improved performance and battery usage on older devices

lib/api/request_cache.dart

@@ -1,10 +1,9 @@
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:localstore/localstore.dart';
-
 import '../model/account_data.dart';
 import 'api_response.dart';
+import 'cache_store.dart';
 import 'errors/parse_exception.dart';
 import 'errors/stale_session_exception.dart';
 
@@ -14,12 +13,10 @@ abstract class RequestCache<T extends ApiResponse?> {
   static const int cacheHour = 60 * 60;
   static const int cacheDay = 60 * 60 * 24;
 
-  static String collection = 'MarianumMobile';
-
   int maxCacheTime;
   void Function(T)? onUpdate;
 
-  /// Called only when [start] finds a cached payload in localstore. Use this
+  /// Called only when [start] finds a cached payload in the [CacheStore]. Use this
   /// (instead of [onUpdate]) when callers need to distinguish stale-but-fast
   /// cache hits from authoritative network responses.
   void Function(T)? onCacheData;
@@ -52,47 +49,55 @@ abstract class RequestCache<T extends ApiResponse?> {
   Future<void> start(String document) async {
     final epoch = AccountData().sessionEpoch;
     try {
-      final tableData = await Localstore.instance
-          .collection(collection)
-          .doc(document)
-          .get();
-      if (tableData != null) {
-        final cached = onLocalData(tableData['json'] as String);
+      final entry = await CacheStore.instance.read(document);
+      var lastUpdate = entry?.lastUpdate ?? 0;
+      T? cached;
+      if (entry != null) {
+        try {
+          cached = fromCacheJson(await CacheStore.decode(entry.json));
+        } on Object {
+          // A payload from an older model version: treat it as a miss.
+          await CacheStore.instance.delete(document);
+          lastUpdate = 0;
+        }
+      }
+      if (cached != null) {
         onUpdate?.call(cached);
         onCacheData?.call(cached);
       }
-
-      final lastUpdate = (tableData?['lastupdate'] as num?) ?? 0;
-      if (DateTime.now().millisecondsSinceEpoch - (maxCacheTime * 1000) <
-          lastUpdate) {
-        if (renew == null || !renew!) return;
-      }
-
-      try {
-        final newValue = await onLoad();
-        // The collection is shared, so a late response of a signed-out
-        // account would otherwise be cached for the next one.
-        if (!AccountData().isCurrentSession(epoch)) {
-          onError(const StaleSessionException());
-          return;
-        }
-        onUpdate?.call(newValue);
-        onNetworkData?.call(newValue);
-        unawaited(
-          Localstore.instance.collection(collection).doc(document).set({
-            'json': jsonEncode(newValue),
-            'lastupdate': DateTime.now().millisecondsSinceEpoch,
-          }),
-        );
-      } on Exception catch (e) {
-        onError(e);
-      }
+      await _load(document, epoch, lastUpdate: lastUpdate);
     } finally {
       if (!_ready.isCompleted) _ready.complete();
     }
   }
 
-  T onLocalData(String json);
+  Future<void> _load(
+    String document,
+    int epoch, {
+    required int lastUpdate,
+  }) async {
+    if (DateTime.now().millisecondsSinceEpoch - (maxCacheTime * 1000) <
+        lastUpdate) {
+      if (renew == null || !renew!) return;
+    }
+
+    try {
+      final newValue = await onLoad();
+      // The cache is shared, so a late response of a signed-out
+      // account would otherwise be cached for the next one.
+      if (!AccountData().isCurrentSession(epoch)) {
+        onError(const StaleSessionException());
+        return;
+      }
+      onUpdate?.call(newValue);
+      onNetworkData?.call(newValue);
+      unawaited(CacheStore.instance.write(document, jsonEncode(newValue)));
+    } on Exception catch (e) {
+      onError(e);
+    }
+  }
+
+  T fromCacheJson(Map<String, dynamic> json);
   Future<T> onLoad();
 }
 
@@ -126,8 +131,7 @@ class SimpleCache<T extends ApiResponse?> extends RequestCache<T> {
   Future<T> onLoad() => _loader();
 
   @override
-  T onLocalData(String json) =>
-      _fromJson(jsonDecode(json) as Map<String, dynamic>);
+  T fromCacheJson(Map<String, dynamic> json) => _fromJson(json);
 }
 
 /// Captures the latest cache payload (cached or network) and rethrows the