Expose the alignment property for DropdownButton and item (#81282)

packages/flutter/lib/src/material/dropdown.dart

@@ -668,6 +668,7 @@ class _DropdownMenuItemContainer extends StatelessWidget {
   /// The [child] argument is required.
   const _DropdownMenuItemContainer({
     Key? key,
+    this.alignment = AlignmentDirectional.centerStart,
     required this.child,
   }) : assert(child != null),
        super(key: key);
@@ -677,11 +678,23 @@ class _DropdownMenuItemContainer extends StatelessWidget {
   /// Typically a [Text] widget.
   final Widget child;
 
+  /// Defines how the item is positioned within the container.
+  ///
+  /// This property must not be null. It defaults to [AlignmentDirectional.centerStart].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
+  final AlignmentGeometry alignment;
+
   @override
   Widget build(BuildContext context) {
     return Container(
       constraints: const BoxConstraints(minHeight: _kMenuItemHeight),
-      alignment: AlignmentDirectional.centerStart,
+      alignment: alignment,
       child: child,
     );
   }
@@ -700,9 +713,10 @@ class DropdownMenuItem<T> extends _DropdownMenuItemContainer {
     this.onTap,
     this.value,
     this.enabled = true,
+    AlignmentGeometry alignment = AlignmentDirectional.centerStart,
     required Widget child,
   }) : assert(child != null),
-       super(key: key, child: child);
+       super(key: key, alignment:alignment, child: child);
 
   /// Called when the dropdown menu item is tapped.
   final VoidCallback? onTap;
@@ -866,6 +880,7 @@ class DropdownButton<T> extends StatefulWidget {
     this.dropdownColor,
     this.menuMaxHeight,
     this.enableFeedback,
+    this.alignment = AlignmentDirectional.centerStart,
     // When adding new arguments, consider adding similar arguments to
     // DropdownButtonFormField.
   }) : assert(items == null || items.isEmpty || value == null ||
@@ -1138,6 +1153,18 @@ class DropdownButton<T> extends StatefulWidget {
   ///  * [Feedback] for providing platform-specific feedback to certain actions.
   final bool? enableFeedback;
 
+  /// Defines how the hint or the selected item is positioned within the button.
+  ///
+  /// This property must not be null. It defaults to [AlignmentDirectional.centerStart].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
+  final AlignmentGeometry alignment;
+
   @override
   _DropdownButtonState<T> createState() => _DropdownButtonState<T>();
 }
@@ -1407,7 +1434,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
     } else {
       innerItemsWidget = IndexedStack(
         index: _selectedIndex ?? hintIndex,
-        alignment: AlignmentDirectional.centerStart,
+        alignment: widget.alignment,
         children: widget.isDense ? items : items.map((Widget item) {
           return widget.itemHeight != null
             ? SizedBox(height: widget.itemHeight, child: item)
@@ -1537,6 +1564,8 @@ class DropdownButtonFormField<T> extends FormField<T> {
     bool autovalidate = false,
     AutovalidateMode? autovalidateMode,
     double? menuMaxHeight,
+    bool? enableFeedback,
+    AlignmentGeometry alignment = AlignmentDirectional.centerStart,
   }) : assert(items == null || items.isEmpty || value == null ||
               items.where((DropdownMenuItem<T> item) {
                 return item.value == value;
@@ -1606,6 +1635,8 @@ class DropdownButtonFormField<T> extends FormField<T> {
                      autofocus: autofocus,
                      dropdownColor: dropdownColor,
                      menuMaxHeight: menuMaxHeight,
+                     enableFeedback: enableFeedback,
+                     alignment: alignment,
                    ),
                  ),
                );

packages/flutter/test/material/dropdown_form_field_test.dart

@@ -43,6 +43,7 @@ Widget buildFormFrame({
   List<String>? items = menuItems,
   Alignment alignment = Alignment.center,
   TextDirection textDirection = TextDirection.ltr,
+  AlignmentGeometry buttonAlignment = AlignmentDirectional.centerStart,
 }) {
   return TestApp(
     textDirection: textDirection,
@@ -72,6 +73,7 @@ Widget buildFormFrame({
                 child: Text(item, key: ValueKey<String>(item + 'Text')),
               );
             }).toList(),
+            alignment: buttonAlignment,
           ),
         ),
       ),
@@ -815,4 +817,20 @@ void main() {
     expect(() => builder(), throwsAssertionError);
   });
 
+  testWidgets('DropdownButtonFormField - Custom button alignment', (WidgetTester tester) async {
+    await tester.pumpWidget(buildFormFrame(
+      buttonAlignment: AlignmentDirectional.center,
+      items: <String>['one'],
+      value: 'one',
+    ));
+
+    final RenderBox buttonBox = tester.renderObject<RenderBox>(find.byType(IndexedStack));
+    final RenderBox selectedItemBox = tester.renderObject(find.text('one'));
+
+    // Should be center-center aligned.
+    expect(
+      buttonBox.localToGlobal(Offset(buttonBox.size.width / 2.0, buttonBox.size.height / 2.0)),
+      selectedItemBox.localToGlobal(Offset(selectedItemBox.size.width / 2.0, selectedItemBox.size.height / 2.0)),
+    );
+  });
 }

packages/flutter/test/material/dropdown_test.dart

@@ -3253,6 +3253,77 @@ void main() {
     expect(Focus.maybeOf(disabledItem), null, reason: 'Disabled menu item should not be able to request focus');
   });
 
+  testWidgets('alignment test', (WidgetTester tester) async {
+    final Key buttonKey = UniqueKey();
+    Widget buildFrame({AlignmentGeometry? buttonAlignment, AlignmentGeometry? menuAlignment}) {
+      return MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: DropdownButton<String>(
+              key: buttonKey,
+              alignment: buttonAlignment ?? AlignmentDirectional.centerStart,
+              value: 'enabled',
+              onChanged: onChanged,
+              items: <DropdownMenuItem<String>>[
+                DropdownMenuItem<String>(
+                  alignment: buttonAlignment ?? AlignmentDirectional.centerStart,
+                  enabled: false,
+                  child: const Text('disabled'),
+                ),
+                DropdownMenuItem<String>(
+                  alignment: buttonAlignment ?? AlignmentDirectional.centerStart,
+                  value: 'enabled',
+                  child: const Text('enabled'),
+                )
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame());
+
+    final RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
+    RenderBox selectedItemBox = tester.renderObject(find.text('enabled'));
+    // Default to center-start aligned.
+    expect(
+      buttonBox.localToGlobal(Offset(0.0, buttonBox.size.height / 2.0)),
+      selectedItemBox.localToGlobal(Offset(0.0, selectedItemBox.size.height / 2.0)),
+    );
+
+    await tester.pumpWidget(buildFrame(
+      buttonAlignment: AlignmentDirectional.center,
+      menuAlignment: AlignmentDirectional.center,
+    ));
+
+    selectedItemBox = tester.renderObject(find.text('enabled'));
+    // Should be center-center aligned, the icon size is 24.0 pixels.
+    expect(
+      buttonBox.localToGlobal(Offset((buttonBox.size.width -24.0) / 2.0, buttonBox.size.height / 2.0)),
+      selectedItemBox.localToGlobal(Offset(selectedItemBox.size.width / 2.0, selectedItemBox.size.height / 2.0)),
+    );
+
+    // Open dropdown.
+    await tester.tap(find.text('enabled').hitTestable());
+    await tester.pumpAndSettle();
+
+    final RenderBox selectedItemBoxInMenu = tester.renderObjectList<RenderBox>(find.text('enabled')).toList()[1];
+    final Finder menu = find.byWidgetPredicate((Widget widget) {
+      return widget.runtimeType.toString().startsWith('_DropdownMenu<');
+    });
+    final Rect menuRect = tester.getRect(menu);
+    final Offset center = selectedItemBoxInMenu.localToGlobal(
+      Offset(selectedItemBoxInMenu.size.width / 2.0, selectedItemBoxInMenu.size.height / 2.0)
+    );
+
+    expect(center.dx, menuRect.topCenter.dx,);
+    expect(
+      center.dy,
+      selectedItemBox.localToGlobal(Offset(selectedItemBox.size.width / 2.0, selectedItemBox.size.height / 2.0)).dy,
+    );
+  });
+
   group('feedback', () {
     late FeedbackTester feedback;