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

In the current Xcode 26.6 toolchain, AudioAccessoryKit is present in the device SDK but not the iPhone Simulator 26.5 SDK. Use a physical-device destination for this target. If the rest of the app must build for Simulator, isolate target membership or guard the import and implementation with #if canImport(AudioAccessoryKit) and provide a simulator stub.

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)

Treat this as a gated write workflow: confirm registration declared the capability, copy and mutate device.configuration, then try await update(_:). The method returns no configuration value; update an app-side mirror only after the call succeeds. On failure, use the disposition in Error Handling. 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 .audioSwitching during the canonical registration flow above. Include .placement and an initial placement only when the accessory can report ongoing placement changes.

Capabilities

Automatic switching commonly uses these AccessoryControlDevice.Capabilities:

Combine capabilities as needed. 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:

Updating Placement

config.devicePlacement = .inEar

Apply this mutation within the canonical current→copy→update sequence above.

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

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

Update these identifiers when the Bluetooth connection state changes (new device connects, existing device disconnects), then call the canonical update(_:) sequence.

Configuration Properties

Automatic switching uses these configuration fields:

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:

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, surface, or propagate; Apple does not classify this as transient
        throw error
    @unknown default:
        throw error
    }
}

Do not infer that .invalidated or .unknown is transient. Correct invalid capabilities or request parameters, discard an invalidated handle and notify the container app to re-evaluate registration where appropriate, and surface unspecified errors. Load Error Recovery Patterns for the complete disposition and invalidation handoff.

Common Mistakes

DON'T: Register before pairing with AccessorySetupKit

Register only the ASAccessory returned by a completed AccessorySetupKit pairing.

DON'T: Declare placement capability without updating placement

If registration declares .placement, the extension must update placement on every detected transition using the canonical update sequence.

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

Clear or replace primary and secondary source identifiers whenever Bluetooth connections change; stale identifiers reduce switching accuracy.

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 -- discard the handle and let the container re-evaluate registration
do {
    try await device.update(config)
} catch AccessoryControlDevice.Error.invalidated {
    await notifyContainerAppToReevaluateRegistration(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 registration
• AccessoryControlDevice lookup
• AccessoryControlDevice update
• AccessoryControlDevice.Configuration
• AccessoryControlDevice.Capabilities
• AccessoryControlDevice.Placement
• AccessorySetupKit framework (prerequisite for pairing)