, cameraInformation: cameraInfo)
// (called at 1 fps)

// CORRECT -- call at 10-30 fps
// Hook into AVCaptureVideoDataOutputSampleBufferDelegate for per-frame calls

DON'T: Forget to restore tracking after animations

// WRONG -- tracking stays disabled after animation
try await DockAccessoryManager.shared.setSystemTrackingEnabled(false)
let progress = try await accessory.animate(motion: .kapow)

// CORRECT -- restore tracking when animation completes
try await DockAccessoryManager.shared.setSystemTrackingEnabled(false)
let progress = try await accessory.animate(motion: .kapow)
while !progress.isFinished && !progress.isCancelled {
    try await Task.sleep(for: .milliseconds(100))
}
try await DockAccessoryManager.shared.setSystemTrackingEnabled(true)

DON'T: Use DockKit in Simulator

DockKit requires a physical DockKit-compatible accessory. Guard initialization and provide fallback behavior when no accessory is available.

Review Checklist

  • import DockKit present where needed
  • Subscribed to accessoryStateChanges to detect dock/undock events
  • Handled both .docked and .undocked states
  • System tracking disabled before custom tracking or motor control
  • System tracking restored after animations complete
  • Custom observations supplied at 10-30 fps
  • Observation rect uses normalized coordinates (lower-left origin)
  • Camera information matches the active capture device
  • @unknown default handled in all switch statements over DockKit enums
  • Motion limits set if restricting accessory range of motion
  • Tracking state re-applied after app returns to foreground
  • Accessory events handled for physical button integration
  • Battery state monitored when showing dock status to user
  • No DockKit code paths executed in Simulator builds

References

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