[Light Switch] Light Switch should detect changes in Windows Settings and treat as manual override (same as using shortcut) (#42882)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
This PR ensures that Light Switch detects changes to app/system theme
from Windows Settings. This PR also introduces new behavior where
switching the schedule will cause an instant update to the theme if
necessary.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #42878
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

src/modules/LightSwitch/LightSwitchService/LightSwitchService.cpp

@@ -1,4 +1,4 @@
-#include <windows.h>
+#include <windows.h>
 #include <tchar.h>
 #include "ThemeScheduler.h"
 #include "ThemeHelper.h"
@@ -11,11 +11,12 @@
 #include <logger/logger_settings.h>
 #include <logger/logger.h>
 #include <utils/logger_helper.h>
+#include <LightSwitchServiceObserver.h>
 
 SERVICE_STATUS g_ServiceStatus = {};
 SERVICE_STATUS_HANDLE g_StatusHandle = nullptr;
 HANDLE g_ServiceStopEvent = nullptr;
-static int g_lastUpdatedDay = -1;
+extern int g_lastUpdatedDay = -1;
 static ScheduleMode prevMode = ScheduleMode::Off;
 static std::wstring prevLat, prevLon;
 
@@ -161,25 +162,18 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
     Logger::info(L"[LightSwitchService] Worker thread starting...");
     Logger::info(L"[LightSwitchService] Parent PID: {}", parentPid);
 
-    // Initialize settings system
     LightSwitchSettings::instance().InitFileWatcher();
 
-    // Open the manual override event created by the module interface
+    LightSwitchServiceObserver observer({ SettingId::LightTime,
+                                          SettingId::DarkTime,
+                                          SettingId::ScheduleMode,
+                                          SettingId::Sunrise_Offset,
+                                          SettingId::Sunset_Offset });
+
     HANDLE hManualOverride = OpenEventW(SYNCHRONIZE | EVENT_MODIFY_STATE, FALSE, L"POWERTOYS_LIGHTSWITCH_MANUAL_OVERRIDE");
 
     auto applyTheme = [](int nowMinutes, int lightMinutes, int darkMinutes, const auto& settings) {
-        bool isLightActive = false;
+        bool isLightActive = (lightMinutes < darkMinutes) ? (nowMinutes >= lightMinutes && nowMinutes < darkMinutes) : (nowMinutes >= lightMinutes || nowMinutes < darkMinutes);
-
-        if (lightMinutes < darkMinutes)
-        {
-            // Normal case: sunrise < sunset
-            isLightActive = (nowMinutes >= lightMinutes && nowMinutes < darkMinutes);
-        }
-        else
-        {
-            // Wraparound case: e.g. light at 21:00, dark at 06:00
-            isLightActive = (nowMinutes >= lightMinutes || nowMinutes < darkMinutes);
-        }
 
         bool isSystemCurrentlyLight = GetCurrentSystemTheme();
         bool isAppsCurrentlyLight = GetCurrentAppsTheme();
@@ -212,33 +206,155 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
         }
     };
 
-    // --- Initial settings load ---
     LightSwitchSettings::instance().LoadSettings();
     auto& settings = LightSwitchSettings::instance().settings();
 
-    // --- Initial theme application (if schedule enabled) ---
-    if (settings.scheduleMode != ScheduleMode::Off)
-    {
     SYSTEMTIME st;
     GetLocalTime(&st);
     int nowMinutes = st.wHour * 60 + st.wMinute;
-        applyTheme(nowMinutes, settings.lightTime + settings.sunrise_offset, settings.darkTime + settings.sunset_offset, settings);
+
+    if (settings.scheduleMode != ScheduleMode::Off)
+    {
+        applyTheme(nowMinutes,
+                   settings.lightTime + settings.sunrise_offset,
+                   settings.darkTime + settings.sunset_offset,
+                   settings);
+        Logger::trace(L"[LightSwitchService] Initialized g_lastUpdatedDay = {}", g_lastUpdatedDay);
     }
     else
     {
         Logger::info(L"[LightSwitchService] Schedule mode is OFF - ticker suspended, waiting for manual action or mode change.");
     }
 
-    // --- Main loop ---
+    g_lastUpdatedDay = st.wDay;
+    ULONGLONG lastSettingsReload = 0;
+
     for (;;)
     {
         HANDLE waits[2] = { g_ServiceStopEvent, hParent };
         DWORD count = hParent ? 2 : 1;
+        bool skipRest = false;
 
-        LightSwitchSettings::instance().LoadSettings();
         const auto& settings = LightSwitchSettings::instance().settings();
 
-        // Check for changes in schedule mode or coordinates
+        bool scheduleJustEnabled = (prevMode == ScheduleMode::Off && settings.scheduleMode != ScheduleMode::Off);
+        prevMode = settings.scheduleMode;
+
+        // ─── Handle "Schedule Off" Mode ─────────────────────────────────────────────
+        if (settings.scheduleMode == ScheduleMode::Off)
+        {
+            Logger::info(L"[LightSwitchService] Schedule mode OFF - suspending scheduler but keeping service alive.");
+
+            if (!hManualOverride)
+                hManualOverride = OpenEventW(SYNCHRONIZE | EVENT_MODIFY_STATE, FALSE, L"POWERTOYS_LIGHTSWITCH_MANUAL_OVERRIDE");
+
+            HANDLE waitsOff[4];
+            DWORD countOff = 0;
+            waitsOff[countOff++] = g_ServiceStopEvent;
+            if (hParent)
+                waitsOff[countOff++] = hParent;
+            if (hManualOverride)
+                waitsOff[countOff++] = hManualOverride;
+            waitsOff[countOff++] = LightSwitchSettings::instance().GetSettingsChangedEvent();
+
+            for (;;)
+            {
+                DWORD wait = WaitForMultipleObjects(countOff, waitsOff, FALSE, INFINITE);
+
+                if (wait == WAIT_OBJECT_0)
+                {
+                    Logger::info(L"[LightSwitchService] Stop event triggered - exiting worker loop.");
+                    goto cleanup;
+                }
+                if (hParent && wait == WAIT_OBJECT_0 + 1)
+                {
+                    Logger::info(L"[LightSwitchService] Parent exited - stopping service.");
+                    goto cleanup;
+                }
+
+                if (wait == WAIT_OBJECT_0 + (hParent ? 2 : 1))
+                {
+                    Logger::info(L"[LightSwitchService] Manual override received while schedule OFF.");
+                    ResetEvent(hManualOverride);
+                    continue;
+                }
+
+                if (wait == WAIT_OBJECT_0 + (hParent ? 3 : 2))
+                {
+                    Logger::trace(L"[LightSwitchService] Settings change event triggered, reloading settings...");
+                    ResetEvent(LightSwitchSettings::instance().GetSettingsChangedEvent());
+                    LightSwitchSettings::instance().LoadSettings();
+                    const auto& newSettings = LightSwitchSettings::instance().settings();
+                    lastSettingsReload = GetTickCount64();
+
+                    if (newSettings.scheduleMode != ScheduleMode::Off)
+                    {
+                        Logger::info(L"[LightSwitchService] Schedule re-enabled, resuming normal loop.");
+                        break;
+                    }
+                }
+            }
+
+            continue;
+        }
+
+        // ─── Normal Schedule Loop ───────────────────────────────────────────────────
+        ULONGLONG nowTick = GetTickCount64();
+        bool recentSettingsReload = (nowTick - lastSettingsReload < 5000);
+
+        if (g_lastUpdatedDay != -1)
+        {
+            bool manualOverrideActive = (hManualOverride && WaitForSingleObject(hManualOverride, 0) == WAIT_OBJECT_0);
+
+            if (settings.scheduleMode != ScheduleMode::Off && !recentSettingsReload && !scheduleJustEnabled)
+            {
+                Logger::debug(L"[LightSwitchService] Checking if manual override is active...");
+                bool manualOverrideActive = (hManualOverride && WaitForSingleObject(hManualOverride, 0) == WAIT_OBJECT_0);
+                Logger::debug(L"[LightSwitchService] Manual override active = {}", manualOverrideActive);
+
+                if (!manualOverrideActive)
+                {
+                    bool currentSystemTheme = GetCurrentSystemTheme();
+                    bool currentAppsTheme = GetCurrentAppsTheme();
+
+                    SYSTEMTIME st;
+                    GetLocalTime(&st);
+                    int nowMinutes = st.wHour * 60 + st.wMinute;
+
+                    bool shouldBeLight = (settings.lightTime < settings.darkTime) ? (nowMinutes >= settings.lightTime && nowMinutes < settings.darkTime) : (nowMinutes >= settings.lightTime || nowMinutes < settings.darkTime);
+
+                    Logger::debug(L"[LightSwitchService] shouldBeLight = {}", shouldBeLight);
+
+                    if ((settings.changeSystem && (currentSystemTheme != shouldBeLight)) ||
+                        (settings.changeApps && (currentAppsTheme != shouldBeLight)))
+                    {
+                        Logger::debug(L"[LightSwitchService] External theme change detected - enabling manual override");
+
+                        if (!hManualOverride)
+                        {
+                            hManualOverride = OpenEventW(SYNCHRONIZE | EVENT_MODIFY_STATE, FALSE, L"POWERTOYS_LIGHTSWITCH_MANUAL_OVERRIDE");
+                            if (!hManualOverride)
+                                hManualOverride = CreateEventW(nullptr, TRUE, FALSE, L"POWERTOYS_LIGHTSWITCH_MANUAL_OVERRIDE");
+                        }
+
+                        if (hManualOverride)
+                        {
+                            SetEvent(hManualOverride);
+                            Logger::info(L"[LightSwitchService] Detected manual theme change outside of LightSwitch. Triggering manual override.");
+                            skipRest = true;
+                        }
+                    }
+                }
+            }
+            else
+            {
+                Logger::debug(L"[LightSwitchService] Skipping external-change detection (schedule off, recent reload, or just enabled).");
+            }
+        }
+
+        // ─── Apply Schedule Logic ───────────────────────────────────────────────────
+        if (!skipRest)
+        {
             bool modeChangedToSunset = (prevMode != settings.scheduleMode &&
                                         settings.scheduleMode == ScheduleMode::SunsetToSunrise);
             bool coordsChanged = (prevLat != settings.latitude || prevLon != settings.longitude);
@@ -255,75 +371,10 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
                 prevLon = settings.longitude;
             }
 
-        // If schedule is off, idle but keep watching settings and manual override
-        if (settings.scheduleMode == ScheduleMode::Off)
-        {
-            Logger::info(L"[LightSwitchService] Schedule mode OFF - suspending scheduler but keeping service alive.");
-
-            if (!hManualOverride)
-            {
-                hManualOverride = OpenEventW(SYNCHRONIZE | EVENT_MODIFY_STATE, FALSE, L"POWERTOYS_LIGHTSWITCH_MANUAL_OVERRIDE");
-            }
-
-            HANDLE waits[4];
-            DWORD count = 0;
-            waits[count++] = g_ServiceStopEvent;
-            if (hParent)
-                waits[count++] = hParent;
-            if (hManualOverride)
-                waits[count++] = hManualOverride;
-            waits[count++] = LightSwitchSettings::instance().GetSettingsChangedEvent();
-
-            for (;;)
-            {
-                DWORD wait = WaitForMultipleObjects(count, waits, FALSE, INFINITE);
-
-                // --- Handle exit signals ---
-                if (wait == WAIT_OBJECT_0) // stop event
-                {
-                    Logger::info(L"[LightSwitchService] Stop event triggered - exiting worker loop.");
-                    break;
-                }
-                if (hParent && wait == WAIT_OBJECT_0 + 1)
-                {
-                    Logger::info(L"[LightSwitchService] Parent exited - stopping service.");
-                    break;
-                }
-
-                // --- Manual override triggered ---
-                if (wait == WAIT_OBJECT_0 + (hParent ? 2 : 1))
-                {
-                    Logger::info(L"[LightSwitchService] Manual override received while schedule OFF.");
-                    ResetEvent(hManualOverride);
-                    continue;
-                }
-
-                // --- Settings file changed ---
-                if (wait == WAIT_OBJECT_0 + (hParent ? 3 : 2))
-                {
-                    Logger::trace(L"[LightSwitchService] Settings change event triggered, reloading settings...");
-
-                    ResetEvent(LightSwitchSettings::instance().GetSettingsChangedEvent());
-
-                    LightSwitchSettings::instance().LoadSettings();
-                    const auto& newSettings = LightSwitchSettings::instance().settings();
-
-                    if (newSettings.scheduleMode != ScheduleMode::Off)
-                    {
-                        Logger::info(L"[LightSwitchService] Schedule re-enabled, resuming normal loop.");
-                        break;
-                    }
-                }
-            }
-        }
-
-
-        // --- When schedule is active, run once per minute ---
             SYSTEMTIME st;
             GetLocalTime(&st);
             int nowMinutes = st.wHour * 60 + st.wMinute;
 
-        // Refresh suntimes at day boundary
             if ((g_lastUpdatedDay != st.wDay) && (settings.scheduleMode == ScheduleMode::SunsetToSunrise))
             {
                 update_sun_times(settings);
@@ -331,7 +382,6 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
                 Logger::info(L"[LightSwitchService] Recalculated sun times at new day boundary.");
             }
 
-        // Have to do this again in case settings got updated in the refresh suntimes chunk
             LightSwitchSettings::instance().LoadSettings();
             const auto& currentSettings = LightSwitchSettings::instance().settings();
 
@@ -347,12 +397,9 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
                        static_cast<int>(currentSettings.scheduleMode));
             Logger::info(msg);
 
-        // --- Manual override check ---
             bool manualOverrideActive = false;
             if (hManualOverride)
-        {
                 manualOverrideActive = (WaitForSingleObject(hManualOverride, 0) == WAIT_OBJECT_0);
-        }
 
             if (manualOverrideActive)
             {
@@ -365,13 +412,19 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
                 else
                 {
                     Logger::info(L"[LightSwitchService] Skipping schedule due to manual override");
-                goto sleep_until_next_minute;
+                    skipRest = true;
                 }
             }
 
-        applyTheme(nowMinutes, currentSettings.lightTime + currentSettings.sunrise_offset, currentSettings.darkTime + currentSettings.sunset_offset, currentSettings);
+            if (!skipRest)
+                applyTheme(nowMinutes,
+                           currentSettings.lightTime + currentSettings.sunrise_offset,
+                           currentSettings.darkTime + currentSettings.sunset_offset,
+                           currentSettings);
+        }
 
-    sleep_until_next_minute:
+        // ─── Wait For Next Minute Tick Or Stop Event ────────────────────────────────
+        SYSTEMTIME st;
         GetLocalTime(&st);
         int msToNextMinute = (60 - st.wSecond) * 1000 - st.wMilliseconds;
         if (msToNextMinute < 50)
@@ -390,6 +443,7 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
         }
     }
 
+cleanup:
     if (hManualOverride)
         CloseHandle(hManualOverride);
     if (hParent)
@@ -398,6 +452,53 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
     return 0;
 }
 
+void ApplyThemeNow()
+{
+    LightSwitchSettings::instance().LoadSettings();
+    const auto& settings = LightSwitchSettings::instance().settings();
+
+    SYSTEMTIME st;
+    GetLocalTime(&st);
+    int nowMinutes = st.wHour * 60 + st.wMinute;
+
+    bool shouldBeLight = false;
+    if (settings.lightTime < settings.darkTime)
+        shouldBeLight = (nowMinutes >= settings.lightTime && nowMinutes < settings.darkTime);
+    else
+        shouldBeLight = (nowMinutes >= settings.lightTime || nowMinutes < settings.darkTime);
+
+    bool isSystemCurrentlyLight = GetCurrentSystemTheme();
+    bool isAppsCurrentlyLight = GetCurrentAppsTheme();
+
+    Logger::info(L"[LightSwitchService] Applying (if needed) theme immediately due to schedule change.");
+
+    if (shouldBeLight)
+    {
+        if (settings.changeSystem && !isSystemCurrentlyLight)
+        {
+            SetSystemTheme(true);
+            Logger::info(L"[LightSwitchService] Changing system theme to light mode.");
+        }
+        if (settings.changeApps && !isAppsCurrentlyLight)
+        {
+            SetAppsTheme(true);
+            Logger::info(L"[LightSwitchService] Changing apps theme to light mode.");
+        }
+    }
+    else
+    {
+        if (settings.changeSystem && isSystemCurrentlyLight)
+        {
+            SetSystemTheme(false);
+            Logger::info(L"[LightSwitchService] Changing system theme to dark mode.");
+        }
+        if (settings.changeApps && isAppsCurrentlyLight)
+        {
+            SetAppsTheme(false);
+            Logger::info(L"[LightSwitchService] Changing apps theme to dark mode.");
+        }
+    }
+}
 
 int APIENTRY wWinMain(HINSTANCE, HINSTANCE, PWSTR, int)
 {

src/modules/LightSwitch/LightSwitchService/LightSwitchService.vcxproj

@@ -74,6 +74,7 @@
   </ItemDefinitionGroup>
   <ItemGroup>
     <ClCompile Include="LightSwitchService.cpp" />
+    <ClCompile Include="LightSwitchServiceObserver.cpp" />
     <ClCompile Include="LightSwitchSettings.cpp" />
     <ClCompile Include="SettingsConstants.cpp" />
     <ClCompile Include="ThemeHelper.cpp" />
@@ -84,6 +85,7 @@
     <ResourceCompile Include="LightSwitchService.rc" />
   </ItemGroup>
   <ItemGroup>
+    <ClInclude Include="LightSwitchServiceObserver.h" />
     <ClInclude Include="LightSwitchSettings.h" />
     <ClInclude Include="SettingsConstants.h" />
     <ClInclude Include="SettingsObserver.h" />

src/modules/LightSwitch/LightSwitchService/LightSwitchService.vcxproj.filters

@@ -33,6 +33,9 @@
     <ClCompile Include="WinHookEventIDs.cpp">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="LightSwitchServiceObserver.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="ThemeScheduler.h">
@@ -53,6 +56,9 @@
     <ClInclude Include="WinHookEventIDs.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="LightSwitchServiceObserver.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <Natvis Include="$(MSBuildThisFileDirectory)..\..\natvis\wil.natvis" />

src/modules/LightSwitch/LightSwitchService/LightSwitchServiceObserver.cpp

@@ -0,0 +1,29 @@
+#include "LightSwitchServiceObserver.h"
+#include <logger.h>
+#include "LightSwitchSettings.h"
+
+// These are defined elsewhere in your service module (ServiceWorkerThread.cpp)
+extern int g_lastUpdatedDay;
+void ApplyThemeNow();
+
+void LightSwitchServiceObserver::SettingsUpdate(SettingId id)
+{
+    Logger::info(L"[LightSwitchService] Setting changed: {}", static_cast<int>(id));
+    g_lastUpdatedDay = -1;
+    ApplyThemeNow();
+}
+
+bool LightSwitchServiceObserver::WantsToBeNotified(SettingId id) const noexcept
+{
+    switch (id)
+    {
+    case SettingId::LightTime:
+    case SettingId::DarkTime:
+    case SettingId::ScheduleMode:
+    case SettingId::Sunrise_Offset:
+    case SettingId::Sunset_Offset:
+        return true;
+    default:
+        return false;
+    }
+}

src/modules/LightSwitch/LightSwitchService/LightSwitchServiceObserver.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "SettingsObserver.h"
+
+// The LightSwitchServiceObserver reacts when LightSwitchSettings changes.
+class LightSwitchServiceObserver : public SettingsObserver
+{
+public:
+    explicit LightSwitchServiceObserver(std::unordered_set<SettingId> observedSettings) :
+        SettingsObserver(std::move(observedSettings))
+    {
+    }
+
+    void SettingsUpdate(SettingId id) override;
+    bool WantsToBeNotified(SettingId id) const noexcept override;
+};

src/modules/LightSwitch/LightSwitchService/LightSwitchSettings.cpp

@@ -39,6 +39,7 @@ void LightSwitchSettings::InitFileWatcher()
             GetSettingsFileName(),
             [this]() {
                 Logger::info(L"[LightSwitchSettings] Settings file changed, signaling event.");
+                LoadSettings();
                 SetEvent(m_settingsChangedEvent);
             });
     }
@@ -65,6 +66,11 @@ void LightSwitchSettings::NotifyObservers(SettingId id) const
     }
 }
 
+HANDLE LightSwitchSettings::GetSettingsChangedEvent() const
+{
+    return m_settingsChangedEvent;
+}
+
 void LightSwitchSettings::LoadSettings()
 {
     try

src/modules/LightSwitch/LightSwitchService/LightSwitchSettings.h

@@ -79,7 +79,7 @@ public:
 
     void LoadSettings();
 
-    HANDLE GetSettingsChangedEvent() const { return m_settingsChangedEvent; }
+    HANDLE GetSettingsChangedEvent() const;
 
 private:
     LightSwitchSettings();

src/modules/LightSwitch/LightSwitchService/SettingsObserver.h

@@ -2,6 +2,7 @@
 
 #include <unordered_set>
 #include "SettingsConstants.h"
+#include "LightSwitchSettings.h"
 
 class LightSwitchSettings;
 
@@ -22,7 +23,7 @@ public:
     // Override this in your class to respond to updates
     virtual void SettingsUpdate(SettingId type) {}
 
-    bool WantsToBeNotified(SettingId type) const noexcept
+    virtual bool WantsToBeNotified(SettingId type) const noexcept
     {
         return m_observedSettings.contains(type);
     }