Enable aligning a dropdown button's width with its menu's width (#14849)

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

@@ -64,16 +64,19 @@ class ButtonTheme extends InheritedWidget {
     double height: 36.0,
     EdgeInsetsGeometry padding,
     ShapeBorder shape,
+    bool alignedDropdown: false,
     Widget child,
   }) : assert(textTheme != null),
        assert(minWidth != null && minWidth >= 0.0),
        assert(height != null && height >= 0.0),
+       assert(alignedDropdown != null),
        data = new ButtonThemeData(
          textTheme: textTheme,
          minWidth: minWidth,
          height: height,
          padding: padding,
          shape: shape,
+         alignedDropdown: alignedDropdown
        ),
        super(key: key, child: child);
 
@@ -98,16 +101,19 @@ class ButtonTheme extends InheritedWidget {
     double height: 36.0,
     EdgeInsetsGeometry padding: const EdgeInsets.symmetric(horizontal: 8.0),
     ShapeBorder shape,
+    bool alignedDropdown: false,
     Widget child,
   }) : assert(textTheme != null),
        assert(minWidth != null && minWidth >= 0.0),
        assert(height != null && height >= 0.0),
+       assert(alignedDropdown != null),
        data = new ButtonThemeData(
          textTheme: textTheme,
          minWidth: minWidth,
          height: height,
          padding: padding,
          shape: shape,
+         alignedDropdown: alignedDropdown,
        ),
        super(key: key, child: child);
 
@@ -146,9 +152,11 @@ class ButtonThemeData extends Diagnosticable {
     this.height: 36.0,
     EdgeInsetsGeometry padding,
     ShapeBorder shape,
+    this.alignedDropdown: false,
   }) : assert(textTheme != null),
        assert(minWidth != null && minWidth >= 0.0),
        assert(height != null && height >= 0.0),
+       assert(alignedDropdown != null),
        _padding = padding,
        _shape = shape;
 
@@ -229,6 +237,17 @@ class ButtonThemeData extends Diagnosticable {
   }
   final ShapeBorder _shape;
 
+  /// If true, then a [DropdownButton] menu's width will match the button's
+  /// width.
+  ///
+  /// If false (the default), then the dropdown's menu will be wider than
+  /// its button. In either case the dropdown button will line up the leading
+  /// edge of the menu's value with the leading edge of the values
+  /// displayed by the menu items.
+  ///
+  /// This property only affects [DropdownButton] and its menu.
+  final bool alignedDropdown;
+
   @override
   bool operator ==(dynamic other) {
     if (other.runtimeType != runtimeType)
@@ -238,7 +257,8 @@ class ButtonThemeData extends Diagnosticable {
         && minWidth == typedOther.minWidth
         && height == typedOther.height
         && padding == typedOther.padding
-        && shape == typedOther.shape;
+        && shape == typedOther.shape
+        && alignedDropdown == typedOther.alignedDropdown;
   }
 
   @override
@@ -249,6 +269,7 @@ class ButtonThemeData extends Diagnosticable {
       height,
       padding,
       shape,
+      alignedDropdown,
     );
   }
 
@@ -256,13 +277,15 @@ class ButtonThemeData extends Diagnosticable {
   void debugFillProperties(DiagnosticPropertiesBuilder description) {
     super.debugFillProperties(description);
     final ButtonThemeData defaultTheme = const ButtonThemeData();
-    description.add(new EnumProperty<ButtonTextTheme>('textTheme', textTheme,
+    description.add(new EnumProperty<ButtonTextTheme>('textTheme', textTheme, defaultValue: defaultTheme.textTheme));
-        defaultValue: defaultTheme.textTheme));
     description.add(new DoubleProperty('minWidth', minWidth, defaultValue: defaultTheme.minWidth));
     description.add(new DoubleProperty('height', height, defaultValue: defaultTheme.height));
-    description.add(new DiagnosticsProperty<EdgeInsetsGeometry>('padding', padding,
+    description.add(new DiagnosticsProperty<EdgeInsetsGeometry>('padding', padding, defaultValue: defaultTheme.padding));
-        defaultValue: defaultTheme.padding));
+    description.add(new DiagnosticsProperty<ShapeBorder>('shape', shape, defaultValue: defaultTheme.shape));
-    description.add(
+    description.add(new FlagProperty('alignedDropdown',
-        new DiagnosticsProperty<ShapeBorder>('shape', shape, defaultValue: defaultTheme.shape));
+      value: alignedDropdown,
+      defaultValue: defaultTheme.alignedDropdown,
+      ifTrue: 'dropdown width matches button',
+    ));
   }
 }

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

@@ -7,6 +7,7 @@ import 'dart:math' as math;
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
+import 'button_theme.dart';
 import 'colors.dart';
 import 'constants.dart';
 import 'debug.dart';
@@ -21,7 +22,11 @@ import 'theme.dart';
 const Duration _kDropdownMenuDuration = const Duration(milliseconds: 300);
 const double _kMenuItemHeight = 48.0;
 const double _kDenseButtonHeight = 24.0;
-const EdgeInsets _kMenuHorizontalPadding = const EdgeInsets.symmetric(horizontal: 16.0);
+const EdgeInsets _kMenuItemPadding = const EdgeInsets.symmetric(horizontal: 16.0);
+const EdgeInsetsGeometry _kAlignedButtonPadding = const EdgeInsetsDirectional.only(start: 16.0, end: 4.0);
+const EdgeInsets _kUnalignedButtonPadding = EdgeInsets.zero;
+const EdgeInsets _kAlignedMenuMargin = EdgeInsets.zero;
+const EdgeInsetsGeometry _kUnalignedMenuMargin = const EdgeInsetsDirectional.only(start: 16.0, end: 24.0);
 
 class _DropdownMenuPainter extends CustomPainter {
   _DropdownMenuPainter({
@@ -91,10 +96,12 @@ class _DropdownScrollBehavior extends ScrollBehavior {
 class _DropdownMenu<T> extends StatefulWidget {
   const _DropdownMenu({
     Key key,
+    this.padding,
     this.route,
   }) : super(key: key);
 
   final _DropdownRoute<T> route;
+  final EdgeInsets padding;
 
   @override
   _DropdownMenuState<T> createState() => new _DropdownMenuState<T>();
@@ -149,7 +156,7 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
         opacity: opacity,
         child: new InkWell(
           child: new Container(
-            padding: _kMenuHorizontalPadding,
+            padding: widget.padding,
             child: route.items[itemIndex],
           ),
           onTap: () => Navigator.pop(
@@ -212,7 +219,7 @@ class _DropdownMenuRouteLayout<T> extends SingleChildLayoutDelegate {
     final double maxHeight = math.max(0.0, constraints.maxHeight - 2 * _kMenuItemHeight);
     // The width of a menu should be at most the view width. This ensures that
     // the menu does not extend past the left and right edges of the screen.
-    final double width = math.min(constraints.maxWidth, buttonRect.width + 8.0);
+    final double width = math.min(constraints.maxWidth, buttonRect.width);
     return new BoxConstraints(
       minWidth: width,
       maxWidth: width,
@@ -238,7 +245,7 @@ class _DropdownMenuRouteLayout<T> extends SingleChildLayoutDelegate {
     double left;
     switch (textDirection) {
       case TextDirection.rtl:
-        left = buttonRect.right.clamp(0.0, size.width - childSize.width) - childSize.width;
+      left = buttonRect.right.clamp(0.0, size.width) - childSize.width;
         break;
       case TextDirection.ltr:
         left = buttonRect.left.clamp(0.0, size.width - childSize.width);
@@ -279,6 +286,7 @@ class _DropdownRouteResult<T> {
 class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
   _DropdownRoute({
     this.items,
+    this.padding,
     this.buttonRect,
     this.selectedIndex,
     this.elevation: 8,
@@ -288,6 +296,7 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
   }) : assert(style != null);
 
   final List<DropdownMenuItem<T>> items;
+  final EdgeInsetsGeometry padding;
   final Rect buttonRect;
   final int selectedIndex;
   final int elevation;
@@ -336,7 +345,12 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
       scrollController = new ScrollController(initialScrollOffset: scrollOffset);
     }
 
-    Widget menu = new _DropdownMenu<T>(route: this);
+    final TextDirection textDirection = Directionality.of(context);
+    Widget menu = new _DropdownMenu<T>(
+      route: this,
+      padding: padding.resolve(textDirection),
+    );
+
     if (theme != null)
       menu = new Theme(data: theme, child: menu);
 
@@ -353,7 +367,7 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
               buttonRect: buttonRect,
               menuTop: menuTop,
               menuHeight: menuHeight,
-              textDirection: Directionality.of(context),
+              textDirection: textDirection,
             ),
             child: menu,
           );
@@ -566,11 +580,16 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
   void _handleTap() {
     final RenderBox itemBox = context.findRenderObject();
     final Rect itemRect = itemBox.localToGlobal(Offset.zero) & itemBox.size;
+    final TextDirection textDirection = Directionality.of(context);
+    final EdgeInsetsGeometry menuMargin = ButtonTheme.of(context).alignedDropdown
+      ?_kAlignedMenuMargin
+      : _kUnalignedMenuMargin;
 
     assert(_dropdownRoute == null);
     _dropdownRoute = new _DropdownRoute<T>(
       items: widget.items,
-      buttonRect: _kMenuHorizontalPadding.inflateRect(itemRect),
+      buttonRect: menuMargin.resolve(textDirection).inflateRect(itemRect),
+      padding: _kMenuItemPadding.resolve(textDirection),
       selectedIndex: _selectedIndex ?? 0,
       elevation: widget.elevation,
       theme: Theme.of(context, shadowThemeOnly: true),
@@ -613,9 +632,14 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
       ));
     }
 
+    final EdgeInsetsGeometry padding = ButtonTheme.of(context).alignedDropdown
+      ? _kAlignedButtonPadding
+      : _kUnalignedButtonPadding;
+
     Widget result = new DefaultTextStyle(
       style: _textStyle,
-      child: new SizedBox(
+      child: new Container(
+        padding: padding.resolve(Directionality.of(context)),
         height: widget.isDense ? _denseButtonHeight : null,
         child: new Row(
           mainAxisAlignment: MainAxisAlignment.spaceBetween,

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

@@ -491,7 +491,7 @@ class ThemeData extends Diagnosticable {
     Color unselectedWidgetColor,
     Color disabledColor,
     Color buttonColor,
-    Color buttonTheme,
+    ButtonThemeData buttonTheme,
     Color secondaryHeaderColor,
     Color textSelectionColor,
     Color textSelectionHandleColor,

packages/flutter/test/material/button_theme_test.dart

@@ -14,6 +14,7 @@ void main() {
     expect(theme.shape, const RoundedRectangleBorder(
       borderRadius: const BorderRadius.all(const Radius.circular(2.0)),
     ));
+    expect(theme.alignedDropdown, false);
   });
 
   test('ButtonThemeData default overrides', () {
@@ -23,11 +24,13 @@ void main() {
       height: 200.0,
       padding: EdgeInsets.zero,
       shape: const RoundedRectangleBorder(),
+      alignedDropdown: true,
     );
     expect(theme.textTheme, ButtonTextTheme.primary);
     expect(theme.constraints, const BoxConstraints(minWidth: 100.0, minHeight: 200.0));
     expect(theme.padding, EdgeInsets.zero);
     expect(theme.shape, const RoundedRectangleBorder());
+    expect(theme.alignedDropdown, true);
   });
 
   testWidgets('ButtonTheme defaults', (WidgetTester tester) async {
@@ -173,4 +176,109 @@ void main() {
     expect(tester.widget<Material>(find.byType(Material)).color, const Color(0xFF00FF00));
     expect(tester.getSize(find.byType(Material)), const Size(100.0, 200.0));
   });
+
+  testWidgets('ButtonTheme alignedDropdown', (WidgetTester tester) async {
+    final Key dropdownKey = new UniqueKey();
+
+    Widget buildFrame({ bool alignedDropdown, TextDirection textDirection }) {
+      return new MaterialApp(
+        builder: (BuildContext context, Widget child) {
+          return new Directionality(
+            textDirection: textDirection,
+            child: child,
+          );
+        },
+        home: new ButtonTheme(
+          alignedDropdown: alignedDropdown,
+          child: new Material(
+            child: new Builder(
+              builder: (BuildContext context) {
+                return new Container(
+                  alignment: Alignment.center,
+                  child: new DropdownButtonHideUnderline(
+                    child: new Container(
+                      width: 200.0,
+                      child: new DropdownButton<String>(
+                        key: dropdownKey,
+                        onChanged: (String value) { },
+                        value: 'foo',
+                        items: const <DropdownMenuItem<String>>[
+                          const DropdownMenuItem<String>(
+                            value: 'foo',
+                            child: const Text('foo'),
+                          ),
+                          const DropdownMenuItem<String>(
+                            value: 'bar',
+                            child: const Text('bar'),
+                          ),
+                        ],
+                      ),
+                    ),
+                  ),
+                );
+              },
+            ),
+          ),
+        ),
+      );
+    }
+
+    final Finder button = find.byKey(dropdownKey);
+    final Finder menu = find.byWidgetPredicate((Widget w) => '${w.runtimeType}' == '_DropdownMenu<String>');
+
+    await tester.pumpWidget(
+      buildFrame(
+        alignedDropdown: false,
+        textDirection: TextDirection.ltr,
+      ),
+    );
+    await tester.tap(button);
+    await tester.pumpAndSettle();
+
+    // 240 = 200.0 (button width) + _kUnalignedMenuMargin (20.0 left and right)
+    expect(tester.getSize(button).width, 200.0);
+    expect(tester.getSize(menu).width, 240.0);
+
+    // Dismiss the menu.
+    await tester.tapAt(Offset.zero);
+    await tester.pumpAndSettle();
+    expect(menu, findsNothing);
+
+    await tester.pumpWidget(
+      buildFrame(
+        alignedDropdown: true,
+        textDirection: TextDirection.ltr,
+      ),
+    );
+    await tester.tap(button);
+    await tester.pumpAndSettle();
+
+    // Aligneddropdown: true means the button and menu widths match
+    expect(tester.getSize(button).width, 200.0);
+    expect(tester.getSize(menu).width, 200.0);
+
+    // There are two 'foo' widgets: the selected menu item's label and the drop
+    // down button's label. The should both appear at the same location.
+    final Finder fooText = find.text('foo');
+    expect(fooText, findsNWidgets(2));
+    expect(tester.getRect(fooText.at(0)), tester.getRect(fooText.at(1)));
+
+    // Dismiss the menu.
+    await tester.tapAt(Offset.zero);
+    await tester.pumpAndSettle();
+    expect(menu, findsNothing);
+
+    // Same test as above execpt RTL
+    await tester.pumpWidget(
+      buildFrame(
+        alignedDropdown: true,
+        textDirection: TextDirection.rtl,
+      ),
+    );
+    await tester.tap(button);
+    await tester.pumpAndSettle();
+
+    expect(fooText, findsNWidgets(2));
+    expect(tester.getRect(fooText.at(0)), tester.getRect(fooText.at(1)));
+  });
 }