implemented scheduled updates for widgets

lib/background_tasks/scheduledTask.dart

@@ -0,0 +1,64 @@
+import 'dart:developer';
+
+import 'package:background_fetch/background_fetch.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import '../app.dart';
+import '../homescreen_widgets/timetable/timetableHomeWidget.dart';
+
+class ScheduledTask {
+  static final String fetchApiLastRunTimestampKey = 'fetchApiLastRunTimestamp';
+
+  static Future<void> configure() async {
+    var status = await BackgroundFetch.configure(BackgroundFetchConfig(
+        minimumFetchInterval: 15,
+        stopOnTerminate: false,
+        enableHeadless: true,
+        requiresBatteryNotLow: false,
+        requiresCharging: false,
+        requiresStorageNotLow: false,
+        requiresDeviceIdle: false,
+        requiredNetworkType: NetworkType.ANY,
+        startOnBoot: true,
+    ), (String taskId) async {
+      log('Background fetch started with id $taskId');
+      await ScheduledTask.backgroundFetch();
+      BackgroundFetch.finish(taskId);
+    }, (String taskId) async {
+      log('Background fetch stopped because of an timeout with id $taskId');
+      BackgroundFetch.finish(taskId);
+    });
+
+    log('Background Fetch-API status: $status');
+  }
+
+  // called periodically, iOS and Android
+  static Future<void> backgroundFetch() async {
+    var sp = await SharedPreferences.getInstance();
+    var history = sp.getStringList(fetchApiLastRunTimestampKey) ?? List.empty(growable: true);
+    history.add(DateTime.now().toIso8601String());
+    try {
+      TimetableHomeWidget.update(App.appContext.currentContext!);
+    } on Exception catch(e) {
+      history.add('Got Error:');
+      history.add(e.toString());
+      history.add('--- EXCEPTION END ---');
+    }
+    sp.setStringList(fetchApiLastRunTimestampKey, history.take(100).toList());
+  }
+
+  // only Android, starts when app is terminated
+  @pragma('vm:entry-point')
+  static Future<void> headless(HeadlessTask task) async {
+    var taskId = task.taskId;
+    var isTimeout = task.timeout;
+    if (isTimeout) {
+      log('Background fetch headless task timed-out: $taskId');
+      BackgroundFetch.finish(taskId);
+      return;
+    }
+    log('Background fetch headless event received.');
+    await backgroundFetch();
+    BackgroundFetch.finish(taskId);
+  }
+}