implement theme variant auto-switching for Windows

app/app_windows.go

@@ -12,6 +12,7 @@ import (
 	"syscall"
 
 	"fyne.io/fyne/v2"
+	internalapp "fyne.io/fyne/v2/internal/app"
 )
 
 const notificationTemplate = `$title = "%s"
@@ -95,5 +96,5 @@ func runScript(name, script string) {
 	}
 }
 func watchTheme() {
-	// TODO monitor the Windows theme
+	go internalapp.WatchTheme(fyne.CurrentApp().Settings().(*settings).setupTheme)
 }

internal/app/theme_windows.go

@@ -3,12 +3,16 @@
 package app
 
 import (
+	"syscall"
+
 	"golang.org/x/sys/windows/registry"
 
 	"fyne.io/fyne/v2"
 	"fyne.io/fyne/v2/theme"
 )
 
+const themeRegKey = `SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize`
+
 // DefaultVariant returns the systems default fyne.ThemeVariant.
 // Normally, you should not need this. It is extracted out of the root app package to give the
 // settings app access to it.
@@ -20,7 +24,7 @@ func DefaultVariant() fyne.ThemeVariant {
 }
 
 func isDark() bool {
-	k, err := registry.OpenKey(registry.CURRENT_USER, `SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize`, registry.QUERY_VALUE)
+	k, err := registry.OpenKey(registry.CURRENT_USER, themeRegKey, registry.QUERY_VALUE)
 	if err != nil { // older version of Windows will not have this key
 		return false
 	}
@@ -33,3 +37,24 @@ func isDark() bool {
 
 	return useLight == 0
 }
+
+func WatchTheme(onChanged func()) {
+	var regNotifyChangeKeyValue *syscall.Proc
+	if advapi32, err := syscall.LoadDLL("Advapi32.dll"); err == nil {
+		if p, err := advapi32.FindProc("RegNotifyChangeKeyValue"); err == nil {
+			regNotifyChangeKeyValue = p
+		}
+	}
+	if regNotifyChangeKeyValue == nil {
+		return
+	}
+	k, err := registry.OpenKey(registry.CURRENT_USER, themeRegKey, syscall.KEY_NOTIFY|registry.QUERY_VALUE)
+	if err != nil {
+		return // on older versions of windows the key may not exist
+	}
+	for {
+		// blocks until the reigstry key has been changed
+		regNotifyChangeKeyValue.Call(uintptr(k), 0, 0x00000001|0x00000004, 0, 0)
+		onChanged()
+	}
+}