DropdownButton hint (#7114)

examples/flutter_gallery/lib/demo/buttons_demo.dart

@@ -132,7 +132,8 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
 
   // https://en.wikipedia.org/wiki/Free_Four
   String dropdown1Value = 'Free';
-  String dropdown2Value = 'Four';
+  String dropdown2Value;
+  String dropdown3Value = 'Four';
 
   Widget buildDropdownButton() {
     return new Padding(
@@ -141,7 +142,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
         mainAxisAlignment: MainAxisAlignment.start,
         children: <Widget>[
           new ListItem(
-            title: new Text('Scrollable dropdown:'),
+            title: new Text('Simple dropdown:'),
             trailing: new DropdownButton<String>(
               value: dropdown1Value,
               onChanged: (String newValue) {
@@ -149,26 +150,22 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
                   dropdown1Value = newValue;
                 });
               },
-              items: <String>[
-                  'One', 'Two', 'Free', 'Four', 'Can', 'I', 'Have', 'A', 'Little',
-                  'Bit', 'More', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten'
-                 ]
-                .map((String value) {
+              items: <String>['One', 'Two', 'Free', 'Four'].map((String value) {
                 return new DropdownMenuItem<String>(
                   value: value,
                   child: new Text(value),
                 );
-                })
-                .toList(),
+              }).toList(),
             ),
           ),
           new SizedBox(
             height: 24.0,
           ),
           new ListItem(
-            title: new Text('Simple dropdown:'),
+            title: new Text('Dropdown with a hint:'),
             trailing: new DropdownButton<String>(
               value: dropdown2Value,
+              hint: new Text('Choose'),
               onChanged: (String newValue) {
                 setState(() {
                   dropdown2Value = newValue;
@@ -181,7 +178,32 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
                 );
               }).toList(),
             ),
-          )
+          ),
+          new SizedBox(
+            height: 24.0,
+          ),
+          new ListItem(
+            title: new Text('Scrollable dropdown:'),
+            trailing: new DropdownButton<String>(
+              value: dropdown3Value,
+              onChanged: (String newValue) {
+                setState(() {
+                  dropdown3Value = newValue;
+                });
+              },
+              items: <String>[
+                  'One', 'Two', 'Free', 'Four', 'Can', 'I', 'Have', 'A', 'Little',
+                  'Bit', 'More', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten'
+                 ]
+                .map((String value) {
+                  return new DropdownMenuItem<String>(
+                    value: value,
+                    child: new Text(value),
+                  );
+                })
+                .toList(),
+             ),
+          ),
         ],
       ),
     );

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

@@ -420,6 +420,7 @@ class DropdownButton<T> extends StatefulWidget {
     Key key,
     @required this.items,
     this.value,
+    this.hint,
     @required this.onChanged,
     this.elevation: 8,
     this.style,
@@ -439,6 +440,9 @@ class DropdownButton<T> extends StatefulWidget {
   /// selected.
   final T value;
 
+  /// Displayed if [value] is null.
+  final Widget hint;
+
   /// Called when the user selects an item.
   final ValueChanged<T> onChanged;
 
@@ -530,6 +534,21 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> {
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterial(context));
+
+    // The width of the button and the menu are defined by the widest
+    // item and the width of the hint.
+    final List<Widget> items = new List<Widget>.from(config.items);
+    int hintIndex;
+    if (config.hint != null) {
+      hintIndex = items.length;
+      items.add(new DefaultTextStyle(
+        style: _textStyle.copyWith(color: Theme.of(context).hintColor),
+        child: new IgnorePointer(
+          child: config.hint,
+        ),
+      ));
+    }
+
     Widget result = new DefaultTextStyle(
       style: _textStyle,
       child: new SizedBox(
@@ -538,12 +557,12 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> {
           mainAxisAlignment: MainAxisAlignment.spaceBetween,
           mainAxisSize: MainAxisSize.min,
           children: <Widget>[
-            // The button's size is defined by its largest menu item. If value is
-            // null then an item does not appear.
+            // If value is null (then _selectedIndex is null) then we display
+            // the hint or nothing at all.
             new IndexedStack(
-              index: _selectedIndex,
+              index: _selectedIndex ?? hintIndex,
               alignment: FractionalOffset.centerLeft,
-              children: config.items,
+              children: items,
             ),
             new Icon(Icons.arrow_drop_down,
               size: config.iconSize,
@@ -568,10 +587,10 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> {
               height: 1.0,
               decoration: const BoxDecoration(
                 border: const Border(bottom: const BorderSide(color: const Color(0xFFBDBDBD), width: 0.0))
-              )
-            )
-          )
-        ]
+              ),
+            ),
+          ),
+        ],
       );
     }
 

packages/flutter/test/material/drop_down_test.dart

@@ -9,13 +9,20 @@ import 'package:flutter/material.dart';
 
 final List<String> menuItems = <String>['one', 'two', 'three', 'four'];
 
-Widget buildFrame({ Key buttonKey, String value: 'two',  ValueChanged<String> onChanged, bool isDense: false }) {
+Widget buildFrame({
+    Key buttonKey,
+    String value: 'two',
+    ValueChanged<String> onChanged,
+    bool isDense: false,
+    Widget hint,
+  }) {
   return new MaterialApp(
     home: new Material(
       child: new Center(
         child: new DropdownButton<String>(
           key: buttonKey,
           value: value,
+          hint: hint,
           onChanged: onChanged,
           isDense: isDense,
           items: menuItems.map((String item) {
@@ -317,4 +324,28 @@ void main() {
     expect(value, equals('one'));
   });
 
+  testWidgets('Size of DropdownButton with null value and a hint', (WidgetTester tester) async {
+    Key buttonKey = new UniqueKey();
+    String value;
+
+    // The hint will define the dropdown's width
+    Widget build() => buildFrame(buttonKey: buttonKey, value: value, hint: new Text('onetwothree'));
+
+    await tester.pumpWidget(build());
+    expect(find.text('onetwothree'), findsOneWidget);
+    RenderBox buttonBoxHintValue = tester.renderObject(find.byKey(buttonKey));
+    assert(buttonBoxHintValue.attached);
+
+
+    value = 'three';
+    await tester.pumpWidget(build());
+    RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
+    assert(buttonBox.attached);
+
+    // A DropDown button with a null value and a hint should be the same size as a
+    // one with a non-null value.
+    expect(buttonBox.localToGlobal(Point.origin), equals(buttonBoxHintValue.localToGlobal(Point.origin)));
+    expect(buttonBox.size, equals(buttonBoxHintValue.size));
+  });
+
 }