Keep host alive across audio device changes

packages/host/Sources/SoundBridgeHost/Devices/DeviceMonitor.swift

@@ -4,6 +4,13 @@ import os.log
 
 private let logger = Logger(subsystem: "com.soundbridge.host", category: "DeviceMonitor")
 
+enum HostState: String {
+    case starting
+    case idle
+    case active
+    case stopping
+}
+
 class DeviceMonitor {
     private let registry: DeviceRegistry
     private let proxyManager: ProxyDeviceManager
@@ -16,6 +23,7 @@ class DeviceMonitor {
     private var listenersRegistered = false
     private var devicesListenerRegistered = false
     private var defaultOutputListenerRegistered = false
+    private(set) var hostState: HostState = .starting
 
     init(
         registry: DeviceRegistry,
@@ -31,6 +39,13 @@ class DeviceMonitor {
         self.audioEngine = audioEngine
     }
 
+    func setHostState(_ newState: HostState) {
+        guard hostState != newState else { return }
+
+        print("[HostState] \(hostState.rawValue) -> \(newState.rawValue)")
+        hostState = newState
+    }
+
     func registerListeners() {
         guard !listenersRegistered else { return }
 
@@ -136,6 +151,23 @@ class DeviceMonitor {
             !newDevices.contains { $0.uid == old.uid }
         }
 
+        let startingFromIdle = hostState == .idle && !newDevices.isEmpty
+        let enteringIdle = hostState == .active && newDevices.isEmpty
+
+        if startingFromIdle {
+            let preferredDevice =
+                proxyManager.resolveCurrentOutputDevice(in: newDevices)
+                ?? newDevices.first!
+
+            let sampleRate = discovery.getDeviceNominalSampleRate(preferredDevice.id)
+            SoundBridgeConfig.activeSampleRate = sampleRate
+
+            print(
+                "[DeviceMonitor] Leaving idle mode: \(sampleRate) Hz " +
+                "(from \(preferredDevice.name))"
+            )
+        }
+
         // 1. Create shared memory for new devices
         for device in addedDevices {
             print("Device added: \(device.name) (\(discovery.transportTypeName(device.transportType)))")
@@ -149,6 +181,37 @@ class DeviceMonitor {
         // 2. Update registry (writes control file + sends Darwin notification)
         registry.update(newDevices)
 
+        if enteringIdle {
+            print("[DeviceMonitor] No output devices remain; entering idle mode")
+
+            audioEngine.stop()
+            memoryManager.stopHeartbeat()
+            setHostState(.idle)
+        }
+
+        if startingFromIdle {
+            memoryManager.startHeartbeat()
+
+            do {
+                let preferredDevice =
+                    proxyManager.resolveCurrentOutputDevice(in: newDevices)
+                    ?? newDevices.first!
+
+                try audioEngine.setup(
+                    devices: newDevices,
+                    preferredDeviceID: preferredDevice.id
+                )
+                try audioEngine.start()
+
+                setHostState(.active)
+                logger.info("Audio engine started after leaving idle mode")
+            } catch {
+                logger.error(
+                    "Failed to start audio engine after leaving idle mode: \(error.localizedDescription)"
+                )
+            }
+        }
+
         // 3. Delay shared memory removal for removed devices
         if !removedDevices.isEmpty {
             let removedUIDs = removedDevices.map { $0.uid }
@@ -311,8 +374,15 @@ private func deviceListChangedCallbackC(
     _ clientData: UnsafeMutableRawPointer?
 ) -> OSStatus {
     guard let clientData else { return noErr }
-    Unmanaged<DeviceMonitor>.fromOpaque(clientData).takeUnretainedValue()
-        .handleDeviceListChanged()
+
+    let monitor = Unmanaged<DeviceMonitor>
+        .fromOpaque(clientData)
+        .takeUnretainedValue()
+
+    DispatchQueue.main.async {
+        monitor.handleDeviceListChanged()
+    }
+
     return noErr
 }
 
@@ -323,7 +393,14 @@ private func defaultOutputChangedCallbackC(
     _ clientData: UnsafeMutableRawPointer?
 ) -> OSStatus {
     guard let clientData else { return noErr }
-    Unmanaged<DeviceMonitor>.fromOpaque(clientData).takeUnretainedValue()
-        .handleDefaultOutputChanged()
+
+    let monitor = Unmanaged<DeviceMonitor>
+        .fromOpaque(clientData)
+        .takeUnretainedValue()
+
+    DispatchQueue.main.async {
+        monitor.handleDefaultOutputChanged()
+    }
+
     return noErr
 }

packages/host/Sources/SoundBridgeHost/main.swift

@@ -58,12 +58,25 @@ func main() {
         exit(1)
     }
 
+    setupSignalHandlers()
+    logger.info("Signal handlers installed")
+
+    setupSleepWakeMonitoring()
+
     print("[Step 1] Discovering physical audio devices...")
     let devices = deviceDiscovery.enumeratePhysicalDevices()
 
-    guard !devices.isEmpty else {
-        logger.error("No physical output devices found")
-        exit(1)
+    if devices.isEmpty {
+        logger.info("No physical output devices currently available; entering idle mode")
+        deviceMonitor.setHostState(.idle)
+    }
+
+    print("[Step 2] Registering device change listeners...")
+    deviceMonitor.registerListeners()
+
+    if devices.isEmpty {
+        RunLoop.current.run()
+        return
     }
 
     let validatedDevices = devices.filter { $0.validationPassed }
@@ -91,9 +104,6 @@ func main() {
 
     deviceRegistry.update(devices)
 
-    print("[Step 2] Registering device change listeners...")
-    deviceMonitor.registerListeners()
-
     print("[Step 3] Creating shared memory files...")
     memoryManager.createMemory(for: devices)
 
@@ -117,31 +127,6 @@ func main() {
         }
     }
 
-    print("[Step 7.5] Registering sleep/wake handler...")
-    sleepWakeMonitor.onSleep = {
-        print("[SleepWake] System entering sleep, stopping AudioEngine...")
-        audioEngine.stop()
-        logger.info("AudioEngine stopped before sleep")
-    }
-    sleepWakeMonitor.onWake = {
-        print("[SleepWake] Recovering after wake...")
-        deviceMonitor.reregisterListeners()
-        deviceMonitor.resetDebounce()
-        proxyManager.reregisterVolumeForwarding()
-
-        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
-            do {
-                try audioEngine.setup(devices: deviceRegistry.devices,
-                                      preferredDeviceID: proxyManager.activePhysicalDeviceID)
-                try audioEngine.start()
-                logger.info("AudioEngine restarted after wake")
-            } catch {
-                logger.error("AudioEngine restart failed: \(error.localizedDescription)")
-            }
-        }
-    }
-    sleepWakeMonitor.start()
-
     print("[Step 8] Setting up audio engine with device fallback...")
 
     // Get the user's preferred device from proxy manager (set during autoSelectProxy)
@@ -157,6 +142,8 @@ func main() {
     do {
         try audioEngine.setup(devices: devices, preferredDeviceID: preferredDeviceID != 0 ? preferredDeviceID : nil)
         try audioEngine.start()
+
+        deviceMonitor.setHostState(.active)
         logger.info("Audio engine started successfully")
 
         // Post-start volume sync: After IO starts, the driver maps shared memory.
@@ -186,10 +173,6 @@ func main() {
         exit(1)
     }
 
-    setupSignalHandlers()
-
-    logger.info("Signal handlers installed")
-
     // Listen for bounce requests from App via Darwin notification
     var bounceToken: Int32 = 0
     let bounceStatus = _notify_register_dispatch(
@@ -207,6 +190,51 @@ func main() {
     RunLoop.current.run()
 }
 
+func setupSleepWakeMonitoring() {
+    print("[Startup] Registering sleep/wake handler...")
+
+    sleepWakeMonitor.onSleep = {
+        guard deviceMonitor.hostState == .active else {
+            logger.info("Host is idle; no AudioEngine to stop before sleep")
+            return
+        }
+
+        print("[SleepWake] System entering sleep, stopping AudioEngine...")
+        audioEngine.stop()
+        logger.info("AudioEngine stopped before sleep")
+    }
+
+    sleepWakeMonitor.onWake = {
+        print("[SleepWake] Recovering after wake...")
+
+        deviceMonitor.reregisterListeners()
+        deviceMonitor.resetDebounce()
+        proxyManager.reregisterVolumeForwarding()
+
+        guard deviceMonitor.hostState == .active else {
+            logger.info("Host is idle after wake; skipping AudioEngine restart")
+            return
+        }
+
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
+            do {
+                try audioEngine.setup(
+                    devices: deviceRegistry.devices,
+                    preferredDeviceID: proxyManager.activePhysicalDeviceID
+                )
+                try audioEngine.start()
+                logger.info("AudioEngine restarted after wake")
+            } catch {
+                logger.error(
+                    "AudioEngine restart failed: \(error.localizedDescription)"
+                )
+            }
+        }
+    }
+
+    sleepWakeMonitor.start()
+}
+
 func setupSignalHandlers() {
     let sigintSource = DispatchSource.makeSignalSource(signal: SIGINT, queue: .main)
     sigintSource.setEventHandler {
@@ -229,6 +257,7 @@ func setupSignalHandlers() {
 }
 
 func cleanup() {
+    deviceMonitor.setHostState(.stopping)
     print("\n[Cleanup] Starting cleanup process...")
 
     sleepWakeMonitor.stop()