Expose alwaysShowMiddle in CupertinoSliverNavigationBar (#113544)

* SLFLF-5: Expose `alwaysShowMiddle` in `CupertinoSliverNavigationBar`

* SLFLF-5: Remove space in docstring

* SLFLF-5: Add test and update documentation

* SLFLF-5: Remove trailing whitespace (again)

Author: campovski

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -586,6 +586,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
     this.leading,
     this.automaticallyImplyLeading = true,
     this.automaticallyImplyTitle = true,
+    this.alwaysShowMiddle = true,
     this.previousPageTitle,
     this.middle,
     this.trailing,
@@ -645,13 +646,25 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   /// This value cannot be null.
   final bool automaticallyImplyTitle;
 
+  /// Controls whether [middle] widget should always be visible (even in
+  /// expanded state).
+  ///
+  /// If true (default) and [middle] is not null, [middle] widget is always
+  /// visible. If false, [middle] widget is visible only in collapsed state if
+  /// it is provided.
+  ///
+  /// This should be set to false if you only want to show [largeTitle] in
+  /// expanded state and [middle] in collapsed state.
+  final bool alwaysShowMiddle;
+
   /// {@macro flutter.cupertino.CupertinoNavigationBar.previousPageTitle}
   final String? previousPageTitle;
 
   /// A widget to place in the middle of the static navigation bar instead of
   /// the [largeTitle].
   ///
-  /// This widget is visible in both collapsed and expanded states. The text
+  /// This widget is visible in both collapsed and expanded states if
+  /// [alwaysShowMiddle] is true, otherwise just in collapsed state. The text
   /// supplied in [largeTitle] will no longer appear in collapsed state if a
   /// [middle] widget is provided.
   final Widget? middle;
@@ -742,7 +755,7 @@ class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigation
           transitionBetweenRoutes: widget.transitionBetweenRoutes,
           heroTag: widget.heroTag,
           persistentHeight: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
-          alwaysShowMiddle: widget.middle != null,
+          alwaysShowMiddle: widget.alwaysShowMiddle && widget.middle != null,
           stretchConfiguration: widget.stretch ? OverScrollHeaderStretchConfiguration() : null,
         ),
       ),

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -532,6 +532,52 @@ void main() {
     );
   });
 
+  testWidgets('User specified middle is only visible when sliver is collapsed if alwaysShowMiddle is false', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoPageScaffold(
+          child: CustomScrollView(
+            controller: scrollController,
+            slivers: const <Widget>[
+              CupertinoSliverNavigationBar(
+                largeTitle: Text('Large'),
+                middle: Text('Middle'),
+                alwaysShowMiddle: false,
+              ),
+              SliverToBoxAdapter(
+                child: SizedBox(
+                  height: 1200.0,
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(scrollController.offset, 0.0);
+    expect(find.text('Middle'), findsOneWidget);
+
+    // Initially (in expanded state) middle widget is not visible.
+    RenderAnimatedOpacity middleOpacity = tester.element(find.text('Middle')).findAncestorRenderObjectOfType<RenderAnimatedOpacity>()!;
+    expect(middleOpacity.opacity.value, 0.0);
+
+    scrollController.jumpTo(600.0);
+    await tester.pumpAndSettle();
+
+    // Middle widget is visible when nav bar is collapsed.
+    middleOpacity = tester.element(find.text('Middle')).findAncestorRenderObjectOfType<RenderAnimatedOpacity>()!;
+    expect(middleOpacity.opacity.value, 1.0);
+
+    scrollController.jumpTo(0.0);
+    await tester.pumpAndSettle();
+
+    // Middle widget is not visible when nav bar is again expanded.
+    middleOpacity = tester.element(find.text('Middle')).findAncestorRenderObjectOfType<RenderAnimatedOpacity>()!;
+    expect(middleOpacity.opacity.value, 0.0);
+  });
+
   testWidgets('Small title can be overridden', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     await tester.pumpWidget(