Add collapsed height param to SliverAppBar (#58593)

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

@@ -783,7 +783,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
   final double _bottomHeight;
 
   @override
-  double get minExtent => collapsedHeight ?? (topPadding + kToolbarHeight + _bottomHeight);
+  double get minExtent => collapsedHeight;
 
   @override
   double get maxExtent => math.max(topPadding + (expandedHeight ?? kToolbarHeight + _bottomHeight), minExtent);
@@ -797,20 +797,12 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
   @override
   Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
     final double visibleMainHeight = maxExtent - shrinkOffset - topPadding;
+    final double extraToolbarHeight = math.max(minExtent - _bottomHeight - topPadding - kToolbarHeight, 0.0);
+    final double visibleToolbarHeight = visibleMainHeight - _bottomHeight - extraToolbarHeight;
 
-    // Truth table for `toolbarOpacity`:
-    // pinned | floating | bottom != null || opacity
-    // ----------------------------------------------
-    //    0   |    0     |        0       ||  fade
-    //    0   |    0     |        1       ||  fade
-    //    0   |    1     |        0       ||  fade
-    //    0   |    1     |        1       ||  fade
-    //    1   |    0     |        0       ||  1.0
-    //    1   |    0     |        1       ||  1.0
-    //    1   |    1     |        0       ||  1.0
-    //    1   |    1     |        1       ||  fade
-    final double toolbarOpacity = !pinned || (floating && bottom != null)
-      ? ((visibleMainHeight - _bottomHeight) / kToolbarHeight).clamp(0.0, 1.0) as double
+    final bool isPinnedWithOpacityFade = pinned && floating && bottom != null && extraToolbarHeight == 0.0;
+    final double toolbarOpacity = !pinned || isPinnedWithOpacityFade
+      ? (visibleToolbarHeight / kToolbarHeight).clamp(0.0, 1.0) as double
       : 1.0;
 
     final Widget appBar = FlexibleSpaceBar.createSettings(
@@ -979,6 +971,7 @@ class SliverAppBar extends StatefulWidget {
     this.centerTitle,
     this.excludeHeaderSemantics = false,
     this.titleSpacing = NavigationToolbar.kMiddleSpacing,
+    this.collapsedHeight,
     this.expandedHeight,
     this.floating = false,
     this.pinned = false,
@@ -997,6 +990,7 @@ class SliverAppBar extends StatefulWidget {
        assert(stretch != null),
        assert(floating || !snap, 'The "snap" argument only makes sense for floating app bars.'),
        assert(stretchTriggerOffset > 0.0),
+       assert(collapsedHeight == null || collapsedHeight > kToolbarHeight, 'The "collapsedHeight" argument has to be larger than [kToolbarHeight].'),
        super(key: key);
 
   /// A widget to display before the [title].
@@ -1154,6 +1148,18 @@ class SliverAppBar extends StatefulWidget {
   /// Defaults to [NavigationToolbar.kMiddleSpacing].
   final double titleSpacing;
 
+  /// Defines the height of the app bar when it is collapsed.
+  ///
+  /// By default, the collapsed height is [kToolbarHeight]. If [bottom] widget
+  /// is specified, then its [bottom.preferredSize.height] is added to the
+  /// height. If [primary] is true, then the [MediaQuery] top padding,
+  /// [MediaQueryData.padding.top], is added as well.
+  ///
+  /// If [pinned] and [floating] are true, with [bottom] set, the default
+  /// collapsed height is only [bottom.preferredSize.height] with the
+  /// [MediaQuery] top padding.
+  final double collapsedHeight;
+
   /// The size of the app bar when it is fully expanded.
   ///
   /// By default, the total height of the toolbar and the bottom widget (if
@@ -1310,9 +1316,11 @@ class _SliverAppBarState extends State<SliverAppBar> with TickerProviderStateMix
   @override
   Widget build(BuildContext context) {
     assert(!widget.primary || debugCheckHasMediaQuery(context));
+    final double bottomHeight = widget.bottom?.preferredSize?.height ?? 0.0;
     final double topPadding = widget.primary ? MediaQuery.of(context).padding.top : 0.0;
     final double collapsedHeight = (widget.pinned && widget.floating && widget.bottom != null)
-      ? widget.bottom.preferredSize.height + topPadding : null;
+      ? (widget.collapsedHeight ?? 0.0) + bottomHeight + topPadding
+      : (widget.collapsedHeight ?? kToolbarHeight) + bottomHeight + topPadding;
 
     return MediaQuery.removePadding(
       context: context,

packages/flutter/test/material/app_bar_test.dart

@@ -10,7 +10,13 @@ import 'package:flutter_test/flutter_test.dart';
 
 import '../widgets/semantics_tester.dart';
 
-Widget buildSliverAppBarApp({ bool floating, bool pinned, double expandedHeight, bool snap = false }) {
+Widget buildSliverAppBarApp({
+  bool floating,
+  bool pinned,
+  double collapsedHeight,
+  double expandedHeight,
+  bool snap = false,
+}) {
   return Localizations(
     locale: const Locale('en', 'US'),
     delegates: const <LocalizationsDelegate<dynamic>>[
@@ -31,6 +37,7 @@ Widget buildSliverAppBarApp({ bool floating, bool pinned, double expandedHeight,
                   title: const Text('AppBar Title'),
                   floating: floating,
                   pinned: pinned,
+                  collapsedHeight: collapsedHeight,
                   expandedHeight: expandedHeight,
                   snap: snap,
                   bottom: TabBar(
@@ -844,6 +851,46 @@ void main() {
     expect(appBarBottom(tester), kTextTabBarHeight);
   });
 
+  testWidgets('SliverAppBar expandedHeight, collapsedHeight', (WidgetTester tester) async {
+    const double expandedAppBarHeight = 400.0;
+    const double collapsedAppBarHeight = 200.0;
+
+    await tester.pumpWidget(buildSliverAppBarApp(
+      floating: false,
+      pinned: false,
+      collapsedHeight: collapsedAppBarHeight,
+      expandedHeight: expandedAppBarHeight,
+    ));
+
+    final ScrollController controller = primaryScrollController(tester);
+    expect(controller.offset, 0.0);
+    expect(find.byType(SliverAppBar), findsOneWidget);
+    expect(appBarHeight(tester), expandedAppBarHeight);
+
+    final double initialTabBarHeight = tabBarHeight(tester);
+
+    // Scroll the not-pinned appbar partially out of view.
+    controller.jumpTo(50.0);
+    await tester.pump();
+    expect(find.byType(SliverAppBar), findsOneWidget);
+    expect(appBarHeight(tester), expandedAppBarHeight - 50.0);
+    expect(tabBarHeight(tester), initialTabBarHeight);
+
+    // Scroll the not-pinned appbar out of view, to its collapsed height.
+    controller.jumpTo(600.0);
+    await tester.pump();
+    expect(find.byType(SliverAppBar), findsNothing);
+    expect(appBarHeight(tester), collapsedAppBarHeight + initialTabBarHeight);
+    expect(tabBarHeight(tester), initialTabBarHeight);
+
+    // Scroll the not-pinned appbar back into view.
+    controller.jumpTo(0.0);
+    await tester.pump();
+    expect(find.byType(SliverAppBar), findsOneWidget);
+    expect(appBarHeight(tester), expandedAppBarHeight);
+    expect(tabBarHeight(tester), initialTabBarHeight);
+  });
+
   testWidgets('AppBar dimensions, with and without bottom, primary', (WidgetTester tester) async {
     const MediaQueryData topPadding100 = MediaQueryData(padding: EdgeInsets.only(top: 100.0));
 

packages/flutter/test/widgets/sliver_appbar_opacity.dart → packages/flutter/test/widgets/sliver_appbar_opacity_test.dart

@@ -142,7 +142,7 @@ void main() {
     expect(render.text.style.color.opacity, 1.0);
   });
 
-  testWidgets('pinned && floating && bottom ==> fade opactiy', (WidgetTester tester) async {
+  testWidgets('pinned && floating && bottom && extraToolbarHeight == 0.0 ==> fade opactiy', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/25993.
 
     final ScrollController controller = ScrollController();
@@ -162,16 +162,64 @@ void main() {
     await tester.pumpAndSettle();
     expect(render.text.style.color.opacity, 0.0);
   });
+
+  testWidgets('pinned && floating && bottom && extraToolbarHeight != 0.0 ==> 1.0 opacity', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    await tester.pumpWidget(
+      _TestWidget(
+        pinned: true,
+        floating: true,
+        bottom: true,
+        collapsedHeight: 100.0,
+        controller: controller,
+      ),
+    );
+
+    final RenderParagraph render = tester.renderObject(find.text('Hallo Welt!!1'));
+    expect(render.text.style.color.opacity, 1.0);
+
+    controller.jumpTo(200.0);
+    await tester.pumpAndSettle();
+    expect(render.text.style.color.opacity, 1.0);
+  });
+
+  testWidgets('!pinned && !floating && !bottom && extraToolbarHeight != 0.0 ==> fade opacity', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    const double collapsedHeight = 100.0;
+    await tester.pumpWidget(
+        _TestWidget(
+          pinned: false,
+          floating: false,
+          bottom: false,
+          controller: controller,
+          collapsedHeight: collapsedHeight,
+        ),
+    );
+
+    final RenderParagraph render = tester.renderObject(find.text('Hallo Welt!!1'));
+    expect(render.text.style.color.opacity, 1.0);
+
+    controller.jumpTo(collapsedHeight);
+    await tester.pumpAndSettle();
+    expect(render.text.style.color.opacity, 0.0);
+  });
 }
 
 class _TestWidget extends StatelessWidget {
 
-  const _TestWidget({this.pinned, this.floating, this.bottom, this.controller});
+  const _TestWidget({
+    this.pinned,
+    this.floating,
+    this.bottom,
+    this.controller,
+    this.collapsedHeight,
+  });
 
   final bool pinned;
   final bool floating;
   final bool bottom;
   final ScrollController controller;
+  final double collapsedHeight;
 
   @override
   Widget build(BuildContext context) {
@@ -183,6 +231,7 @@ class _TestWidget extends StatelessWidget {
             pinned: pinned,
             floating: floating,
             expandedHeight: 120.0,
+            collapsedHeight: collapsedHeight,
             title: const Text('Hallo Welt!!1'),
             bottom: !bottom ? null :  PreferredSize(
               preferredSize: const Size.fromHeight(35.0),