improved performance and battery usage on older devices

lib/api/cache_store.dart

@@ -0,0 +1,185 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:path_provider/path_provider.dart';
+
+/// One cached response: the raw JSON payload and when it was stored.
+class CacheEntry {
+  final String json;
+  final int lastUpdate;
+
+  const CacheEntry({required this.json, required this.lastUpdate});
+}
+
+/// File-per-key store behind [RequestCache] and friends.
+///
+/// Replaces `localstore` for response caching: localstore reads and decodes
+/// the entire collection synchronously on first access, keeps every document
+/// in memory for the whole session and JSON-encodes payloads a second time.
+/// Here every access is async and touches exactly one file, and the payload is
+/// stored verbatim behind a one-line header (`<lastUpdate>\n<json>`).
+class CacheStore {
+  CacheStore._();
+
+  static final CacheStore instance = CacheStore._();
+
+  /// Payloads above this size are decoded on a background isolate.
+  static const int isolateDecodeThreshold = 64 * 1024;
+
+  Future<Directory>? _dir;
+  final Map<String, Future<void>> _writes = {};
+
+  Future<Directory> _directory() => _dir ??= () async {
+    final base = await getApplicationCacheDirectory();
+    final dir = Directory('${base.path}/request_cache');
+    await dir.create(recursive: true);
+    return dir;
+  }();
+
+  Future<File> _file(String key) async =>
+      File('${(await _directory()).path}/${_safeName(key)}');
+
+  /// Keys are internal ids (`nc-chat-<token>`, `wd-folder-<md5>`); only path
+  /// separators would be a problem in a file name.
+  static String _safeName(String key) => key.replaceAll(RegExp(r'[/\\]'), '_');
+
+  Future<CacheEntry?> read(String key) async {
+    try {
+      await _writes[key];
+      return parse(await (await _file(key)).readAsString());
+    } on Object {
+      // Missing (PathNotFoundException) or unreadable: a cache miss.
+      return null;
+    }
+  }
+
+  /// All entries whose key starts with [prefix], read concurrently.
+  Future<Map<String, CacheEntry>> readAll({String prefix = ''}) async {
+    final keys = await this.keys(prefix: prefix);
+    final entries = await Future.wait(keys.map(read));
+    return {for (var i = 0; i < keys.length; i++) keys[i]: ?entries[i]};
+  }
+
+  Future<void> write(String key, String json) {
+    final previous = _writes[key] ?? Future<void>.value();
+    late final Future<void> current;
+    current = previous.then((_) => _write(key, json)).whenComplete(() {
+      if (identical(_writes[key], current)) _writes.remove(key);
+    });
+    return _writes[key] = current;
+  }
+
+  Future<void> _write(String key, String json) async {
+    try {
+      final file = await _file(key);
+      // Write-then-rename so a reader (or the widget background isolate)
+      // never sees a half-written file.
+      final tmp = File('${file.path}.tmp');
+      await tmp.writeAsString(
+        '${DateTime.now().millisecondsSinceEpoch}\n$json',
+        flush: true,
+      );
+      await tmp.rename(file.path);
+    } on Object catch (e) {
+      debugPrint('CacheStore.write($key) failed: $e');
+    }
+  }
+
+  Future<void> delete(String key) async {
+    try {
+      await _writes[key];
+      await (await _file(key)).delete();
+    } on Object {
+      // A missing or locked file is as good as deleted for a cache.
+    }
+  }
+
+  Future<void> clear() async {
+    try {
+      final dir = await _directory();
+      if (dir.existsSync()) await dir.delete(recursive: true);
+    } on Object catch (e) {
+      debugPrint('CacheStore.clear failed: $e');
+    }
+    _dir = null;
+  }
+
+  /// All stored keys, optionally filtered by [prefix].
+  Future<List<String>> keys({String prefix = ''}) async {
+    try {
+      final dir = await _directory();
+      return [
+        await for (final entity in dir.list())
+          if (entity is File && !entity.path.endsWith('.tmp'))
+            if (entity.uri.pathSegments.last case final name
+                when name.startsWith(prefix))
+              name,
+      ];
+    } on Object {
+      return const [];
+    }
+  }
+
+  /// Removes entries not written for [maxAge], judged by file mtime so nothing
+  /// has to be read or decoded.
+  Future<void> deleteOlderThan(Duration maxAge) async {
+    try {
+      final dir = await _directory();
+      final cutoff = DateTime.now().subtract(maxAge);
+      await for (final entity in dir.list()) {
+        if (entity is! File) continue;
+        // ignore: avoid_slow_async_io
+        final modified = (await entity.stat()).modified;
+        if (modified.isBefore(cutoff)) await entity.delete();
+      }
+    } on Object catch (e) {
+      debugPrint('CacheStore.deleteOlderThan failed: $e');
+    }
+  }
+
+  /// Total size of all entries in bytes.
+  Future<int> totalSize() async {
+    try {
+      final dir = await _directory();
+      var sum = 0;
+      await for (final entity in dir.list()) {
+        if (entity is File) sum += await entity.length();
+      }
+      return sum;
+    } on Object {
+      return 0;
+    }
+  }
+
+  /// Parses the stored file format; public for tests.
+  static CacheEntry? parse(String raw) {
+    final newline = raw.indexOf('\n');
+    if (newline < 0) return null;
+    final lastUpdate = int.tryParse(raw.substring(0, newline));
+    if (lastUpdate == null) return null;
+    return CacheEntry(json: raw.substring(newline + 1), lastUpdate: lastUpdate);
+  }
+
+  /// Decodes a cached payload, off the UI isolate when it is large.
+  static Future<Map<String, dynamic>> decode(String json) =>
+      json.length > isolateDecodeThreshold
+      ? compute(_decodeMap, json)
+      : Future.value(_decodeMap(json));
+
+  static Map<String, dynamic> _decodeMap(String json) =>
+      jsonDecode(json) as Map<String, dynamic>;
+
+  /// Deletes the directory the old localstore-based cache lived in. It is
+  /// never read again and would otherwise linger with up to 200 days of data.
+  static Future<void> deleteLegacyLocalstoreCache() async {
+    try {
+      final docs = await getApplicationDocumentsDirectory();
+      final legacy = Directory('${docs.path}/MarianumMobile');
+      if (legacy.existsSync()) await legacy.delete(recursive: true);
+    } on Object catch (e) {
+      debugPrint('Legacy cache cleanup failed: $e');
+    }
+  }
+}