dpearson2699/audioaccessorykit

AudioAccessoryKit

Automatic audio switching support and intelligent audio routing inputs for third-party audio accessories. Enables companion apps to register audio accessory configuration with the system, and app extensions to report placement and connected source changes that help the system switch audio output. Available iOS 26.4+ / iPadOS 26.4+.

Beta-sensitive. AudioAccessoryKit is new in iOS 26.4. 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 registers a Configuration from the container app and applies ongoing configuration updates from the app extension.

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 the frameworks where needed in the container app and extension:

import AccessorySetupKit
import AudioAccessoryKit

Framework Availability

| Platform | Minimum Version | |---|---| | iOS | 26.4+ | | iPadOS | 26.4+ |

Session Management

Registering an Accessory

After pairing via AccessorySetupKit, register the accessory from the container app by passing an AccessoryControlDevice.Configuration that describes the capabilities and any initial state the accessory supports:

let accessory: ASAccessory  // Obtained from AccessorySetupKit pairing

let configuration = AccessoryControlDevice.Configuration(
    devicePlacement: .offHead,
    deviceCapabilities: [.audioSwitching, .placement]
)

try await AccessoryControlDevice.register(accessory, configuration)

Registration activates the specified capabilities and gives the system the configuration it needs to participate in audio routing decisions.

Retrieving the Current Configuration

In the app extension, access the device's current configuration 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. Apple marks current(for:) as app-extension-only.

Updating Configuration

In the app extension, push configuration changes to the system with update(_:). Only update fields for capabilities that were declared during registration:

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. Apple marks update(_:) as app-extension-only.

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 in the registration configuration:

let configuration = AccessoryControlDevice.Configuration(
    deviceCapabilities: [.audioSwitching]
)

try await AccessoryControlDevice.register(accessory, configuration)

For Apple's automatic switching workflow, include both .audioSwitching and .placement when the accessory can report placement:

let configuration = AccessoryControlDevice.Configuration(
    devicePlacement: .offHead,
    deviceCapabilities: [.audioSwitching, .placement]
)

try await AccessoryControlDevice.register(accessory, configuration)

Capabilities

AccessoryControlDevice.Capabilities is an option set with two members:

| Capability | Purpose | |---|---| | .audioSwitching | Device supports automatic audio switching | | .placement | Device can report its physical placement |

Both capabilities can be combined. Do not declare .placement unless the accessory can keep the system updated with real placement state.

Device Placement

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

Placement Values

AccessoryControlDevice.Placement defines four cases:

| Placement | Meaning | |---|---| | .inEar | Accessory is seated in the ear (e.g., earbuds) | | .onHead | Accessory is on the head (e.g., headband headphones) | | .overTheEar | Accessory is over the ear (e.g., over-ear headphones) | | .offHead | Accessory is not being worn |

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 from the app extension 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:

| Property | Type | Purpose | |---|---|---| | deviceCapabilities | Capabilities | Declared device capabilities | | devicePlacement | Placement? | Current physical placement | | primaryAudioSourceDeviceIdentifier | Data? | Primary connected Bluetooth device address | | secondaryAudioSourceDeviceIdentifier | Data? | Secondary connected Bluetooth device address |

Feature Discovery

Querying Capabilities

In the app extension, 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:

| Error | Cause | |---|---| | .accessoryNotCapable | Accessory does not support the requested capability | | .invalidRequest | Request parameters are invalid | | .invalidated | Device registration has been invalidated | | .unknown | An unspecified error occurred |

Handle errors from registration and update calls:

let configuration = AccessoryControlDevice.Configuration(
    devicePlacement: .offHead,
    deviceCapabilities: [.audioSwitching, .placement]
)

do {
    try await AccessoryControlDevice.register(accessory, configuration)
} 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:
        // Coordinate container-app registration again
        break
    case .unknown:
        // Log and retry
        break
    @unknown default:
        break
    }
}

Common Mistakes

DON'T: Register before pairing with AccessorySetupKit

// WRONG -- no ASAccessory from a completed AccessorySetupKit pairing
try await AccessoryControlDevice.register(unknownAccessory, configuration)

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

DON'T: Declare placement capability without updating placement

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

// CORRECT -- extension updates placement when state changes
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 ask the container app to re-register
do {
    try await device.update(config)
} catch AccessoryControlDevice.Error.invalidated {
    await notifyContainerAppToRegisterAgain(accessory)
}

Review Checklist

• [ ] Accessory paired via AccessorySetupKit before AudioAccessoryKit registration
• [ ] Both AccessorySetupKit and AudioAccessoryKit imported
• [ ] Container app calls register(_: _:) with AccessoryControlDevice.Configuration
• [ ] App extension calls current(for:) and update(_:)
• [ ] Capabilities in the registration configuration match actual hardware support
• [ ] Updates only touch fields for capabilities declared during registration
• [ ] .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 trigger container-app registration recovery
• [ ] 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.register(::)
• AccessoryControlDevice.current(for:)
• AccessoryControlDevice.update(_:)
• AccessoryControlDevice.Configuration
• AccessoryControlDevice.Capabilities
• AccessoryControlDevice.Placement
• AccessorySetupKit framework (prerequisite for pairing)