Add menuController to DropdownMenu (flutter#175039)

Fixes flutter#174780

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Author: ValentinVignal

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

@@ -197,6 +197,7 @@ class DropdownMenu<T> extends StatefulWidget {
     this.textInputAction,
     this.cursorHeight,
     this.restorationId,
+    this.menuController,
   }) : assert(filterCallback == null || enableFilter),
        assert(
          inputDecorationTheme == null ||
@@ -595,6 +596,10 @@ class DropdownMenu<T> extends StatefulWidget {
   /// {@macro flutter.material.textfield.restorationId}
   final String? restorationId;
 
+  /// An optional controller that allows opening and closing of the menu from
+  /// other widgets.
+  final MenuController? menuController;
+
   @override
   State<DropdownMenu<T>> createState() => _DropdownMenuState<T>();
 }
@@ -603,7 +608,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
   final GlobalKey _anchorKey = GlobalKey();
   final GlobalKey _leadingKey = GlobalKey();
   late List<GlobalKey> buttonItemKeys;
-  final MenuController _controller = MenuController();
+  late MenuController _controller;
   bool _enableFilter = false;
   late bool _enableSearch;
   late List<DropdownMenuEntry<T>> filteredEntries;
@@ -642,6 +647,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
       _selectedEntryIndex = index;
     }
     refreshLeadingPadding();
+    _controller = widget.menuController ?? MenuController();
   }
 
   @override
@@ -712,6 +718,9 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
         _selectedEntryIndex = index;
       }
     }
+    if (oldWidget.menuController != widget.menuController) {
+      _controller = widget.menuController ?? MenuController();
+    }
   }
 
   bool canRequestFocus() {

packages/flutter/test/material/dropdown_menu_test.dart

@@ -4740,6 +4740,27 @@ void main() {
     final EditableText editableText = tester.widget(find.byType(EditableText));
     expect(editableText.cursorHeight, cursorHeight);
   });
+
+  testWidgets('DropdownMenu accepts a MenuController', (WidgetTester tester) async {
+    final MenuController menuController = MenuController();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<TestMenu>(
+            menuController: menuController,
+            dropdownMenuEntries: menuChildren,
+          ),
+        ),
+      ),
+    );
+    expect(findMenuItemButton('Item 0').hitTestable(), findsNothing);
+    menuController.open();
+    await tester.pumpAndSettle();
+    expect(findMenuItemButton('Item 0').hitTestable(), findsOne);
+    menuController.close();
+    await tester.pumpAndSettle();
+    expect(findMenuItemButton('Item 0').hitTestable(), findsNothing);
+  });
 }
 
 enum TestMenu {