audioaccessorykit

Support for third-party audio accessory features - notably automatic audio switching and intelligent routing inputs - using Apple's AudioAccessoryKit, available on iOS and iPadOS 26.4+. The framework builds on AccessorySetupKit: an accessory must first be paired over Bluetooth (yielding an ASAccessory) before it can be registered for audio features. Its central type, AccessoryControlDevice, lets the container app register a Configuration describing the accessory's capabilities and initial state, and lets an app extension apply ongoing configuration updates that help the system decide where to route audio. Registration is performed with AccessoryControlDevice.register(accessory, configuration); the app extension retrieves the current device with the app-extension-only current(for:) and pushes changes with the async, throwing update(_:). Capabilities are an option set with two members - .audioSwitching (the device supports automatic switching) and .placement (the device can report its physical position) - which may be combined, though .placement should only be declared when the accessory can keep the system updated with real placement state. Placement is reported through four cases: .inEar, .onHead, .overTheEar, and .offHead, updated promptly on every detected change for responsive routing. For accessories connected to multiple Bluetooth devices at once, the primary and secondary audio-source device identifiers are supplied as Data holding Bluetooth addresses and updated as connection state changes. The Configuration exposes deviceCapabilities, devicePlacement, and the primary and secondary source identifiers, and feature discovery reads deviceCapabilities.contains(...) to check what a paired device supports. Because the framework is new in 26.4 and beta-sensitive, the guidance advises re-checking current Apple documentation before relying on specific API details.