Add Fallback URLs (for #1203)

example/lib/main.dart

@@ -6,6 +6,8 @@ import 'package:flutter_map_example/pages/custom_crs/custom_crs.dart';
 import 'package:flutter_map_example/pages/epsg3413_crs.dart';
 import 'package:flutter_map_example/pages/epsg4326_crs.dart';
 import 'package:flutter_map_example/pages/esri.dart';
+import 'package:flutter_map_example/pages/fallback_url_network_page.dart';
+import 'package:flutter_map_example/pages/fallback_url_offline_page.dart';
 import 'package:flutter_map_example/pages/home.dart';
 import 'package:flutter_map_example/pages/interactive_test_page.dart';
 import 'package:flutter_map_example/pages/latlng_to_screen_point.dart';
@@ -87,6 +89,10 @@ class MyApp extends StatelessWidget {
         PointToLatLngPage.route: (context) => const PointToLatLngPage(),
         LatLngScreenPointTestPage.route: (context) =>
             const LatLngScreenPointTestPage(),
+        FallbackUrlNetworkPage.route: (context) =>
+            const FallbackUrlNetworkPage(),
+        FallbackUrlOfflinePage.route: (context) =>
+            const FallbackUrlOfflinePage(),
       },
     );
   }

example/lib/pages/fallback_url/fallback_url.dart

@@ -0,0 +1,57 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_map_example/widgets/drawer.dart';
+import 'package:latlong2/latlong.dart';
+
+class FallbackUrlPage extends StatelessWidget {
+  final String route;
+  final TileLayer tileLayer;
+  final String title;
+  final String description;
+  final double zoom;
+  final double? maxZoom;
+  final double? minZoom;
+  final LatLng center;
+
+  const FallbackUrlPage({
+    Key? key,
+    required this.route,
+    required this.tileLayer,
+    required this.title,
+    required this.description,
+    required this.center,
+    this.zoom = 13,
+    this.maxZoom,
+    this.minZoom,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(title)),
+      drawer: buildDrawer(context, route),
+      body: Padding(
+        padding: const EdgeInsets.all(8),
+        child: Column(
+          children: [
+            Padding(
+              padding: const EdgeInsets.only(top: 8, bottom: 8),
+              child: Text(description),
+            ),
+            Flexible(
+              child: FlutterMap(
+                options: MapOptions(
+                  center: center,
+                  zoom: zoom,
+                  maxZoom: maxZoom,
+                  minZoom: minZoom,
+                ),
+                children: [tileLayer],
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}

example/lib/pages/fallback_url_network_page.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_map_example/pages/fallback_url/fallback_url.dart';
+import 'package:latlong2/latlong.dart';
+
+class FallbackUrlNetworkPage extends StatelessWidget {
+  static const String route = '/fallback_url_network';
+
+  const FallbackUrlNetworkPage({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return FallbackUrlPage(
+      route: route,
+      tileLayer: TileLayer(
+        urlTemplate: 'https://fake-tile-provider.org/{z}/{x}/{y}.png',
+        fallbackUrl: 'https://tile.openstreetmap.org/{z}/{x}/{y}.png',
+        subdomains: const ['a', 'b', 'c'],
+        userAgentPackageName: 'dev.fleaflet.flutter_map.example',
+      ),
+      title: 'Fallback URL NetworkTileProvider',
+      description:
+          'Map with a fake url should use the fallback, showing (51.5, -0.9).',
+      zoom: 5,
+      center: LatLng(51.5, -0.09),
+    );
+  }
+}

example/lib/pages/fallback_url_offline_page.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_map_example/pages/fallback_url/fallback_url.dart';
+import 'package:latlong2/latlong.dart';
+
+class FallbackUrlOfflinePage extends StatelessWidget {
+  static const String route = '/fallback_url_offline';
+
+  const FallbackUrlOfflinePage({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return FallbackUrlPage(
+      route: route,
+      tileLayer: TileLayer(
+        tileProvider: AssetTileProvider(),
+        maxZoom: 14,
+        urlTemplate: 'assets/fake/tiles/{z}/{x}/{y}.png',
+        fallbackUrl: 'assets/map/anholt_osmbright/{z}/{x}/{y}.png',
+      ),
+      title: 'Fallback URL AssetTileProvider',
+      description:
+          'Map with a fake asset path, should be using the fallback to show Anholt Island, Denmark.',
+      maxZoom: 14,
+      minZoom: 12,
+      center: LatLng(56.704173, 11.543808),
+    );
+  }
+}

example/lib/pages/offline_map.dart

@@ -28,7 +28,6 @@ class OfflineMapPage extends StatelessWidget {
                   center: LatLng(56.704173, 11.543808),
                   minZoom: 12,
                   maxZoom: 14,
-                  zoom: 13,
                   swPanBoundary: LatLng(56.6877, 11.5089),
                   nePanBoundary: LatLng(56.7378, 11.6644),
                 ),

example/lib/widgets/drawer.dart

@@ -6,6 +6,8 @@ import 'package:flutter_map_example/pages/custom_crs/custom_crs.dart';
 import 'package:flutter_map_example/pages/epsg3413_crs.dart';
 import 'package:flutter_map_example/pages/epsg4326_crs.dart';
 import 'package:flutter_map_example/pages/esri.dart';
+import 'package:flutter_map_example/pages/fallback_url_network_page.dart';
+import 'package:flutter_map_example/pages/fallback_url_offline_page.dart';
 import 'package:flutter_map_example/pages/home.dart';
 import 'package:flutter_map_example/pages/interactive_test_page.dart';
 import 'package:flutter_map_example/pages/latlng_to_screen_point.dart';
@@ -257,6 +259,18 @@ Drawer buildDrawer(BuildContext context, String currentRoute) {
             PointToLatLngPage.route, currentRoute),
         _buildMenuItem(context, const Text('LatLng to ScreenPoint'),
             LatLngScreenPointTestPage.route, currentRoute),
+        _buildMenuItem(
+          context,
+          const Text('Fallback URL NetworkTileProvider'),
+          FallbackUrlNetworkPage.route,
+          currentRoute,
+        ),
+        _buildMenuItem(
+          context,
+          const Text('Fallback URL AssetTileProvider'),
+          FallbackUrlOfflinePage.route,
+          currentRoute,
+        ),
       ],
     ),
   );

lib/flutter_map.dart

@@ -35,6 +35,7 @@ export 'package:flutter_map/src/layer/tile_layer/tile.dart';
 export 'package:flutter_map/src/layer/tile_layer/tile_builder.dart';
 export 'package:flutter_map/src/layer/tile_layer/tile_layer.dart';
 export 'package:flutter_map/src/layer/tile_layer/tile_provider/base_tile_provider.dart';
+export 'package:flutter_map/src/layer/tile_layer/tile_provider/asset_tile_provider.dart';
 export 'package:flutter_map/src/layer/tile_layer/tile_provider/file_tile_provider_io.dart'
     if (dart.library.html) 'package:flutter_map/src/layer/tile_layer/tile_provider/file_tile_provider_web.dart';
 export 'package:flutter_map/src/layer/tile_layer/tile_provider/tile_provider_io.dart'

lib/src/layer/tile_layer/tile_layer.dart

@@ -39,6 +39,10 @@ class TileLayer extends StatefulWidget {
   /// https://a.tile.openstreetmap.org/12/2177/1259.png
   final String? urlTemplate;
 
+  /// Follows the same structure as [urlTemplate]. If specified, this URL is
+  /// used only if an error occurs when loading the [urlTemplate].
+  final String? fallbackUrl;
+
   /// If `true`, inverses Y axis numbering for tiles (turn this on for
   /// [TMS](https://en.wikipedia.org/wiki/Tile_Map_Service) services).
   final bool tms;
@@ -239,6 +243,7 @@ class TileLayer extends StatefulWidget {
   TileLayer({
     super.key,
     this.urlTemplate,
+    this.fallbackUrl,
     double tileSize = 256.0,
     double minZoom = 0.0,
     double maxZoom = 18.0,
@@ -340,6 +345,9 @@ class _TileLayerState extends State<TileLayer> with TickerProviderStateMixin {
     if (widget.reset != null) {
       _resetSub = widget.reset?.listen((_) => _resetTiles());
     }
+
+    //TODO fix
+    // _initThrottleUpdate();
   }
 
   @override

lib/src/layer/tile_layer/tile_provider/asset_tile_provider.dart

@@ -0,0 +1,48 @@
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_map/src/layer/tile_layer/coords.dart';
+import 'package:flutter_map/src/layer/tile_layer/tile_layer.dart';
+import 'package:flutter_map/src/layer/tile_layer/tile_provider/base_tile_provider.dart';
+
+class AssetTileProvider extends TileProvider {
+  @override
+  AssetImage getImage(Coords<num> coords, TileLayer options) {
+    return AssetImage(
+      getTileUrl(coords, options),
+      bundle: _FlutterMapAssetBundle(
+        fallbackKey: getTileFallbackUrl(coords, options),
+      ),
+    );
+  }
+}
+
+/// Used to load a fallback asset when the main asset is not found.
+class _FlutterMapAssetBundle extends CachingAssetBundle {
+  final String? fallbackKey;
+
+  _FlutterMapAssetBundle({required this.fallbackKey});
+
+  Future<ByteData?> _loadAsset(String key) async {
+    final Uint8List encoded =
+        utf8.encoder.convert(Uri(path: Uri.encodeFull(key)).path);
+    final ByteData? asset = await ServicesBinding
+        .instance.defaultBinaryMessenger
+        .send('flutter/assets', encoded.buffer.asByteData());
+    return asset;
+  }
+
+  @override
+  Future<ByteData> load(String key) async {
+    final asset = await _loadAsset(key);
+    if (asset != null && asset.lengthInBytes > 0) return asset;
+
+    if (fallbackKey != null) {
+      final fallbackAsset = await _loadAsset(fallbackKey!);
+      if (fallbackAsset != null) return fallbackAsset;
+    }
+
+    throw FlutterError('_FlutterMapAssetBundle - Unable to load asset: $key');
+  }
+}

lib/src/layer/tile_layer/tile_provider/base_tile_provider.dart

@@ -22,13 +22,7 @@ abstract class TileProvider {
   /// Called when the [TileLayerWidget] is disposed
   void dispose() {}
 
-  /// Generate a valid URL for a tile, based on it's coordinates and the current [TileLayerOptions]
-  String getTileUrl(Coords coords, TileLayer options) {
-    final urlTemplate = (options.wmsOptions != null)
-        ? options.wmsOptions!
-            .getUrl(coords, options.tileSize.toInt(), options.retinaMode)
-        : options.urlTemplate;
-
+  String _getTileUrl(String urlTemplate, Coords coords, TileLayer options) {
     final z = _getZoomForUrl(coords, options);
 
     final data = <String, String>{
@@ -43,7 +37,25 @@ abstract class TileProvider {
     }
     final allOpts = Map<String, String>.from(data)
       ..addAll(options.additionalOptions);
-    return options.templateFunction(urlTemplate!, allOpts);
+    return options.templateFunction(urlTemplate, allOpts);
+  }
+
+  /// Generate a valid URL for a tile, based on it's coordinates and the current
+  /// [TileLayerOptions]
+  String getTileUrl(Coords coords, TileLayer options) {
+    final urlTemplate = (options.wmsOptions != null)
+        ? options.wmsOptions!
+            .getUrl(coords, options.tileSize.toInt(), options.retinaMode)
+        : options.urlTemplate;
+
+    return _getTileUrl(urlTemplate!, coords, options);
+  }
+
+  /// Generates a valid URL for the [fallbackUrl].
+  String? getTileFallbackUrl(Coords coords, TileLayer options) {
+    final urlTemplate = options.fallbackUrl;
+    if (urlTemplate == null) return null;
+    return _getTileUrl(urlTemplate, coords, options);
   }
 
   double _getZoomForUrl(Coords coords, TileLayer options) {

lib/src/layer/tile_layer/tile_provider/network_image_provider.dart

@@ -2,24 +2,29 @@ import 'dart:ui';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
-import 'package:http/http.dart';
+import 'package:http/http.dart' as http;
 import 'package:http/retry.dart';
 
 class FMNetworkImageProvider extends ImageProvider<FMNetworkImageProvider> {
   /// The URL from which the image will be fetched.
   final String url;
 
-  /// The http RetryClient that is used for the requests
-  final RetryClient retryClient;
+  /// The fallback URL from which the image will be fetched.
+  final String? fallbackUrl;
+
+  /// The http client that is used for the requests. Defaults to a [RetryClient]
+  /// with a [http.Client].
+  final http.Client httpClient;
 
   /// Custom headers to add to the image fetch request
   final Map<String, String> headers;
 
   FMNetworkImageProvider(
     this.url, {
-    RetryClient? retryClient,
+    required this.fallbackUrl,
+    http.Client? httpClient,
     this.headers = const {},
-  }) : retryClient = retryClient ?? RetryClient(Client());
+  }) : httpClient = httpClient ?? RetryClient(http.Client());
 
   @override
   ImageStreamCompleter loadBuffer(
@@ -38,22 +43,31 @@ class FMNetworkImageProvider extends ImageProvider<FMNetworkImageProvider> {
 
   Future<ImageInfo> _loadWithRetry(
     FMNetworkImageProvider key,
-    DecoderBufferCallback decode,
-  ) async {
+    DecoderBufferCallback decode, [
+    bool useFallback = false,
+    ]) async {
     assert(key == this);
+    assert(useFallback == false || fallbackUrl != null);
 
-    final uri = Uri.parse(url);
-    final response = await retryClient.get(uri, headers: headers);
+    try {
+      final uri = Uri.parse(useFallback ? fallbackUrl! : url);
+      final response = await httpClient.get(uri, headers: headers);
 
-    if (response.statusCode != 200) {
-      throw NetworkImageLoadException(
-          statusCode: response.statusCode, uri: uri);
-    }
+      if (response.statusCode != 200) {
+        throw NetworkImageLoadException(
+            statusCode: response.statusCode, uri: uri);
+      }
 
     final codec =
         await decode(await ImmutableBuffer.fromUint8List(response.bodyBytes));
     final image = (await codec.getNextFrame()).image;
 
-    return ImageInfo(image: image);
+      return ImageInfo(image: image);
+    } catch (e) {
+      if (!useFallback && fallbackUrl != null) {
+        return _loadWithRetry(key, decode, true);
+      }
+      rethrow;
+    }
   }
 }