feat(Settings): enhance writeSettings to skip unchanged files and serialize config into JSON strings

src/app/settings.ts

@@ -164,34 +164,59 @@ export async function readSettings(): Promise<ReadSettingsResult | null> {
   }
 }
 
+export interface SavedJson {
+  configJson:  string;
+  devicesJson: string;
+}
+
+/** Serialize config into the two canonical JSON strings without writing anything. */
+export function serializeConfig(config: SettingsConfig): SavedJson {
+  const { devices, ...mainConfig } = config;
+  return {
+    configJson:  JSON.stringify(mainConfig,  null, 2) + '\n',
+    devicesJson: JSON.stringify({ devices }, null, 2) + '\n',
+  };
+}
+
 /**
  * Persist the settings object to the WebDAV server, creating the
  * containing `/.sys/` directory if needed. Pretty-prints with 2-space
  * indentation so the file is diff-friendly.
+ *
+ * Pass `lastSaved` to skip writing files whose content hasn't changed.
+ * Returns the canonical JSON strings that were written (or would have been).
  */
-export async function writeSettings(config: SettingsConfig): Promise<void> {
+export async function writeSettings(
+  config: SettingsConfig,
+  lastSaved?: SavedJson | null,
+): Promise<SavedJson> {
+  const next = serializeConfig(config);
+
+  const writeConfig  = !lastSaved || next.configJson  !== lastSaved.configJson;
+  const writeDevices = !lastSaved || next.devicesJson !== lastSaved.devicesJson;
+
+  if (!writeConfig && !writeDevices) return next;
+
   try { await createFolder(SETTINGS_DIR, true); } catch { /* exists */ }
 
-  const { devices, ...mainConfig } = config;
+  const writes: Promise<void>[] = [];
-  const configJson  = JSON.stringify(mainConfig,   null, 2) + '\n';
+  if (writeConfig)  writes.push(putFileContents(SETTINGS_PATH, next.configJson));
-  const devicesJson = JSON.stringify({ devices },  null, 2) + '\n';
+  if (writeDevices) writes.push(putFileContents(DEVICES_PATH,  next.devicesJson));
-
+  await Promise.all(writes);
-  await Promise.all([
-    putFileContents(SETTINGS_PATH, configJson),
-    putFileContents(DEVICES_PATH,  devicesJson),
-  ]);
 
   // Mirror to /sd/.sys/ if the SD card is mounted.
   try {
     const sdStat = await stat('/sd');
     if (sdStat) {
       try { await createFolder(SD_SYS_DIR, true); } catch { /* exists */ }
-      await Promise.all([
+      const sdWrites: Promise<void>[] = [];
-        putFileContents(SD_SYS_DIR + '/config.json',  configJson),
+      if (writeConfig)  sdWrites.push(putFileContents(SD_SYS_DIR + '/config.json',  next.configJson));
-        putFileContents(SD_SYS_DIR + '/devices.json', devicesJson),
+      if (writeDevices) sdWrites.push(putFileContents(SD_SYS_DIR + '/devices.json', next.devicesJson));
-      ]);
+      await Promise.all(sdWrites);
     }
   } catch { /* /sd unreachable — skip mirror */ }
+
+  return next;
 }
 
 export type SaveStatus =
@@ -246,6 +271,7 @@ export function useSettings(): UseSettingsResult {
   const dirtyRef       = useRef<boolean>(false);
   const pendingRef     = useRef<number>(0);
   const savedTimerRef  = useRef<number | null>(null);
+  const lastSavedRef   = useRef<SavedJson | null>(null);
 
   const load = useCallback(async () => {
     setSaveStatus('loading');
@@ -257,7 +283,7 @@ export function useSettings(): UseSettingsResult {
         // Persist the migrated config immediately so the device is updated.
         setSaveStatus('saving');
         try {
-          await writeSettings(result.config);
+          lastSavedRef.current = await writeSettings(result.config);
           setSaveStatus('saved');
           savedTimerRef.current = window.setTimeout(() => {
             setSaveStatus((s) => (s === 'saved' ? 'idle' : s));
@@ -273,6 +299,8 @@ export function useSettings(): UseSettingsResult {
         setLoaded(true);
         return;
       }
+      // Record what the server holds so flushNow can skip unchanged files.
+      lastSavedRef.current = serializeConfig(result.config);
     }
     loadedRef.current = true;
     setLoaded(true);
@@ -299,7 +327,7 @@ export function useSettings(): UseSettingsResult {
     setPendingCount(0);
     setSaveStatus('saving');
     try {
-      await writeSettings(configRef.current);
+      lastSavedRef.current = await writeSettings(configRef.current, lastSavedRef.current);
       setSaveStatus('saved');
       // Drop the "saved" badge after a short delay so it doesn't linger.
       savedTimerRef.current = window.setTimeout(() => {
@@ -324,19 +352,20 @@ export function useSettings(): UseSettingsResult {
       // Modern browsers: `fetch` with `keepalive: true` continues even
       // after the page is being unloaded. We don't await it.
       try {
-        const { devices, ...mainConfig } = configRef.current;
+        const next = serializeConfig(configRef.current);
+        const last = lastSavedRef.current;
+        if (!last || next.configJson  !== last.configJson) {
           void fetch(absoluteUrl(SETTINGS_PATH), {
-          method: 'PUT',
+            method: 'PUT', body: next.configJson,
-          body: JSON.stringify(mainConfig,  null, 2) + '\n',
+            headers: { 'Content-Type': 'application/json' }, keepalive: true,
-          headers: { 'Content-Type': 'application/json' },
-          keepalive: true,
           });
+        }
+        if (!last || next.devicesJson !== last.devicesJson) {
           void fetch(absoluteUrl(DEVICES_PATH), {
-          method: 'PUT',
+            method: 'PUT', body: next.devicesJson,
-          body: JSON.stringify({ devices }, null, 2) + '\n',
+            headers: { 'Content-Type': 'application/json' }, keepalive: true,
-          headers: { 'Content-Type': 'application/json' },
-          keepalive: true,
           });
+        }
       } catch {
         /* ignore */
       }