Add disabledHint to DropdownButton (#18770)

If `items` or `onChanged` is null the button will be disabled, the down arrow will be grayed out, and
the new `disabledHint` will be shown (if provided).

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

@@ -493,6 +493,8 @@ class DropdownButton<T> extends StatefulWidget {
   /// Creates a dropdown button.
   ///
   /// The [items] must have distinct values and if [value] isn't null it must be among them.
+  /// If [items] or [onChanged] is null, the button will be disabled, the down arrow will be grayed out, and
+  /// the [disabledHint] will be shown (if provided).
   ///
   /// The [elevation] and [iconSize] arguments must not be null (they both have
   /// defaults, so do not need to be specified).
@@ -501,14 +503,14 @@ class DropdownButton<T> extends StatefulWidget {
     @required this.items,
     this.value,
     this.hint,
+    this.disabledHint,
     @required this.onChanged,
     this.elevation = 8,
     this.style,
     this.iconSize = 24.0,
     this.isDense = false,
     this.isExpanded = false,
-  }) : assert(items != null),
+  }) : assert(items == null || value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1),
-       assert(value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1),
       super(key: key);
 
   /// The list of possible items to select among.
@@ -522,6 +524,11 @@ class DropdownButton<T> extends StatefulWidget {
   /// Displayed if [value] is null.
   final Widget hint;
 
+  /// A message to show when the dropdown is disabled.
+  ///
+  /// Displayed if [items] or [onChanged] is null.
+  final Widget disabledHint;
+
   /// Called when the user selects an item.
   final ValueChanged<T> onChanged;
 
@@ -600,6 +607,10 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
   }
 
   void _updateSelectedIndex() {
+    if (!_enabled) {
+      return;
+    }
+
     assert(widget.value == null ||
       widget.items.where((DropdownMenuItem<T> item) => item.value == widget.value).length == 1);
     _selectedIndex = null;
@@ -650,6 +661,25 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
     return math.max(_textStyle.fontSize, math.max(widget.iconSize, _kDenseButtonHeight));
   }
 
+  Color get _downArrowColor {
+    // These colors are not defined in the Material Design spec.
+    if (_enabled) {
+      if (Theme.of(context).brightness == Brightness.light) {
+        return Colors.grey.shade700;
+      } else {
+        return Colors.white70;
+      }
+    } else {
+      if (Theme.of(context).brightness == Brightness.light) {
+        return Colors.grey.shade400;
+      } else {
+        return Colors.white10;
+      }
+    }
+  }
+
+  bool get _enabled => widget.items != null && widget.items.isNotEmpty && widget.onChanged != null;
+
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterial(context));
@@ -657,14 +687,16 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
 
     // The width of the button and the menu are defined by the widest
     // item and the width of the hint.
-    final List<Widget> items = List<Widget>.from(widget.items);
+    final List<Widget> items = _enabled ? List<Widget>.from(widget.items) : <Widget>[];
     int hintIndex;
-    if (widget.hint != null) {
+    if (widget.hint != null || (!_enabled && widget.disabledHint != null)) {
+      final Widget emplacedHint =
+        _enabled ? widget.hint : DropdownMenuItem<Widget>(child: widget.disabledHint ?? widget.hint);
       hintIndex = items.length;
       items.add(DefaultTextStyle(
         style: _textStyle.copyWith(color: Theme.of(context).hintColor),
         child: IgnorePointer(
-          child: widget.hint,
+          child: emplacedHint,
           ignoringSemantics: false,
         ),
       ));
@@ -674,10 +706,10 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
       ? _kAlignedButtonPadding
       : _kUnalignedButtonPadding;
 
-    // If value is null (then _selectedIndex is null) then we display
+    // If value is null (then _selectedIndex is null) or if disabled then we
-    // the hint or nothing at all.
+    // display the hint or nothing at all.
     final IndexedStack innerItemsWidget = IndexedStack(
-      index: _selectedIndex ?? hintIndex,
+      index: _enabled ? (_selectedIndex ?? hintIndex) : hintIndex,
       alignment: AlignmentDirectional.centerStart,
       children: items,
     );
@@ -694,8 +726,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
             widget.isExpanded ? Expanded(child: innerItemsWidget) : innerItemsWidget,
             Icon(Icons.arrow_drop_down,
               size: widget.iconSize,
-              // These colors are not defined in the Material Design spec.
+              color: _downArrowColor,
-              color: Theme.of(context).brightness == Brightness.light ? Colors.grey.shade700 : Colors.white70
             ),
           ],
         ),
@@ -725,7 +756,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
     return Semantics(
       button: true,
       child: GestureDetector(
-        onTap: _handleTap,
+        onTap: _enabled ? _handleTap : null,
         behavior: HitTestBehavior.opaque,
         child: result
       ),

packages/flutter/test/material/dropdown_test.dart

@@ -13,6 +13,7 @@ import 'package:flutter/rendering.dart';
 import '../widgets/semantics_tester.dart';
 
 const List<String> menuItems = <String>['one', 'two', 'three', 'four'];
+final ValueChanged<String> onChanged = (_) {};
 
 final Type dropdownButtonType = DropdownButton<String>(
   onChanged: (_) { },
@@ -26,6 +27,7 @@ Widget buildFrame({
   bool isDense = false,
   bool isExpanded = false,
   Widget hint,
+  Widget disabledHint,
   List<String> items = menuItems,
   Alignment alignment = Alignment.center,
   TextDirection textDirection = TextDirection.ltr,
@@ -40,10 +42,11 @@ Widget buildFrame({
             key: buttonKey,
             value: value,
             hint: hint,
+            disabledHint: disabledHint,
             onChanged: onChanged,
             isDense: isDense,
             isExpanded: isExpanded,
-            items: items.map<DropdownMenuItem<String>>((String item) {
+            items: items == null ? null : items.map<DropdownMenuItem<String>>((String item) {
               return DropdownMenuItem<String>(
                 key: ValueKey<String>(item),
                 value: item,
@@ -115,7 +118,7 @@ bool sameGeometry(RenderBox box1, RenderBox box2) {
 void main() {
   testWidgets('Default dropdown golden', (WidgetTester tester) async {
     final Key buttonKey = UniqueKey();
-    Widget build() => buildFrame(buttonKey: buttonKey, value: 'two');
+    Widget build() => buildFrame(buttonKey: buttonKey, value: 'two', onChanged: onChanged);
     await tester.pumpWidget(build());
     final Finder buttonFinder = find.byKey(buttonKey);
     assert(tester.renderObject(buttonFinder).attached);
@@ -128,7 +131,7 @@ void main() {
 
   testWidgets('Expanded dropdown golden', (WidgetTester tester) async {
     final Key buttonKey = UniqueKey();
-    Widget build() => buildFrame(buttonKey: buttonKey, value: 'two', isExpanded: true);
+    Widget build() => buildFrame(buttonKey: buttonKey, value: 'two', isExpanded: true, onChanged: onChanged);
     await tester.pumpWidget(build());
     final Finder buttonFinder = find.byKey(buttonKey);
     assert(tester.renderObject(buttonFinder).attached);
@@ -326,7 +329,7 @@ void main() {
       final Key buttonKey = UniqueKey();
       const String value = 'two';
 
-      Widget build() => buildFrame(buttonKey: buttonKey, value: value, textDirection: textDirection);
+      Widget build() => buildFrame(buttonKey: buttonKey, value: value, textDirection: textDirection, onChanged: onChanged);
 
       await tester.pumpWidget(build());
       final RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
@@ -371,7 +374,7 @@ void main() {
   testWidgets('Arrow icon aligns with the edge of button when expanded', (WidgetTester tester) async {
     final Key buttonKey = UniqueKey();
 
-    Widget build() => buildFrame(buttonKey: buttonKey, value: 'two', isExpanded: true);
+    Widget build() => buildFrame(buttonKey: buttonKey, value: 'two', isExpanded: true, onChanged: onChanged);
 
     await tester.pumpWidget(build());
     final RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
@@ -389,7 +392,7 @@ void main() {
     final Key buttonKey = UniqueKey();
     const String value = 'two';
 
-    Widget build() => buildFrame(buttonKey: buttonKey, value: value, isDense: true);
+    Widget build() => buildFrame(buttonKey: buttonKey, value: value, isDense: true, onChanged: onChanged);
 
     await tester.pumpWidget(build());
     final RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
@@ -428,7 +431,8 @@ void main() {
     await tester.pumpWidget(buildFrame(
       buttonKey: buttonKey,
       value: null, // nothing selected
-      items: List<String>.generate(/*length=*/ 100, (int index) => index.toString())
+      items: List<String>.generate(/*length=*/ 100, (int index) => index.toString()),
+      onChanged: onChanged,
     ));
     await tester.tap(find.byKey(buttonKey));
     await tester.pump();
@@ -455,7 +459,8 @@ void main() {
     await tester.pumpWidget(buildFrame(
       buttonKey: buttonKey,
       value: '50',
-      items: List<String>.generate(/*length=*/ 100, (int index) => index.toString())
+      items: List<String>.generate(/*length=*/ 100, (int index) => index.toString()),
+      onChanged: onChanged,
     ));
     final RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
     await tester.tap(find.byKey(buttonKey));
@@ -477,7 +482,7 @@ void main() {
     final Key buttonKey = UniqueKey();
     String value;
 
-    Widget build() => buildFrame(buttonKey: buttonKey, value: value);
+    Widget build() => buildFrame(buttonKey: buttonKey, value: value, onChanged: onChanged);
 
     await tester.pumpWidget(build());
     final RenderBox buttonBoxNullValue = tester.renderObject(find.byKey(buttonKey));
@@ -594,19 +599,19 @@ void main() {
     // so that it fits within the frame.
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.topLeft, value: menuItems.last)
+      buildFrame(alignment: Alignment.topLeft, value: menuItems.last, onChanged: onChanged)
     );
     expect(menuRect.topLeft, Offset.zero);
     expect(menuRect.topRight, Offset(menuRect.width, 0.0));
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.topCenter, value: menuItems.last)
+      buildFrame(alignment: Alignment.topCenter, value: menuItems.last, onChanged: onChanged)
     );
     expect(menuRect.topLeft, Offset(buttonRect.left, 0.0));
     expect(menuRect.topRight, Offset(buttonRect.right, 0.0));
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.topRight, value: menuItems.last)
+      buildFrame(alignment: Alignment.topRight, value: menuItems.last, onChanged: onChanged)
     );
     expect(menuRect.topLeft, Offset(800.0 - menuRect.width, 0.0));
     expect(menuRect.topRight, const Offset(800.0, 0.0));
@@ -616,19 +621,19 @@ void main() {
     // is selected) and shifted horizontally so that it fits within the frame.
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.centerLeft, value: menuItems.first)
+      buildFrame(alignment: Alignment.centerLeft, value: menuItems.first, onChanged: onChanged)
     );
     expect(menuRect.topLeft, Offset(0.0, buttonRect.top));
     expect(menuRect.topRight, Offset(menuRect.width, buttonRect.top));
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.center, value: menuItems.first)
+      buildFrame(alignment: Alignment.center, value: menuItems.first, onChanged: onChanged)
     );
     expect(menuRect.topLeft, buttonRect.topLeft);
     expect(menuRect.topRight, buttonRect.topRight);
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.centerRight, value: menuItems.first)
+      buildFrame(alignment: Alignment.centerRight, value: menuItems.first, onChanged: onChanged)
     );
     expect(menuRect.topLeft, Offset(800.0 - menuRect.width, buttonRect.top));
     expect(menuRect.topRight, Offset(800.0, buttonRect.top));
@@ -638,26 +643,26 @@ void main() {
     // so that it fits within the frame.
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.bottomLeft, value: menuItems.first)
+      buildFrame(alignment: Alignment.bottomLeft, value: menuItems.first, onChanged: onChanged)
     );
     expect(menuRect.bottomLeft, const Offset(0.0, 600.0));
     expect(menuRect.bottomRight, Offset(menuRect.width, 600.0));
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.bottomCenter, value: menuItems.first)
+      buildFrame(alignment: Alignment.bottomCenter, value: menuItems.first, onChanged: onChanged)
     );
     expect(menuRect.bottomLeft, Offset(buttonRect.left, 600.0));
     expect(menuRect.bottomRight, Offset(buttonRect.right, 600.0));
 
     await popUpAndDown(
-      buildFrame(alignment: Alignment.bottomRight, value: menuItems.first)
+      buildFrame(alignment: Alignment.bottomRight, value: menuItems.first, onChanged: onChanged)
     );
     expect(menuRect.bottomLeft, Offset(800.0 - menuRect.width, 600.0));
     expect(menuRect.bottomRight, const Offset(800.0, 600.0));
   });
 
   testWidgets('Dropdown menus are dismissed on screen orientation changes', (WidgetTester tester) async {
-    await tester.pumpWidget(buildFrame());
+    await tester.pumpWidget(buildFrame(onChanged: onChanged));
     await tester.tap(find.byType(dropdownButtonType));
     await tester.pumpAndSettle();
     expect(find.byType(ListView), findsOneWidget);
@@ -670,7 +675,7 @@ void main() {
 
   testWidgets('Semantics Tree contains only selected element', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);
-    await tester.pumpWidget(buildFrame(items: menuItems));
+    await tester.pumpWidget(buildFrame(items: menuItems, onChanged: onChanged));
 
     expect(semantics, isNot(includesNodeWith(label: menuItems[0])));
     expect(semantics, includesNodeWith(label: menuItems[1]));
@@ -702,7 +707,7 @@ void main() {
       buttonKey: key,
       value: 'three',
       items: menuItems,
-      onChanged: null,
+      onChanged: onChanged,
       hint: const Text('test'),
     ));
 
@@ -722,6 +727,7 @@ void main() {
       buttonKey: key,
       value: null,
       items: menuItems,
+      onChanged: onChanged,
     ));
     await tester.tap(find.byKey(key));
     await tester.pumpAndSettle();
@@ -778,6 +784,42 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('disabledHint displays on empty items or onChanged', (WidgetTester tester) async {
+    final Key buttonKey = UniqueKey();
+
+    Widget build({List<String> items, ValueChanged<String> onChanged}) => buildFrame(
+      items: items,
+      onChanged: onChanged,
+      buttonKey: buttonKey, value: null,
+      hint: const Text('enabled'),
+      disabledHint: const Text('disabled'));
+
+    // [disabledHint] should display when [items] is null
+    await tester.pumpWidget(build(items: null, onChanged: onChanged));
+    expect(find.text('enabled'), findsNothing);
+    expect(find.text('disabled'), findsOneWidget);
+
+    // [disabledHint] should display when [items] is an empty list.
+    await tester.pumpWidget(build(items: <String>[], onChanged: onChanged));
+    expect(find.text('enabled'), findsNothing);
+    expect(find.text('disabled'), findsOneWidget);
+
+    // [disabledHint] should display when [onChanged] is null
+    await tester.pumpWidget(build(items: menuItems, onChanged: null));
+    expect(find.text('enabled'), findsNothing);
+    expect(find.text('disabled'), findsOneWidget);
+    final RenderBox disabledHintBox = tester.renderObject(find.byKey(buttonKey));
+
+    // A Dropdown button with a disabled hint should be the same size as a
+    // one with a regular enabled hint.
+    await tester.pumpWidget(build(items: menuItems, onChanged: onChanged));
+    expect(find.text('disabled'), findsNothing);
+    expect(find.text('enabled'), findsOneWidget);
+    final RenderBox enabledHintBox = tester.renderObject(find.byKey(buttonKey));
+    expect(enabledHintBox.localToGlobal(Offset.zero), equals(disabledHintBox.localToGlobal(Offset.zero)));
+    expect(enabledHintBox.size, equals(disabledHintBox.size));
+  });
+
   testWidgets('Dropdown in middle showing middle item', (WidgetTester tester) async {
     final List<DropdownMenuItem<int>> items =
       List<DropdownMenuItem<int>>.generate(100, (int i) =>