Expose showButtonMenu of PopupMenuButtonState (#50670)

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

@@ -1037,10 +1037,22 @@ class PopupMenuButton<T> extends StatefulWidget {
   final bool captureInheritedThemes;
 
   @override
-  _PopupMenuButtonState<T> createState() => _PopupMenuButtonState<T>();
+  PopupMenuButtonState<T> createState() => PopupMenuButtonState<T>();
 }
 
-class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
+/// The [State] for a [PopupMenuButton].
+///
+/// See [showButtonMenu] for a way to programmatically open the popup menu
+/// of your button state.
+class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
+  /// A method to show a popup menu with the items supplied to
+  /// [PopupMenuButton.itemBuilder] at the position of your [PopupMenuButton].
+  ///
+  /// By default, it is called when the user taps the button and [PopupMenuButton.enabled]
+  /// is set to `true`. Moreover, you can open the button by calling the method manually.
+  ///
+  /// You would access your [PopupMenuButtonState] using a [GlobalKey] and
+  /// show the menu of the button with `globalKey.currentState.showButtonMenu`.
   void showButtonMenu() {
     final PopupMenuThemeData popupMenuTheme = PopupMenuTheme.of(context);
     final RenderBox button = context.findRenderObject() as RenderBox;

packages/flutter/test/material/popup_menu_test.dart

@@ -1197,6 +1197,41 @@ void main() {
     expect(rootObserver.menuCount, 1);
     expect(nestedObserver.menuCount, 0);
   });
+
+  testWidgets('PopupMenuButton calling showButtonMenu manually', (WidgetTester tester) async {
+    final GlobalKey<PopupMenuButtonState<int>> globalKey = GlobalKey();
+
+    await tester.pumpWidget(
+        MaterialApp(
+          home: Material(
+            child: Column(
+              children: <Widget>[
+                PopupMenuButton<int>(
+                  key: globalKey,
+                  itemBuilder: (BuildContext context) {
+                    return <PopupMenuEntry<int>>[
+                      const PopupMenuItem<int>(
+                        value: 1,
+                        child: Text('Tap me please!'),
+                      ),
+                    ];
+                  },
+                ),
+              ],
+            ),
+          ),
+        )
+    );
+
+    expect(find.text('Tap me please!'), findsNothing);
+
+    globalKey.currentState.showButtonMenu();
+    // The PopupMenuItem will appear after an animation, hence,
+    // we have to first wait for the tester to settle.
+    await tester.pumpAndSettle();
+
+    expect(find.text('Tap me please!'), findsOneWidget);
+  });
 }
 
 class TestApp extends StatefulWidget {