Add DropDownButtonFormField convenience class (#23611)

* Adds DropDownFormField Convenience Class

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

@@ -12,6 +12,7 @@ import 'constants.dart';
 import 'debug.dart';
 import 'icons.dart';
 import 'ink_well.dart';
+import 'input_decorator.dart';
 import 'material.dart';
 import 'material_localizations.dart';
 import 'scrollbar.dart';
@@ -763,3 +764,62 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
     );
   }
 }
+
+/// A convenience widget that wraps a [DropdownButton] in a [FormField].
+class DropdownButtonFormField<T> extends FormField<T> {
+  /// Creates a [DropdownButton] widget wrapped in an [InputDecorator] and
+  /// [FormField].
+  ///
+  /// The [DropdownButton] [items] parameters must not be null.
+  DropdownButtonFormField({
+    Key key,
+    T value,
+    @required List<DropdownMenuItem<T>> items,
+    this.onChanged,
+    InputDecoration decoration = const InputDecoration(),
+    FormFieldSetter<T> onSaved,
+    FormFieldValidator<T> validator,
+    Widget hint,
+  }) : assert(decoration != null),
+       super(
+         key: key,
+         onSaved: onSaved,
+         initialValue: value,
+         validator: validator,
+         builder: (FormFieldState<T> field) {
+           final InputDecoration effectiveDecoration = decoration
+             .applyDefaults(Theme.of(field.context).inputDecorationTheme);
+           return InputDecorator(
+             decoration: effectiveDecoration.copyWith(errorText: field.errorText),
+             isEmpty: value == null,
+             child: DropdownButtonHideUnderline(
+               child: DropdownButton<T>(
+                 isDense: true,
+                 value: value,
+                 items: items,
+                 hint: hint,
+                 onChanged: field.didChange,
+               ),
+             ),
+           );
+         }
+       );
+
+  /// Called when the user selects an item.
+  final ValueChanged<T> onChanged;
+
+  @override
+  FormFieldState<T> createState() => _DropdownButtonFormFieldState<T>();
+}
+
+class _DropdownButtonFormFieldState<T> extends FormFieldState<T> {
+  @override
+  DropdownButtonFormField<T> get widget => super.widget;
+
+  @override
+  void didChange(T value) {
+    super.didChange(value);
+    if (widget.onChanged != null)
+      widget.onChanged(value);
+  }
+}

packages/flutter/test/material/dropdown_test.dart

@@ -237,6 +237,48 @@ void main() {
     expect(value, equals('two'));
   });
 
+  testWidgets('Dropdown form field', (WidgetTester tester) async {
+    String value = 'one';
+
+    await tester.pumpWidget(
+      StatefulBuilder(
+        builder: (BuildContext context, StateSetter setState) {
+          return MaterialApp(
+            home: Material(
+              child: DropdownButtonFormField<String>(
+                value: value,
+                hint: const Text('Select Value'),
+                decoration: const InputDecoration(
+                  prefixIcon: Icon(Icons.fastfood)
+                ),
+                items: menuItems.map((String val) {
+                  return DropdownMenuItem<String>(
+                    value: val,
+                    child: Text(val)
+                  );
+                }).toList(),
+                onChanged: (String v) {
+                  setState(() {
+                    value = v;
+                  });
+                },
+                validator: (String v) => v == null ? 'Must select value' : null,
+              ),
+            ),
+          );
+        }
+      )
+    );
+
+    expect(value, equals('one'));
+    await tester.tap(find.text('one'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('three').last);
+    await tester.pump();
+    await tester.pumpAndSettle();
+    expect(value, equals('three'));
+  });
+
   testWidgets('Dropdown in ListView', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/12053
     // Positions a DropdownButton at the left and right edges of the screen,