Added memory caching support to FlutterMapNetworkImageProvider (#1629)

lib/src/layer/tile_layer/tile_layer.dart

@@ -24,22 +24,18 @@ part 'wms_tile_layer_options.dart';
 /// avoid issues.
 @immutable
 class TileLayer extends StatefulWidget {
-  /// Defines the structure to create the URLs for the tiles. `{s}` means one of
-  /// the available subdomains (can be omitted) `{z}` zoom level `{x}` and `{y}`
-  /// — tile coordinates `{r}` can be used to add "@2x" to the URL to
-  /// load retina tiles (can be omitted)
+  /// The URL template is a string that contains placeholders, which, when filled
+  /// in, create a URL/URI to a specific tile.
   ///
-  /// Example:
-  ///
-  /// https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png
-  ///
-  /// Is translated to this:
-  ///
-  /// https://a.tile.openstreetmap.org/12/2177/1259.png
+  /// For more information, see <https://docs.fleaflet.dev/layers/tile-layer>.
   final String? urlTemplate;
 
-  /// Follows the same structure as [urlTemplate]. If specified, this URL is
-  /// used only if an error occurs when loading the [urlTemplate].
+  /// Fallback URL template, used if an error occurs when fetching tiles from
+  /// the [urlTemplate].
+  ///
+  /// Note that specifying this (non-null) will result in tiles not being cached
+  /// in memory. This is to avoid issues where the [urlTemplate] is flaky, to
+  /// prevent different tilesets being displayed at the same time.
   ///
   /// Avoid specifying this when using [AssetTileProvider] or [FileTileProvider],
   /// as these providers are less performant and efficient when this is

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

@@ -6,6 +6,10 @@ import 'package:flutter/painting.dart';
 import 'package:http/http.dart';
 
 /// Dedicated [ImageProvider] to fetch tiles from the network
+///
+/// Supports falling back to a secondary URL, if the primary URL fetch fails.
+/// Note that specifying a [fallbackUrl] will prevent this image provider from
+/// being cached.
 @immutable
 class FlutterMapNetworkImageProvider
     extends ImageProvider<FlutterMapNetworkImageProvider> {
@@ -14,15 +18,27 @@ class FlutterMapNetworkImageProvider
 
   /// The URL to fetch the tile from (GET request), in the event the original
   /// [url] request fails
+  ///
+  /// If this is non-null, [operator==] will always return `false` (except if
+  /// the two objects are [identical]). Therefore, if this is non-null, this
+  /// image provider will not be cached in memory.
   final String? fallbackUrl;
 
   /// The HTTP client to use to make network requests
+  ///
+  /// Not included in [operator==].
   final BaseClient httpClient;
 
   /// The headers to include with the tile fetch request
+  ///
+  /// Not included in [operator==].
   final Map<String, String> headers;
 
-  /// Dedicated [ImageProvider] to fetch tiles from the network
+  /// Create a dedicated [ImageProvider] to fetch tiles from the network
+  ///
+  /// Supports falling back to a secondary URL, if the primary URL fetch fails.
+  /// Note that specifying a [fallbackUrl] will prevent this image provider from
+  /// being cached.
   const FlutterMapNetworkImageProvider({
     required this.url,
     required this.fallbackUrl,
@@ -75,4 +91,15 @@ class FlutterMapNetworkImageProvider
 
     return decode(await ImmutableBuffer.fromUint8List(bytes));
   }
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      (other is FlutterMapNetworkImageProvider &&
+          fallbackUrl == null &&
+          url == other.url);
+
+  @override
+  int get hashCode =>
+      Object.hashAll([url, if (fallbackUrl != null) fallbackUrl]);
 }