Attempting to send non-Sendable AVFoundation types in a safe way

ExportSession.swift

public import AVFoundation

struct UniqueRef<Value>: ~Copyable, @unchecked Sendable {
    private let lock = NSLock()
    private var unsafeValue: Value?

    init(value: sending Value) {
        self.unsafeValue = value
    }

    mutating func take() -> sending Value? {
        lock.withLock {
            defer { unsafeValue = nil }
            return unsafeValue
        }
    }
}

public struct Settings: ~Copyable, Sendable {
    private var uniqueMix: UniqueRef<AVAudioMix>?

    public init(mix: sending AVAudioMix?) {
        self.uniqueMix = mix.map(UniqueRef.init)
    }

    mutating func takeMix() -> sending AVAudioMix? {
        uniqueMix?.take()
    }
}

public final class ExampleExportSession: Sendable {
    func export(settings: consuming sending Settings) async throws {
        let writer = ExampleWriter(mix: settings.takeMix())
        try await writer.write()
    }
}

actor ExampleWriter {
    let mix: AVAudioMix?

    init(mix: sending AVAudioMix?) {
        self.mix = mix
    }

    func write() async throws {}
}