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pi/ui/lib/services/backend_service.dart

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+import 'dart:async';
+import 'dart:convert';
+import 'package:http/http.dart' as http;
+
+/// Data from Arduino (voltage, rpm, engine temp, gear)
+class ArduinoData {
+  final double? voltage;
+  final int? rpm;
+  final int? engTemp;
+  final int? gear; // 0 = neutral, 1-6 = gear
+
+  ArduinoData({this.voltage, this.rpm, this.engTemp, this.gear});
+
+  factory ArduinoData.fromJson(Map<String, dynamic> json) {
+    return ArduinoData(
+      voltage: (json['voltage'] as num?)?.toDouble(),
+      rpm: (json['rpm'] as num?)?.toInt(),
+      engTemp: (json['eng_temp'] as num?)?.toInt(),
+      gear: (json['gear'] as num?)?.toInt(),
+    );
+  }
+}
+
+/// Data from GPS
+class GpsData {
+  final double? lat;
+  final double? lon;
+  final double? speed; // m/s
+  final double? alt;
+  final double? track;
+  final int? mode; // 0=no fix, 2=2D, 3=3D
+
+  GpsData({this.lat, this.lon, this.speed, this.alt, this.track, this.mode});
+
+  factory GpsData.fromJson(Map<String, dynamic> json) {
+    return GpsData(
+      lat: (json['lat'] as num?)?.toDouble(),
+      lon: (json['lon'] as num?)?.toDouble(),
+      speed: (json['speed'] as num?)?.toDouble(),
+      alt: (json['alt'] as num?)?.toDouble(),
+      track: (json['track'] as num?)?.toDouble(),
+      mode: (json['mode'] as num?)?.toInt(),
+    );
+  }
+}
+
+/// HTTP client for Flask backend - fire-and-forget async fetch, sync cache return
+///
+/// Follows the same pattern as PiIO: never blocks UI, always returns cached data.
+class BackendService {
+  BackendService._() {
+    // Kick off initial fetches
+    _refreshArduino();
+    _refreshGps();
+  }
+  static final instance = BackendService._();
+
+  static const _baseUrl = 'http://127.0.0.1:5000';
+  static const _timeout = Duration(seconds: 2);
+
+  // Caches
+  ArduinoData? _arduinoCache;
+  GpsData? _gpsCache;
+  bool _connected = false;
+
+  // In-progress flags (prevent duplicate requests)
+  bool _arduinoFetchInProgress = false;
+  bool _gpsFetchInProgress = false;
+
+  /// Whether backend is reachable
+  bool get isConnected => _connected;
+
+  /// Get Arduino data (sync, returns cached value)
+  ArduinoData? getArduinoData() {
+    if (!_arduinoFetchInProgress) {
+      _refreshArduino();
+    }
+    return _arduinoCache;
+  }
+
+  /// Get GPS data (sync, returns cached value)
+  GpsData? getGpsData() {
+    if (!_gpsFetchInProgress) {
+      _refreshGps();
+    }
+    return _gpsCache;
+  }
+
+  /// Background fetch for Arduino data
+  Future<void> _refreshArduino() async {
+    if (_arduinoFetchInProgress) return;
+    _arduinoFetchInProgress = true;
+
+    try {
+      final response = await http
+          .get(Uri.parse('$_baseUrl/arduino'))
+          .timeout(_timeout);
+
+      if (response.statusCode == 200) {
+        final json = jsonDecode(response.body) as Map<String, dynamic>;
+        // Skip if backend returns error (no data yet) - keep cached value
+        if (!json.containsKey('error')) {
+          _arduinoCache = ArduinoData.fromJson(json);
+        }
+        _connected = true;
+      }
+      // Non-200: keep cached data, just mark disconnected
+    } catch (e) {
+      // Network error, timeout, etc - keep cached data for transient hiccups
+      _connected = false;
+    } finally {
+      _arduinoFetchInProgress = false;
+    }
+  }
+
+  /// Background fetch for GPS data
+  Future<void> _refreshGps() async {
+    if (_gpsFetchInProgress) return;
+    _gpsFetchInProgress = true;
+
+    try {
+      final response = await http
+          .get(Uri.parse('$_baseUrl/gps'))
+          .timeout(_timeout);
+
+      if (response.statusCode == 200) {
+        final json = jsonDecode(response.body) as Map<String, dynamic>;
+        // Skip if backend returns error (no data yet) - keep cached value
+        if (!json.containsKey('error')) {
+          _gpsCache = GpsData.fromJson(json);
+        }
+        _connected = true;
+      }
+      // Non-200: keep cached data, just mark disconnected
+    } catch (e) {
+      // Network error, timeout, etc - keep cached data for transient hiccups
+      _connected = false;
+    } finally {
+      _gpsFetchInProgress = false;
+    }
+  }
+
+  /// Force clear all caches
+  void clearCache() {
+    _arduinoCache = null;
+    _gpsCache = null;
+    _connected = false;
+  }
+}