Complete all IoT Monitoring functions

- Add methods for retrieving previous/historical telemetry data
- Use timestamp from Realtime Database to update reload time instead of using timestamp generated on device

lib/data/IoT/models/telemetry_data.dart

@@ -12,4 +12,11 @@ class TelemetryData {
       value: data.value,
     );
   }
+
+  factory TelemetryData.from(String timestamp, String value) {
+    return TelemetryData(
+      timestamp: DateTime.fromMillisecondsSinceEpoch(int.parse(timestamp)),
+      value: value,
+    );
+  }
 }

lib/data/IoT/repositories/telemetry_data_repository.dart

@@ -17,8 +17,43 @@ class TelemetryDataRepository {
   Stream<TelemetryData> readData(String data) {
     return _telemetryDb
         .child(data)
-        .orderByValue()
+        .orderByKey()
         .onChildAdded
         .map((event) => TelemetryData.fromRealtimeDatabase(event.snapshot));
   }
+
+  TelemetryData readPreviousReading(String data) {
+    TelemetryData telemetryData;
+    _telemetryDb
+        .child(data)
+        .orderByKey()
+        .limitToLast(1)
+        .onChildAdded
+        .forEach((event) {
+      if (event.snapshot != null) {
+        telemetryData = TelemetryData.fromRealtimeDatabase(event.snapshot);
+      }
+    });
+    return telemetryData;
+  }
+
+  List<TelemetryData> readPreviousReadings(String data, int num) {
+    List<TelemetryData> telemetryDataList = List();
+    _telemetryDb
+        .child(data)
+        .orderByKey()
+        .limitToLast(num)
+        .onValue
+        .forEach((event) {
+      if (event.snapshot != null && event.snapshot.value != null) {
+        Map<dynamic, dynamic> map = event.snapshot.value;
+        map = Map.fromEntries(map.entries.toList()
+          ..sort((e1, e2) => int.parse(e1.key).compareTo(int.parse(e2.key))));
+        map.forEach((key, value) {
+          telemetryDataList.add(TelemetryData.from(key, value));
+        });
+      }
+    });
+    return telemetryDataList;
+  }
 }

lib/ui/IoT/reload_time.dart

@@ -6,8 +6,8 @@ class ReloadTime extends ChangeNotifier {
 
   String get reloadTime => _reloadTime;
 
-  void update() {
-    _reloadTime = DateFormat('dd-MM-yyyy HH:mm:ss').format(DateTime.now());
+  void update(DateTime timestamp) {
+    _reloadTime = DateFormat('dd-MM-yyyy HH:mm:ss').format(timestamp);
     notifyListeners();
   }
 }

lib/ui/IoT/telemetry_data_card.dart

@@ -98,6 +98,7 @@ class TelemetryDataCard extends StatelessWidget {
                           Expanded(
                             flex: 2,
                             child: TelemetryDataReading(
+                              data: cardItem.data,
                               reloadTime: reloadTime,
                             ),
                           ),
@@ -135,6 +136,7 @@ class TelemetryDataCard extends StatelessWidget {
                               bottom: 12,
                             ),
                             child: TelemetryDataChart(
+                              data: cardItem.data,
                               numData: 6,
                               cardItem: cardItem,
                             ),

lib/ui/IoT/telemetry_data_chart.dart

@@ -7,12 +7,17 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:intl/intl.dart';
 
 class TelemetryDataChart extends StatelessWidget {
-  TelemetryDataChart({Key key, @required this.numData, @required this.cardItem})
-      : spots = LineChartSpots(numData),
+  TelemetryDataChart({
+    Key key,
+    @required this.data,
+    @required this.numData,
+    @required this.cardItem,
+  })  : spots = LineChartSpots(numData),
         bottomTitles = FixedSizedQueue(numData),
         xIndexes = Iterable<int>.generate(numData).toList(),
         super(key: key);
 
+  final String data;
   final int numData;
   final LineChartSpots spots;
   final FixedSizedQueue<String> bottomTitles;
@@ -23,9 +28,31 @@ class TelemetryDataChart extends StatelessWidget {
   Widget build(BuildContext context) {
     TelemetryData telemetryData = context.watch<TelemetryData>();
 
+    // If real-time telemetry data is not available,
     if (telemetryData == null) {
-      return Container();
-    } else {
+      // Tries to read previous telemetry data.
+      // Use Provider.of() method due to stricter restrictions imposed by
+      // context.read() method when it is called inside a build function.
+      List<TelemetryData> telemetryDataList =
+          RepositoryProvider.of<TelemetryDataRepository>(
+        context,
+        listen: false,
+      ).readPreviousReadings(data, numData);
+
+      // Returns an empty container if previous telemetry data is also not available.
+      if (telemetryDataList.length == 0) {
+        return Container();
+      }
+      // Adds previous data into line chart points.
+      else {
+        telemetryDataList.forEach((telemetryData) {
+          spots.add(telemetryData.value);
+          bottomTitles.add(DateFormat.ms().format(telemetryData.timestamp));
+        });
+      }
+    }
+    // If real-time telemetry data is available, add data into line chart points.
+    else {
       spots.add(telemetryData.value);
       bottomTitles.add(DateFormat.ms().format(telemetryData.timestamp));
     }

lib/ui/IoT/telemetry_data_reading.dart

@@ -1,22 +1,40 @@
 import 'package:farmassist/app_theme.dart';
 import 'package:farmassist/data/IoT/models/telemetry_data.dart';
+import 'package:farmassist/data/IoT/repositories/telemetry_data_repository.dart';
 import 'package:farmassist/ui/IoT/reload_time.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:provider/provider.dart';
 
 class TelemetryDataReading extends StatelessWidget {
-  const TelemetryDataReading({this.reloadTime});
+  const TelemetryDataReading({@required this.data, this.reloadTime});
 
+  final String data;
   final ReloadTime reloadTime;
 
   @override
   Widget build(BuildContext context) {
     TelemetryData telemetryData = context.watch<TelemetryData>();
 
+    // If real-time telemetry data is not available,
     if (telemetryData == null) {
-      return _buildReading('N/A');
+      // Tries to read previous telemetry data.
+      // Use Provider.of() method due to stricter restrictions imposed by
+      // context.read() method when it is called inside a build function.
+      telemetryData = RepositoryProvider.of<TelemetryDataRepository>(
+        context,
+        listen: false,
+      ).readPreviousReading(data);
+    }
+
+    // Returns "N/A" if previous telemetry data is also not available.
+    if (telemetryData == null) {
+      return _buildReading("N/A");
     } else {
-      Future.delayed(Duration.zero, () => reloadTime.update());
+      Future.delayed(
+        Duration.zero,
+        () => reloadTime.update(telemetryData.timestamp),
+      );
       return _buildReading(telemetryData.value);
     }
   }