Adding SpriteSheet class

lib/spritesheet.dart

@@ -0,0 +1,48 @@
+import 'sprite.dart';
+import 'animation.dart';
+
+class SpriteSheet {
+  String imageName;
+  int textureWidth;
+  int textureHeight;
+  int columns;
+  int rows;
+
+  List<List<Sprite>> _sprites;
+
+  SpriteSheet({ this.imageName, this.textureWidth, this.textureHeight, this.columns, this.rows }) {
+    _sprites = List.generate(rows, (y) => List.generate(columns, (x) =>
+      Sprite(
+        imageName,
+        x: (x * textureWidth).toDouble(),
+        y: (y * textureHeight).toDouble(),
+        width: textureWidth.toDouble(),
+        height: textureHeight.toDouble()
+      ))
+    );
+  }
+
+  Sprite getSprite(int row, int column) => _sprites[row][column];
+
+  /// Creates an animation from this SpriteSheet
+  ///
+  /// An [from]  and a [to]  parameter can be specified to create an animation from a subset of the columns on the row
+  Animation createAnimation(int row, { double stepTime, loop = true, int from = 0, int to }) {
+    List<Sprite> spriteRow = _sprites[row];
+
+    if (spriteRow != null) {
+      to ??= spriteRow.length;
+
+      final spriteList = spriteRow.sublist(from, to);
+
+      return Animation.spriteList(
+        spriteList,
+        stepTime: stepTime,
+        loop: loop,
+      );
+    }
+
+    return null;
+  }
+}
+