Created TimerRepository which saves timers list to sharedpreferences

Signed-off-by: Andreas Fahrecker <AndreasFahrecker@gmail.com>

lib/persistence/timers_repository.dart

@@ -0,0 +1,27 @@
+import 'dart:convert';
+
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:time_progress_calculator/persistence/timer_entity.dart';
+
+class TimersRepository {
+  static const String _key = "timers_repo";
+  final SharedPreferences prefs;
+  final JsonCodec codec;
+
+  TimersRepository(this.prefs, {this.codec = json});
+
+  Future<List<TimerEntity>> loadTimers() {
+    final String jsonString = this.prefs.getString(_key);
+    return codec
+        .decode(jsonString)["timers"]
+        .cast<Map<String, Object>>()
+        .map<TimerEntity>(TimerEntity.fromJson)
+        .toList(growable: false);
+  }
+
+  Future<bool> saveTimers(List<TimerEntity> timers) {
+    final String jsonString = codec
+        .encode({"timers": timers.map((timer) => timer.toJson()).toList()});
+    return this.prefs.setString(_key, jsonString);
+  }
+}