Implement DropdownButton and DropdownButtonFormField onTap callback (#53105)

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

@@ -787,6 +787,7 @@ class DropdownButton<T> extends StatefulWidget {
     this.hint,
     this.disabledHint,
     @required this.onChanged,
+    this.onTap,
     this.elevation = 8,
     this.style,
     this.underline,
@@ -858,6 +859,14 @@ class DropdownButton<T> extends StatefulWidget {
   /// {@endtemplate}
   final ValueChanged<T> onChanged;
 
+  /// Called when the dropdown button is tapped.
+  ///
+  /// This is distinct from [onChanged], which is called when the user
+  /// selects an item from the dropdown.
+  ///
+  /// The callback will not be invoked if the dropdown button is disabled.
+  final VoidCallback onTap;
+
   /// A builder to customize the dropdown buttons corresponding to the
   /// [DropdownMenuItem]s in [items].
   ///
@@ -1179,6 +1188,10 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
       if (widget.onChanged != null)
         widget.onChanged(newValue.result);
     });
+
+    if (widget.onTap != null) {
+      widget.onTap();
+    }
   }
 
   Action _createAction() {
@@ -1402,6 +1415,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
     DropdownButtonBuilder selectedItemBuilder,
     Widget hint,
     @required this.onChanged,
+    VoidCallback onTap,
     this.decoration = const InputDecoration(),
     FormFieldSetter<T> onSaved,
     FormFieldValidator<T> validator,
@@ -1452,6 +1466,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
                  selectedItemBuilder: selectedItemBuilder,
                  hint: hint,
                  onChanged: onChanged == null ? null : state.didChange,
+                 onTap: onTap,
                  disabledHint: disabledHint,
                  elevation: elevation,
                  style: style,

packages/flutter/test/material/dropdown_form_field_test.dart

@@ -31,6 +31,7 @@ Widget buildFormFrame({
   int elevation = 8,
   String value = 'two',
   ValueChanged<String> onChanged,
+  VoidCallback onTap,
   Widget icon,
   Color iconDisabledColor,
   Color iconEnabledColor,
@@ -58,6 +59,7 @@ Widget buildFormFrame({
             hint: hint,
             disabledHint: disabledHint,
             onChanged: onChanged,
+            onTap: onTap,
             icon: icon,
             iconSize: iconSize,
             iconDisabledColor: iconDisabledColor,
@@ -669,4 +671,48 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('Two as an Arabic numeral: 2'), findsOneWidget);
   });
+
+  testWidgets('DropdownButton onTap callback is called when defined', (WidgetTester tester) async {
+    int dropdownButtonTapCounter = 0;
+    String value = 'one';
+    void onChanged(String newValue) { value = newValue; }
+    void onTap() { dropdownButtonTapCounter += 1; }
+
+    Widget build() => buildFormFrame(
+      value: value,
+      onChanged: onChanged,
+      onTap: onTap,
+    );
+    await tester.pumpWidget(build());
+
+    expect(dropdownButtonTapCounter, 0);
+
+    // Tap dropdown button.
+    await tester.tap(find.text('one'));
+    await tester.pumpAndSettle();
+
+    expect(value, equals('one'));
+    expect(dropdownButtonTapCounter, 1); // Should update counter.
+
+    // Tap dropdown menu item.
+    await tester.tap(find.text('three').last);
+    await tester.pumpAndSettle();
+
+    expect(value, equals('three'));
+    expect(dropdownButtonTapCounter, 1); // Should not change.
+
+    // Tap dropdown button again.
+    await tester.tap(find.text('three'));
+    await tester.pumpAndSettle();
+
+    expect(value, equals('three'));
+    expect(dropdownButtonTapCounter, 2); // Should update counter.
+
+    // Tap dropdown menu item.
+    await tester.tap(find.text('two').last);
+    await tester.pumpAndSettle();
+
+    expect(value, equals('two'));
+    expect(dropdownButtonTapCounter, 2); // Should not change.
+  });
 }

packages/flutter/test/material/dropdown_test.dart

@@ -34,6 +34,7 @@ Widget buildFrame({
   Key buttonKey,
   String value = 'two',
   ValueChanged<String> onChanged,
+  VoidCallback onTap,
   Widget icon,
   Color iconDisabledColor,
   Color iconEnabledColor,
@@ -66,6 +67,7 @@ Widget buildFrame({
             hint: hint,
             disabledHint: disabledHint,
             onChanged: onChanged,
+            onTap: onTap,
             icon: icon,
             iconSize: iconSize,
             iconDisabledColor: iconDisabledColor,
@@ -2314,4 +2316,49 @@ void main() {
     // tree, causing it to lose focus.
     expect(Focus.of(tester.element(find.byKey(const ValueKey<int>(91)).last)).hasPrimaryFocus, isFalse);
   }, skip: kIsWeb);
+
+  testWidgets('DropdownButton onTap callback is called when defined', (WidgetTester tester) async {
+    int dropdownButtonTapCounter = 0;
+    String value = 'one';
+
+    void onChanged(String newValue) { value = newValue; }
+    void onTap() { dropdownButtonTapCounter += 1; }
+
+    Widget build() => buildFrame(
+      value: value,
+      onChanged: onChanged,
+      onTap: onTap,
+    );
+    await tester.pumpWidget(build());
+
+    expect(dropdownButtonTapCounter, 0);
+
+    // Tap dropdown button.
+    await tester.tap(find.text('one'));
+    await tester.pumpAndSettle();
+
+    expect(value, equals('one'));
+    expect(dropdownButtonTapCounter, 1); // Should update counter.
+
+    // Tap dropdown menu item.
+    await tester.tap(find.text('three').last);
+    await tester.pumpAndSettle();
+
+    expect(value, equals('three'));
+    expect(dropdownButtonTapCounter, 1); // Should not change.
+
+    // Tap dropdown button again.
+    await tester.tap(find.text('three'));
+    await tester.pumpAndSettle();
+
+    expect(value, equals('three'));
+    expect(dropdownButtonTapCounter, 2); // Should update counter.
+
+    // Tap dropdown menu item.
+    await tester.tap(find.text('two').last);
+    await tester.pumpAndSettle();
+
+    expect(value, equals('two'));
+    expect(dropdownButtonTapCounter, 2); // Should not change.
+  });
 }