add: load_saved_data.dart, not used

app/lib/pages/marker_handler/load_saved_data.dart

+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'marker_data.dart';
+import '../consts.dart';
+import 'package:path_provider/path_provider.dart';
+import 'dart:typed_data';
+
+/// loadSavedData parses json data from a saved file, either to a list
+/// of Measurement objects or a Uint8List object depending if it reads
+/// measurement data or relation data.
+Future<FetchResult> loadSavedData() async {
+  Uint8List relation;
+  List<Measurement> measurements = [];
+
+  // Get latest saved data from file if the server does not respond
+  Directory appDocumentsDirectory = await getApplicationDocumentsDirectory();
+  String filePath = '${appDocumentsDirectory.path}/last_data.sjon';
+
+  // Read measurement file contents
+  File file = File(filePath);
+  if (await file.exists()) {
+    String contents = await file.readAsString();
+    List<dynamic> jsonData = json.decode(contents); // Parse JSON string from file
+    measurements = jsonData.map((data) => Measurement.fromJson(data)).toList();
+  } else {
+    throw Exception('File does not exist');
+  }
+
+  filePath = '${appDocumentsDirectory.path}/relation.json';
+
+  // Read relation file contents
+  file = File(filePath);
+  if (await file.exists()) {
+    String contents = await file.readAsString();
+    List<dynamic> jsonData = json.decode(contents); // Parse JSON string from file
+    relation = Uint8List.fromList(utf8.encode(jsonData.toString()));
+  } else {
+    throw Exception('File does not exist');
+  }
+
+  return FetchResult(relation, measurements, false);
+}
+
+class FetchResult {
+  final Uint8List relation;
+  final List<Measurement> measurements;
+  final bool connected;
+
+  FetchResult(this.relation, this.measurements, this.connected);
+}