AudioAccessoryKit

Automatic audio switching and intelligent audio routing for third-party audio accessories. Enables companion apps to register audio accessories with the system, report device placement, and declare capabilities so the system can seamlessly switch audio output. Available iOS 26.4+ / Swift 6.3.

Beta-sensitive. AudioAccessoryKit is new in iOS 26.4 and may change before GM. Re-check current Apple documentation before relying on specific API details.

AudioAccessoryKit builds on top of AccessorySetupKit. The accessory must first be paired via AccessorySetupKit before it can be registered for audio features. The central type is AccessoryControlDevice, which manages registration, capability declaration, and ongoing state updates.

Contents

• Setup
• Session Management
• Audio Switching
• Device Placement
• Connected Audio Sources
• Feature Discovery
• Error Handling
• Common Mistakes
• Review Checklist
• References

Setup

Prerequisites

1. Pair the accessory over Bluetooth using AccessorySetupKit. This yields an ASAccessory object.
2. Import both frameworks in the companion app:

import AccessorySetupKit
import AudioAccessoryKit

Framework Availability

Session Management

Registering an Accessory

After pairing via AccessorySetupKit, register the accessory with AccessoryControlDevice by specifying the capabilities it supports:

let accessory: ASAccessory  // Obtained from AccessorySetupKit pairing

let capabilities: AccessoryControlDevice.Capabilities = [.audioSwitching, .placement]
try await AccessoryControlDevice.register(accessory, capabilities)

Registration activates the specified capabilities and tells the system to begin routing audio to the accessory.

Retrieving the Current Configuration

Access the device's current configuration at any time using the static current(for:) method:

let device = try AccessoryControlDevice.current(for: accessory)
let currentConfig = device.configuration

This returns the AccessoryControlDevice instance associated with the paired ASAccessory. The device exposes both the accessory reference and the current configuration.

Updating Configuration

Push configuration changes to the system with update(_:):

let device = try AccessoryControlDevice.current(for: accessory)
var config = device.configuration

config.devicePlacement = .onHead
try await device.update(config)

The update call is async and can throw AccessoryControlDevice.Error on failure.

Audio Switching

Automatic audio switching lets the system intelligently route audio output to the correct device based on placement and connected sources.

Enabling Audio Switching

Declare the .audioSwitching capability during registration:

let capabilities: AccessoryControlDevice.Capabilities = [.audioSwitching]
try await AccessoryControlDevice.register(accessory, capabilities)

For full automatic switching (including placement-based routing), include both capabilities:

let capabilities: AccessoryControlDevice.Capabilities = [.audioSwitching, .placement]
try await AccessoryControlDevice.register(accessory, capabilities)

Capabilities

AccessoryControlDevice.Capabilities is an option set with two members:

Both capabilities can be combined. Audio switching works without placement, but providing placement enables more intelligent routing decisions.

Device Placement

Report the physical position of the accessory to help the system make routing decisions. Update placement whenever the accessory detects a position change.

Placement Values

AccessoryControlDevice.Placement defines four cases:

Updating Placement

let device = try AccessoryControlDevice.current(for: accessory)
var config = device.configuration

config.devicePlacement = .inEar
try await device.update(config)

Common transitions:

• .offHead to .onHead or .inEar when the user puts on the accessory
• .onHead or .inEar to .offHead when removed
• Update promptly on every detected change for responsive audio routing

Connected Audio Sources

For accessories that connect to multiple Bluetooth devices simultaneously, inform the system which devices are connected. This lets the system route audio from the appropriate source.

Setting Audio Source Identifiers

Provide the Bluetooth address of connected devices as Data:

let device = try AccessoryControlDevice.current(for: accessory)
var config = device.configuration

let primaryBTAddress = Data([0x12, 0x34, 0x56, 0x78, 0x9A, 0xBC])
config.primaryAudioSourceDeviceIdentifier = primaryBTAddress

let secondaryBTAddress = Data([0xAB, 0xCD, 0xEF, 0x01, 0x23, 0x45])
config.secondaryAudioSourceDeviceIdentifier = secondaryBTAddress

try await device.update(config)

Update these identifiers when the Bluetooth connection state changes (new device connects, existing device disconnects).

Configuration Properties

AccessoryControlDevice.Configuration contains all configurable state:

Feature Discovery

Querying Capabilities

After registration, inspect the device's declared capabilities through its configuration:

let device = try AccessoryControlDevice.current(for: accessory)
let caps = device.configuration.deviceCapabilities

if caps.contains(.audioSwitching) {
    // Device supports automatic audio switching
}

if caps.contains(.placement) {
    // Device reports physical placement
}

Checking Placement

Read the current placement to determine if the accessory is being worn:

let device = try AccessoryControlDevice.current(for: accessory)

if let placement = device.configuration.devicePlacement {
    switch placement {
    case .inEar, .onHead, .overTheEar:
        // Accessory is being worn
        break
    case .offHead:
        // Accessory is not being worn
        break
    @unknown default:
        break
    }
}

Error Handling

AccessoryControlDevice.Error covers failure cases during registration and updates:

Handle errors from registration and update calls:

do {
    try await AccessoryControlDevice.register(accessory, capabilities)
} catch let error as AccessoryControlDevice.Error {
    switch error {
    case .accessoryNotCapable:
        // Accessory hardware does not support requested capabilities
        break
    case .invalidRequest:
        // Check registration parameters
        break
    case .invalidated:
        // Re-register the device
        break
    case .unknown:
        // Log and retry
        break
    @unknown default:
        break
    }
}

Common Mistakes

DON'T: Register before pairing with AccessorySetupKit

// WRONG -- accessory not yet paired
let rawAccessory = ASAccessory()
try await AccessoryControlDevice.register(rawAccessory, [.audioSwitching])

// CORRECT -- use the ASAccessory from a completed pairing session
session.activate(on: .main) { event in
    if event.eventType == .accessoryAdded, let accessory = event.accessory {
        Task {
            try await AccessoryControlDevice.register(accessory, [.audioSwitching])
        }
    }
}

DON'T: Declare placement capability without updating placement

// WRONG -- registers placement but never updates it
try await AccessoryControlDevice.register(accessory, [.audioSwitching, .placement])
// System never receives placement data, reducing switching accuracy

// CORRECT -- update placement promptly after registration
try await AccessoryControlDevice.register(accessory, [.audioSwitching, .placement])
let device = try AccessoryControlDevice.current(for: accessory)
var config = device.configuration
config.devicePlacement = .offHead
try await device.update(config)

DON'T: Ignore connection state changes for multi-device accessories

// WRONG -- set audio source identifiers once and never update
config.primaryAudioSourceDeviceIdentifier = someAddress
try await device.update(config)
// Device disconnects, but system still thinks it's the primary source

// CORRECT -- update identifiers when connections change
func onDeviceDisconnected() {
    var config = device.configuration
    config.primaryAudioSourceDeviceIdentifier = nil
    Task { try await device.update(config) }
}

DON'T: Forget to handle the invalidated error

// WRONG -- ignores invalidation, keeps using stale device reference
try await device.update(config)  // Throws .invalidated, unhandled

// CORRECT -- catch invalidation and re-register
do {
    try await device.update(config)
} catch AccessoryControlDevice.Error.invalidated {
    try await AccessoryControlDevice.register(accessory, capabilities)
}

Review Checklist

• Accessory paired via AccessorySetupKit before AudioAccessoryKit registration
• Both AccessorySetupKit and AudioAccessoryKit imported
• Capabilities declared at registration match actual hardware support
• .placement capability accompanied by ongoing placement updates
• Placement transitions (on/off head) reported promptly
• Audio source device identifiers updated on Bluetooth connection changes
• All AccessoryControlDevice.Error cases handled, including @unknown default
• update(_:) calls use try await and handle errors
• Invalidated device references re-registered when needed
• Deployment target set to iOS 26.4+ or iPadOS 26.4+

References

• Extended patterns (registration flow, placement monitoring, multi-device coordination): references/audioaccessorykit-patterns.md
• AudioAccessoryKit framework
• Supporting automatic audio switching
• AccessoryControlDevice
• AccessoryControlDevice.Configuration
• AccessoryControlDevice.Capabilities
• AccessoryControlDevice.Placement
• AccessorySetupKit framework (prerequisite for pairing)