From 09aa3dbec657494993c78de1635f62fd4826ee84 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nils=20Scho=CC=88nwald?= <nils@schoen.world>
Date: Thu, 14 May 2026 16:51:31 +0200
Subject: [PATCH] Add "follow main output" support for system (alert) device
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Adds a new sub-command "follow" to -t system that restores the
"Selected Sound Output Device" mode for alerts/sound effects:

    SwitchAudioSource -t system follow

This performs the two writes that System Settings → Sound makes when
the user picks "Selected Sound Output Device" from "Play sound effects
through":

  1. AlertsUseMainDevice = true in com.apple.soundpref (CurrentHost).
  2. kAudioHardwarePropertyDefaultSystemOutputDevice ← current
     kAudioHardwarePropertyDefaultOutputDevice.

In addition, when AlertsUseMainDevice is true, -t output now also
mirrors the new device to system output, so alerts stay in sync with
the main output automatically. When the flag is false, behavior is
unchanged (pinned-mode preserved).

System Settings caches com.apple.soundpref on launch — fully quit and
reopen System Settings to see the popup label update.
---
 audio_switch.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
 audio_switch.h |  2 ++
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/audio_switch.c b/audio_switch.c
index 814edce..55b4487 100644
--- a/audio_switch.c
+++ b/audio_switch.c
@@ -40,7 +40,9 @@ void showUsage(const char * appName) {
            "  -n             : cycles the audio device to the next one\n"
            "  -i device_id   : sets the audio device to the given device by id\n"
            "  -u device_uid  : sets the audio device to the given device by uid or a substring of the uid\n"
-           "  -s device_name : sets the audio device to the given device by name\n\n",appName);
+           "  -s device_name : sets the audio device to the given device by name\n\n"
+           "  -t system follow : restore alerts \"follow main output\" mode\n"
+           "                   (\"Selected sound output device\" in System Settings)\n\n",appName);
 }
 
 int runAudioSwitch(int argc, const char * argv[]) {
@@ -170,6 +172,20 @@ int runAudioSwitch(int argc, const char * argv[]) {
         return 0;
     }
 
+    if (typeRequested == kAudioTypeSystemOutput && optind < argc && strcmp(argv[optind], "follow") == 0) {
+        AudioDeviceID mainOutput = getCurrentlySelectedDeviceID(kAudioTypeOutput);
+        if (mainOutput != kAudioDeviceUnknown) {
+            setOneDevice(mainOutput, kAudioTypeSystemOutput);
+        }
+        result = setAlertsFollowMain(true);
+        if (result == 0) {
+            char name[256];
+            getDeviceName(mainOutput, name);
+            printf("alerts follow main output (\"%s\"); reopen System Settings to refresh UI\n", name);
+        }
+        return result;
+    }
+
     if (typeRequested == kAudioTypeUnknown) typeRequested = kAudioTypeOutput;
 
     if (function == kFunctionCycleNext) {
@@ -588,6 +604,15 @@ int setOneDevice(AudioDeviceID newDeviceID, ASDeviceType typeRequested) {
         printf("Failed to set %s", deviceTypeName(typeRequested));
     }
 
+    if (typeRequested == kAudioTypeOutput && getAlertsFollowMain()) {
+        AudioObjectPropertyAddress sysAddr = {
+            kAudioHardwarePropertyDefaultSystemOutputDevice,
+            kAudioObjectPropertyScopeGlobal,
+            kAudioObjectPropertyElementMaster
+        };
+        AudioObjectSetPropertyData(kAudioObjectSystemObject, &sysAddr, 0, NULL, propertySize, &newDeviceID);
+    }
+
     return 0;
 }
 
@@ -796,3 +821,24 @@ void showAllDevices(ASDeviceType typeRequested, ASOutputType outputRequested) {
         }
     }
 }
+
+int setAlertsFollowMain(bool follow) {
+    CFStringRef domain = CFSTR("com.apple.soundpref");
+    CFStringRef key = CFSTR("AlertsUseMainDevice");
+    CFBooleanRef value = follow ? kCFBooleanTrue : kCFBooleanFalse;
+    CFPreferencesSetValue(key, value, domain, kCFPreferencesCurrentUser, kCFPreferencesCurrentHost);
+    if (!CFPreferencesSynchronize(domain, kCFPreferencesCurrentUser, kCFPreferencesCurrentHost)) {
+        printf("Failed to write com.apple.soundpref preference\n");
+        return 1;
+    }
+    return 0;
+}
+
+bool getAlertsFollowMain(void) {
+    CFStringRef domain = CFSTR("com.apple.soundpref");
+    CFStringRef key = CFSTR("AlertsUseMainDevice");
+    Boolean valid = false;
+    Boolean result = CFPreferencesGetAppBooleanValue(key, domain, &valid);
+    if (!valid) return false;
+    return (bool)result;
+}
diff --git a/audio_switch.h b/audio_switch.h
index 4d19cb9..49a51d3 100644
--- a/audio_switch.h
+++ b/audio_switch.h
@@ -84,6 +84,8 @@ AudioDeviceID getRequestedDeviceID(char * requestedDeviceName, ASDeviceType type
 AudioDeviceID getNextDeviceID(AudioDeviceID currentDeviceID, ASDeviceType typeRequested);
 int setDevice(AudioDeviceID newDeviceID, ASDeviceType typeRequested);
 int setOneDevice(AudioDeviceID newDeviceID, ASDeviceType typeRequested);
+int setAlertsFollowMain(bool follow);
+bool getAlertsFollowMain(void);
 int setAllDevicesByName(char * requestedDeviceName);
 int cycleNext(ASDeviceType typeRequested);
 int cycleNextForOneDevice(ASDeviceType typeRequested);