Use state value in DropdownButtonFieldForm (#37145)

The current implementation of DropdownButtonFormField does not pass the initial value to _DropdownButtonFormFieldState. As a result changes made through the child DropdownButton are not made to the FormFieldState and the widget is not updated unless a onChanged function is provided to the DropdownButtomFormField constructor.

This change modifies DropdownButtonFormField to behave more consistently with other FormField widgets in how the Form state is handled.

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

@@ -1427,19 +1427,20 @@ class DropdownButtonFormField<T> extends FormField<T> {
          validator: validator,
          autovalidate: autovalidate,
          builder: (FormFieldState<T> field) {
+           final _DropdownButtonFormFieldState<T> state = field as _DropdownButtonFormFieldState<T>;
            final InputDecoration effectiveDecoration = decoration.applyDefaults(
              Theme.of(field.context).inputDecorationTheme,
            );
            return InputDecorator(
              decoration: effectiveDecoration.copyWith(errorText: field.errorText),
-             isEmpty: value == null,
+             isEmpty: state.value == null,
              child: DropdownButtonHideUnderline(
                child: DropdownButton<T>(
-                 value: value,
+                 value: state.value,
                  items: items,
                  selectedItemBuilder: selectedItemBuilder,
                  hint: hint,
-                 onChanged: onChanged == null ? null : field.didChange,
+                 onChanged: onChanged == null ? null : state.didChange,
                  disabledHint: disabledHint,
                  elevation: elevation,
                  style: style,

packages/flutter/test/material/dropdown_test.dart

@@ -360,6 +360,63 @@ void main() {
     }
   });
 
+  testWidgets('Dropdown form field uses form field state', (WidgetTester tester) async {
+    final Key buttonKey = UniqueKey();
+    final GlobalKey<FormState> formKey = GlobalKey<FormState>();
+    String value;
+    await tester.pumpWidget(
+        StatefulBuilder(
+            builder: (BuildContext context, StateSetter setState) {
+              return MaterialApp(
+                home: Material(
+                  child: Form(
+                    key: formKey,
+                    child: DropdownButtonFormField<String>(
+                      key: buttonKey,
+                      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(),
+                      validator: (String v) => v == null ? 'Must select value' : null,
+                      onChanged: (String newValue) {},
+                      onSaved: (String v) {
+                        setState(() {
+                          value = v;
+                        });
+                      },
+                    ),
+                  ),
+                ),
+              );
+            }
+        )
+    );
+    int getIndex() {
+      final IndexedStack stack = tester.element(find.byType(IndexedStack)).widget as IndexedStack;
+      return stack.index;
+    }
+    // Initial value of null displays hint
+    expect(value, equals(null));
+    expect(getIndex(), 4);
+    await tester.tap(find.text('Select Value'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('three').last);
+    await tester.pumpAndSettle();
+    expect(getIndex(), 2);
+    // Changes only made to FormField state until form saved
+    expect(value, equals(null));
+    final FormState form = formKey.currentState;
+    form.save();
+    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,