perf: replace unstable Tile widget key with dedicated stable TileKey (#2195)

Author: braindamagedman

lib/src/layer/tile_layer/tile_key.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter_map/src/layer/tile_layer/tile_coordinates.dart';
+import 'package:flutter_map/src/layer/tile_layer/tile_image.dart';
+import 'package:flutter_map/src/layer/tile_layer/tile_renderer.dart';
+
+/// A key that identifies rendering of a [TileImage] at given [TileCoordinates].
+///
+/// Two [TileKey]s are equal when they reference the same [TileImage]
+/// instance and have equal [positionCoordinates].
+final class TileKey extends LocalKey {
+  /// Tile image to identify by instance.
+  final TileImage tileImage;
+
+  /// Position where [tileImage] is rendered.
+  final TileCoordinates positionCoordinates;
+
+  /// Creates a [TileKey] for the given [TileRenderer].
+  ///
+  /// The [tileImage] is compared by identity, while [positionCoordinates] are
+  /// compared by value.
+  TileKey(TileRenderer renderer)
+      : tileImage = renderer.tileImage,
+        positionCoordinates = renderer.positionCoordinates;
+
+  @override
+  bool operator ==(Object other) {
+    return other is TileKey &&
+        identical(other.tileImage, tileImage) &&
+        other.positionCoordinates == positionCoordinates;
+  }
+
+  @override
+  int get hashCode => Object.hash(
+        identityHashCode(tileImage),
+        positionCoordinates,
+      );
+}

lib/src/layer/tile_layer/tile_layer.dart

@@ -9,6 +9,7 @@ import 'package:flutter_map/src/layer/tile_layer/tile.dart';
 import 'package:flutter_map/src/layer/tile_layer/tile_bounds/tile_bounds.dart';
 import 'package:flutter_map/src/layer/tile_layer/tile_bounds/tile_bounds_at_zoom.dart';
 import 'package:flutter_map/src/layer/tile_layer/tile_image_manager.dart';
+import 'package:flutter_map/src/layer/tile_layer/tile_key.dart';
 import 'package:flutter_map/src/layer/tile_layer/tile_range.dart';
 import 'package:flutter_map/src/layer/tile_layer/tile_range_calculator.dart';
 import 'package:flutter_map/src/layer/tile_layer/tile_scale_calculator.dart';
@@ -560,7 +561,7 @@ class _TileLayerState extends State<TileLayer> with TickerProviderStateMixin {
         .map((tileRenderer) => Tile(
               // Must be an ObjectKey, not a ValueKey using the coordinates, in
               // case we remove and replace the TileImage with a different one.
-              key: ObjectKey(tileRenderer),
+              key: TileKey(tileRenderer),
               scaledTileDimension: _tileScaleCalculator.scaledTileDimension(
                 map.zoom,
                 tileRenderer.positionCoordinates.z,