perf(prefs): use completers instead of polling

lib/data/prefs.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
@@ -320,12 +321,6 @@ abstract class IPref<T> extends ValueNotifier<T> {
 
   final T defaultValue;
 
-  bool _loaded = false;
-
-  /// Whether this pref has changes that have yet to be saved to disk.
-  @protected
-  bool _saved = true;
-
   IPref(
     this.key,
     this.defaultValue, {
@@ -335,11 +330,12 @@ abstract class IPref<T> extends ValueNotifier<T> {
         deprecatedKeys = deprecatedKeys ?? [],
         super(defaultValue) {
     if (Prefs.testingMode) {
-      _loaded = true;
+      loaded = true;
+
       return;
     } else {
       _load().then((T? loadedValue) {
-        _loaded = true;
+        loaded = true;
         if (loadedValue != null) {
           value = loadedValue;
         }
@@ -367,23 +363,52 @@ abstract class IPref<T> extends ValueNotifier<T> {
     return super.value;
   }
 
+  bool _loaded = false;
+  Completer<void>? _loadedCompleter = Completer();
   bool get loaded => _loaded;
-  bool get saved => _saved;
+  @protected
+  set loaded(bool loaded) {
+    _loaded = loaded;
 
-  Future<void> waitUntilLoaded() async {
-    while (!loaded) {
-      await Future.delayed(const Duration(milliseconds: 10));
+    if (loaded) {
+      if (!_loadedCompleter!.isCompleted) _loadedCompleter!.complete();
+      _loadedCompleter = null;
     }
   }
 
+  final _saved = ValueNotifier<bool>(true);
+
+  /// Whether this pref has changes that have yet to be saved to disk.
+  bool get saved => _saved.value;
+  @protected
+  set saved(bool saved) {
+    _saved.value = saved;
+  }
+
+  Future<void> waitUntilLoaded() async {
+    if (loaded) return;
+    assert(_loadedCompleter != null,
+        '_loadedCompleter should be non-null until loaded');
+    return _loadedCompleter!.future;
+  }
+
   /// Waits until the value has been saved to disk.
   /// Note that there is no guarantee with shared preferences that
   /// the value will actually be saved to disk.
   @visibleForTesting
   Future<void> waitUntilSaved() async {
-    while (!saved) {
-      await Future.delayed(const Duration(milliseconds: 10));
+    if (saved) return;
+
+    final completer = Completer();
+
+    void listener() {
+      if (!saved) return;
+      _saved.removeListener(listener);
+      completer.complete();
     }
+
+    _saved.addListener(listener);
+    return completer.future;
   }
 
   /// Lets us use notifyListeners outside of the class
@@ -451,7 +476,7 @@ class PlainPref<T> extends IPref<T> {
 
   @override
   Future _save() async {
-    _saved = false;
+    saved = false;
     try {
       _prefs ??= await SharedPreferences.getInstance();
 
@@ -512,7 +537,7 @@ class PlainPref<T> extends IPref<T> {
         return await _prefs!.setString(key, value as String);
       }
     } finally {
-      _saved = true;
+      saved = true;
     }
   }
 
@@ -637,7 +662,7 @@ class EncPref<T> extends IPref<T> {
 
   @override
   Future _save() async {
-    _saved = false;
+    saved = false;
     try {
       _storage ??= const FlutterSecureStorage();
       if (T == String)
@@ -646,7 +671,7 @@ class EncPref<T> extends IPref<T> {
         return await _storage!.write(key: key, value: jsonEncode(value));
       return await _storage!.write(key: key, value: jsonEncode(value));
     } finally {
-      _saved = true;
+      saved = true;
     }
   }