fix: Fix SpriteBatch to comply with new drawAtlas requirement

examples/lib/stories/sprites/sprite_batch_load_example.dart

@@ -20,6 +20,11 @@ class SpriteBatchLoadExample extends FlameGame {
 
 class MySpriteBatchComponent extends SpriteBatchComponent
     with HasGameReference<SpriteBatchLoadExample> {
+  MySpriteBatchComponent()
+      : super(
+          blendMode: BlendMode.srcOver,
+        );
+
   @override
   Future<void> onLoad() async {
     final spriteBatch = await game.loadSpriteBatch('boom.png');

packages/flame/lib/src/sprite_batch.dart

@@ -277,9 +277,7 @@ class SpriteBatch {
 
     _sources[index] = newBatchItem.source;
     _transforms[index] = newBatchItem.transform;
-    if (color != null) {
-      _colors[index] = color;
-    }
+    _colors[index] = color ?? _defaultColor;
   }
 
   /// Add a new batch item using a RSTransform.
@@ -328,9 +326,7 @@ class SpriteBatch {
           : batchItem.source,
     );
     _transforms.add(batchItem.transform);
-    if (color != null) {
-      _colors.add(color);
-    }
+    _colors.add(color ?? _defaultColor);
   }
 
   /// Add a new batch item.
@@ -413,13 +409,19 @@ class SpriteBatch {
 
     final renderPaint = paint ?? _emptyPaint;
 
+    final hasNoColors = _colors.every((c) => c == _defaultColor);
+    final actualBlendMode = blendMode ?? defaultBlendMode;
+    if (!hasNoColors && actualBlendMode == null) {
+      throw 'When setting any colors, a blend mode must be provided.';
+    }
+
     if (useAtlas && !_flippedAtlasStatus.isGenerating) {
       canvas.drawAtlas(
         atlas,
         _transforms,
         _sources,
-        _colors.isEmpty ? null : _colors,
-        blendMode ?? defaultBlendMode,
+        hasNoColors ? null : _colors,
+        actualBlendMode,
         cullRect,
         renderPaint,
       );
@@ -441,4 +443,6 @@ class SpriteBatch {
       }
     }
   }
+
+  static const _defaultColor = Color(0x00000000);
 }

packages/flame/test/sprite_batch_test.dart

@@ -1,10 +1,14 @@
 import 'dart:ui';
 
+import 'package:flame/components.dart';
 import 'package:flame/sprite.dart';
+import 'package:flame_test/flame_test.dart';
 import 'package:flutter/material.dart' hide Image;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:mocktail/mocktail.dart';
 
+import '_resources/load_image.dart';
+
 class _MockImage extends Mock implements Image {}
 
 void main() {
@@ -56,5 +60,68 @@ void main() {
             .having((t) => t.ty, 'ty', 1),
       );
     });
+
+    const margin = 2.0;
+    const tileSize = 6.0;
+
+    testGolden(
+      'can render a batch with blend mode',
+      (game) async {
+        final spriteSheet = await loadImage('alphabet.png');
+        final spriteBatch = SpriteBatch(spriteSheet);
+
+        const source = Rect.fromLTWH(3 * tileSize, 0, tileSize, tileSize);
+
+        spriteBatch.add(
+          source: source,
+          color: Colors.redAccent,
+          offset: Vector2.all(margin),
+        );
+
+        spriteBatch.add(
+          source: source,
+          offset: Vector2(2 * margin + tileSize, margin),
+        );
+
+        game.add(
+          SpriteBatchComponent(
+            spriteBatch: spriteBatch,
+            blendMode: BlendMode.srcOver,
+          ),
+        );
+      },
+      size: Vector2(3 * margin + 2 * tileSize, 2 * margin + tileSize),
+      backgroundColor: const Color(0xFFFFFFFF),
+      goldenFile: '_goldens/sprite_batch_test_1.png',
+    );
+
+    testGolden(
+      'can render a batch without blend mode',
+      (game) async {
+        final spriteSheet = await loadImage('alphabet.png');
+        final spriteBatch = SpriteBatch(spriteSheet);
+
+        const source = Rect.fromLTWH(3 * tileSize, 0, tileSize, tileSize);
+
+        spriteBatch.add(
+          source: source,
+          offset: Vector2.all(margin),
+        );
+
+        spriteBatch.add(
+          source: source,
+          offset: Vector2(2 * margin + tileSize, margin),
+        );
+
+        game.add(
+          SpriteBatchComponent(
+            spriteBatch: spriteBatch,
+          ),
+        );
+      },
+      size: Vector2(3 * margin + 2 * tileSize, 2 * margin + tileSize),
+      backgroundColor: const Color(0xFFFFFFFF),
+      goldenFile: '_goldens/sprite_batch_test_2.png',
+    );
   });
 }

packages/flame_test/lib/src/test_golden.dart

@@ -34,13 +34,17 @@ void testGolden(
   PrepareGameFunction testBody, {
   required String goldenFile,
   Vector2? size,
+  Color? backgroundColor,
   FlameGame? game,
   bool skip = false,
 }) {
   testWidgets(
     testName,
     (tester) async {
-      final gameInstance = game ?? FlameGame();
+      final gameInstance = game ??
+          (backgroundColor != null
+              ? GameWithBackgroundColor(backgroundColor)
+              : FlameGame());
       const myKey = ValueKey('game-instance');
 
       await tester.runAsync(() async {
@@ -73,3 +77,12 @@ void testGolden(
 }
 
 typedef PrepareGameFunction = Future<void> Function(FlameGame game);
+
+class GameWithBackgroundColor extends FlameGame {
+  final Color _backgroundColor;
+
+  GameWithBackgroundColor(this._backgroundColor);
+
+  @override
+  Color backgroundColor() => _backgroundColor;
+}