allow promotion to king, needed for Antichess

lib/src/models.dart

@@ -34,6 +34,7 @@ class GameData {
     required this.onPromotionSelection,
     this.isCheck,
     this.premovable,
+    this.canPromoteToKing = false,
   });
 
   /// Side that is allowed to move.
@@ -67,6 +68,9 @@ class GameData {
   ///
   /// If `null`, the board will not allow premoves.
   final Premovable? premovable;
+
+  /// Whether the pawn can be promoted to a king (possible for example in Antichess).
+  final bool canPromoteToKing;
 }
 
 /// State of a premovable chessboard.

lib/src/widgets/board.dart

@@ -487,6 +487,7 @@ class _BoardState extends State<Chessboard> {
                 onCancel: () {
                   widget.game!.onPromotionSelection(null);
                 },
+                canPromoteToKing: widget.game!.canPromoteToKing,
               ),
           ],
         ),

lib/src/widgets/promotion.dart

@@ -8,7 +8,7 @@ import 'piece.dart';
 ///
 /// This widget should be displayed when a pawn reaches the last rank and must be
 /// promoted. The user can select a piece to promote to by tapping on one of
-/// the four pieces displayed.
+/// the pieces displayed in the widget. Normally there are 4 pieces to choose from (queen, rook, bishop and knight), but a king can also be included as an option if `canPromoteToKing` is true (for example in Antichess).
 /// Promotion can be canceled by tapping outside the promotion widget.
 class PromotionSelector extends StatelessWidget with ChessboardGeometry {
   const PromotionSelector({
@@ -21,6 +21,7 @@ class PromotionSelector extends StatelessWidget with ChessboardGeometry {
     required this.onSelect,
     required this.onCancel,
     required this.pieceAssets,
+    required this.canPromoteToKing,
   });
 
   /// The move that is being promoted.
@@ -32,6 +33,9 @@ class PromotionSelector extends StatelessWidget with ChessboardGeometry {
   /// The piece assets to use.
   final PieceAssets pieceAssets;
 
+  /// Whether the pawn can be promoted to a king (possible for example in Antichess).
+  final bool canPromoteToKing;
+
   @override
   final double size;
 
@@ -59,20 +63,18 @@ class PromotionSelector extends StatelessWidget with ChessboardGeometry {
         isPromotionSquareAtTop
             ? square
             : Square.fromCoords(square.file, orientation == Side.white ? Rank.fourth : Rank.fifth);
-    final pieces =
-        isPromotionSquareAtTop
-            ? [
-              Piece(color: color, role: Role.queen, promoted: true),
-              Piece(color: color, role: Role.knight, promoted: true),
-              Piece(color: color, role: Role.rook, promoted: true),
-              Piece(color: color, role: Role.bishop, promoted: true),
-            ]
-            : [
-              Piece(color: color, role: Role.bishop, promoted: true),
-              Piece(color: color, role: Role.rook, promoted: true),
-              Piece(color: color, role: Role.knight, promoted: true),
-              Piece(color: color, role: Role.queen, promoted: true),
-            ];
+
+    final topRoles = [
+      Role.queen,
+      Role.knight,
+      Role.rook,
+      Role.bishop,
+      if (canPromoteToKing) Role.king,
+    ];
+    final roles = isPromotionSquareAtTop ? topRoles : topRoles.reversed.toList(growable: false);
+    final pieces = roles
+        .map((role) => Piece(color: color, role: role, promoted: true))
+        .toList(growable: false);
 
     final offset = squareOffset(anchorSquare);
 
@@ -87,7 +89,7 @@ class PromotionSelector extends StatelessWidget with ChessboardGeometry {
           children: [
             Positioned(
               width: squareSize,
-              height: squareSize * 4,
+              height: squareSize * pieces.length,
               left: offset.dx,
               top: offset.dy,
               child: Column(

test/widgets/board_test.dart

@@ -930,6 +930,100 @@ void main() {
       expect(find.byKey(const Key('a2-blackpawn')), findsNothing);
     });
 
+    testWidgets('default promotion shows 4 pieces without king', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const _TestApp(
+          initialPlayerSide: PlayerSide.both,
+          initialFen: '8/5P2/2RK2P1/8/4k3/8/8/7r w - - 0 1',
+        ),
+      );
+
+      await tester.tap(find.byKey(const Key('f7-whitepawn')));
+      await tester.pump();
+      await tester.tapAt(squareOffset(tester, Square.f8));
+      await tester.pump();
+
+      // wait for promotion selector to show
+      await tester.pump();
+      expect(find.byType(PromotionSelector), findsOneWidget);
+
+      // Find all PieceWidget instances within the PromotionSelector
+      final promotionSelector = find.byType(PromotionSelector);
+      final piecesInSelector = find.descendant(
+        of: promotionSelector,
+        matching: find.byType(PieceWidget),
+      );
+
+      // Should have exactly 4 pieces (queen, knight, rook, bishop)
+      expect(piecesInSelector, findsNWidgets(4));
+
+      // Verify no king piece is present
+      final pieceWidgets = tester.widgetList<PieceWidget>(piecesInSelector);
+      final hasKing = pieceWidgets.any((widget) => widget.piece.role == Role.king);
+      expect(hasKing, false);
+    });
+    testWidgets('promotion with canPromoteToKing shows 5 pieces including king', (
+      WidgetTester tester,
+    ) async {
+      await tester.pumpWidget(
+        const _TestApp(
+          initialPlayerSide: PlayerSide.both,
+          initialFen: '8/5P2/2RK2P1/8/4k3/8/8/7r w - - 0 1',
+          canPromoteToKing: true,
+        ),
+      );
+
+      await tester.tap(find.byKey(const Key('f7-whitepawn')));
+      await tester.pump();
+      await tester.tapAt(squareOffset(tester, Square.f8));
+      await tester.pump();
+
+      // wait for promotion selector to show
+      await tester.pump();
+      expect(find.byType(PromotionSelector), findsOneWidget);
+
+      // Find all PieceWidget instances within the PromotionSelector
+      final promotionSelector = find.byType(PromotionSelector);
+      final piecesInSelector = find.descendant(
+        of: promotionSelector,
+        matching: find.byType(PieceWidget),
+      );
+
+      // has exactly 5 pieces (queen, knight, rook, bishop, king)
+      expect(piecesInSelector, findsNWidgets(5));
+
+      // Verify king piece is present
+      final pieceWidgets = tester.widgetList<PieceWidget>(piecesInSelector);
+      final hasKing = pieceWidgets.any((widget) => widget.piece.role == Role.king);
+      expect(hasKing, true);
+    });
+
+    testWidgets('can promote to king when enabled', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const _TestApp(
+          initialPlayerSide: PlayerSide.both,
+          initialFen: '8/5P2/2RK2P1/8/4k3/8/8/7r w - - 0 1',
+          canPromoteToKing: true,
+        ),
+      );
+
+      await tester.tap(find.byKey(const Key('f7-whitepawn')));
+      await tester.pump();
+      await tester.tapAt(squareOffset(tester, Square.f8));
+      await tester.pump();
+
+      // wait for promotion selector to show
+      await tester.pump();
+      expect(find.byType(PromotionSelector), findsOneWidget);
+
+      // tap on the king is the last option in the promotion selector
+      await tester.tapAt(squareOffset(tester, Square.f4));
+      await tester.pump();
+
+      expect(find.byKey(const Key('f8-whiteking')), findsOneWidget);
+      expect(find.byKey(const Key('f7-whitepawn')), findsNothing);
+    });
+
     testWidgets('cancels promotion', (WidgetTester tester) async {
       await tester.pumpWidget(
         const _TestApp(
@@ -1884,6 +1978,7 @@ class _TestApp extends StatefulWidget {
     this.gameEventStream,
     this.onTouchedSquare,
     this.bottomWidget,
+    this.canPromoteToKing = false,
     super.key,
   });
 
@@ -1907,6 +2002,8 @@ class _TestApp extends StatefulWidget {
 
   final Widget? bottomWidget;
 
+  final bool canPromoteToKing;
+
   @override
   State<_TestApp> createState() => _TestAppState();
 }
@@ -2075,6 +2172,7 @@ class _TestAppState extends State<_TestApp> {
                     });
                   },
                 ),
+                canPromoteToKing: widget.canPromoteToKing,
               ),
               onTouchedSquare: widget.onTouchedSquare,
               shapes: shapes,