Add support for pointer scrolling to trigger floats & snaps (#76145)

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

@@ -1067,58 +1067,6 @@ class _AppBarState extends State<AppBar> {
   }
 }
 
-class _FloatingAppBar extends StatefulWidget {
-  const _FloatingAppBar({ Key? key, required this.child }) : super(key: key);
-
-  final Widget child;
-
-  @override
-  _FloatingAppBarState createState() => _FloatingAppBarState();
-}
-
-// A wrapper for the widget created by _SliverAppBarDelegate that starts and
-// stops the floating app bar's snap-into-view or snap-out-of-view animation.
-class _FloatingAppBarState extends State<_FloatingAppBar> {
-  ScrollPosition? _position;
-
-  @override
-  void didChangeDependencies() {
-    super.didChangeDependencies();
-    if (_position != null)
-      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
-    _position = Scrollable.of(context)?.position;
-    if (_position != null)
-      _position!.isScrollingNotifier.addListener(_isScrollingListener);
-  }
-
-  @override
-  void dispose() {
-    if (_position != null)
-      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
-    super.dispose();
-  }
-
-  RenderSliverFloatingPersistentHeader? _headerRenderer() {
-    return context.findAncestorRenderObjectOfType<RenderSliverFloatingPersistentHeader>();
-  }
-
-  void _isScrollingListener() {
-    if (_position == null)
-      return;
-
-    // When a scroll stops, then maybe snap the appbar into view.
-    // Similarly, when a scroll starts, then maybe stop the snap animation.
-    final RenderSliverFloatingPersistentHeader? header = _headerRenderer();
-    if (_position!.isScrollingNotifier.value)
-      header?.maybeStopSnapAnimation(_position!.userScrollDirection);
-    else
-      header?.maybeStartSnapAnimation(_position!.userScrollDirection);
-  }
-
-  @override
-  Widget build(BuildContext context) => widget.child;
-}
-
 class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
   _SliverAppBarDelegate({
     required this.leading,
@@ -1264,7 +1212,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
         systemOverlayStyle: systemOverlayStyle,
       ),
     );
-    return floating ? _FloatingAppBar(child: appBar) : appBar;
+    return appBar;
   }
 
   @override

packages/flutter/lib/src/rendering/sliver_persistent_header.dart

@@ -550,6 +550,10 @@ abstract class RenderSliverFloatingPersistentHeader extends RenderSliverPersiste
   late Animation<double> _animation;
   double? _lastActualScrollOffset;
   double? _effectiveScrollOffset;
+  // Important for pointer scrolling, which does not have the same concept of
+  // a hold and release scroll movement, like dragging.
+  // This keeps track of the last ScrollDirection when scrolling started.
+  ScrollDirection? _lastStartedScrollDirection;
 
   // Distance from our leading edge to the child's leading edge, in the axis
   // direction. Negative if we're scrolled off the top.
@@ -647,6 +651,11 @@ abstract class RenderSliverFloatingPersistentHeader extends RenderSliverPersiste
     );
   }
 
+  /// Update the last known ScrollDirection when scrolling began.
+  void updateScrollStartDirection(ScrollDirection direction) {
+    _lastStartedScrollDirection = direction;
+  }
+
   /// If the header isn't already fully exposed, then scroll it into view.
   void maybeStartSnapAnimation(ScrollDirection direction) {
     final FloatingHeaderSnapConfiguration? snap = snapConfiguration;
@@ -680,7 +689,8 @@ abstract class RenderSliverFloatingPersistentHeader extends RenderSliverPersiste
          (_effectiveScrollOffset! < maxExtent))) { // some part of it is visible, so should shrink or reveal as appropriate.
       double delta = _lastActualScrollOffset! - constraints.scrollOffset;
 
-      final bool allowFloatingExpansion = constraints.userScrollDirection == ScrollDirection.forward;
+      final bool allowFloatingExpansion = constraints.userScrollDirection == ScrollDirection.forward
+        || (_lastStartedScrollDirection != null && _lastStartedScrollDirection == ScrollDirection.forward);
       if (allowFloatingExpansion) {
         if (_effectiveScrollOffset! > maxExtent) // We're scrolled off-screen, but should reveal, so
           _effectiveScrollOffset = maxExtent; // pretend we're just at the limit.

packages/flutter/lib/src/widgets/nested_scroll_view.dart

@@ -1103,6 +1103,13 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
         delta < 0.0 ? ScrollDirection.forward : ScrollDirection.reverse,
     );
 
+    // Set the isScrollingNotifier. Even if only one position actually receives
+    // the delta, the NestedScrollView's intention is to treat multiple
+    // ScrollPositions as one.
+    _outerPosition!.isScrollingNotifier.value = true;
+    for (final _NestedScrollPosition position in _innerPositions)
+      position.isScrollingNotifier.value = true;
+
     if (_innerPositions.isEmpty) {
       // Does not enter overscroll.
       _outerPosition!.applyClampedPointerSignalUpdate(delta);

packages/flutter/lib/src/widgets/scroll_position_with_single_context.dart

@@ -216,6 +216,7 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
       );
       final double oldPixels = pixels;
       forcePixels(targetPixels);
+      isScrollingNotifier.value = true;
       didStartScroll();
       didUpdateScrollPositionBy(pixels - oldPixels);
       didEndScroll();

packages/flutter/lib/src/widgets/sliver_persistent_header.dart

@@ -7,6 +7,8 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart' show TickerProvider;
 
 import 'framework.dart';
+import 'scroll_position.dart';
+import 'scrollable.dart';
 
 /// Delegate for configuring a [SliverPersistentHeader].
 abstract class SliverPersistentHeaderDelegate {
@@ -185,8 +187,72 @@ class SliverPersistentHeader extends StatelessWidget {
   }
 }
 
+class _FloatingHeader extends StatefulWidget {
+  const _FloatingHeader({ Key? key, required this.child }) : super(key: key);
+
+  final Widget child;
+
+  @override
+  _FloatingHeaderState createState() => _FloatingHeaderState();
+}
+
+// A wrapper for the widget created by _SliverPersistentHeaderElement that
+// starts and stops the floating app bar's snap-into-view or snap-out-of-view
+// animation. It also informs the float when pointer scrolling by updating the
+// last known ScrollDirection when scrolling began.
+class _FloatingHeaderState extends State<_FloatingHeader> {
+  ScrollPosition? _position;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    if (_position != null)
+      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
+    _position = Scrollable.of(context)?.position;
+    if (_position != null)
+      _position!.isScrollingNotifier.addListener(_isScrollingListener);
+  }
+
+  @override
+  void dispose() {
+    if (_position != null)
+      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
+    super.dispose();
+  }
+
+  RenderSliverFloatingPersistentHeader? _headerRenderer() {
+    return context.findAncestorRenderObjectOfType<RenderSliverFloatingPersistentHeader>();
+  }
+
+  void _isScrollingListener() {
+    assert(_position != null);
+
+    // When a scroll stops, then maybe snap the app bar into view.
+    // Similarly, when a scroll starts, then maybe stop the snap animation.
+    // Update the scrolling direction as well for pointer scrolling updates.
+    final RenderSliverFloatingPersistentHeader? header = _headerRenderer();
+    if (_position!.isScrollingNotifier.value) {
+      header?.updateScrollStartDirection(_position!.userScrollDirection);
+      // Only SliverAppBars support snapping, headers will not snap.
+      header?.maybeStopSnapAnimation(_position!.userScrollDirection);
+    } else {
+      // Only SliverAppBars support snapping, headers will not snap.
+      header?.maybeStartSnapAnimation(_position!.userScrollDirection);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) => widget.child;
+}
+
 class _SliverPersistentHeaderElement extends RenderObjectElement {
-  _SliverPersistentHeaderElement(_SliverPersistentHeaderRenderObjectWidget widget) : super(widget);
+  _SliverPersistentHeaderElement(
+    _SliverPersistentHeaderRenderObjectWidget widget, {
+    this.floating = false,
+  }) : assert(floating != null),
+       super(widget);
+
+  final bool floating;
 
   @override
   _SliverPersistentHeaderRenderObjectWidget get widget => super.widget as _SliverPersistentHeaderRenderObjectWidget;
@@ -229,11 +295,13 @@ class _SliverPersistentHeaderElement extends RenderObjectElement {
     owner!.buildScope(this, () {
       child = updateChild(
         child,
-        widget.delegate.build(
+        floating
+          ? _FloatingHeader(child: widget.delegate.build(
             this,
             shrinkOffset,
-          overlapsContent,
-        ),
+            overlapsContent
+          ))
+          : widget.delegate.build(this, shrinkOffset, overlapsContent),
         null,
       );
     });
@@ -273,13 +341,16 @@ abstract class _SliverPersistentHeaderRenderObjectWidget extends RenderObjectWid
   const _SliverPersistentHeaderRenderObjectWidget({
     Key? key,
     required this.delegate,
+    this.floating = false,
   }) : assert(delegate != null),
+       assert(floating != null),
        super(key: key);
 
   final SliverPersistentHeaderDelegate delegate;
+  final bool floating;
 
   @override
-  _SliverPersistentHeaderElement createElement() => _SliverPersistentHeaderElement(this);
+  _SliverPersistentHeaderElement createElement() => _SliverPersistentHeaderElement(this, floating: floating);
 
   @override
   _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context);
@@ -383,6 +454,7 @@ class _SliverFloatingPersistentHeader extends _SliverPersistentHeaderRenderObjec
   }) : super(
     key: key,
     delegate: delegate,
+    floating: true,
   );
 
   @override
@@ -428,6 +500,7 @@ class _SliverFloatingPinnedPersistentHeader extends _SliverPersistentHeaderRende
   }) : super(
     key: key,
     delegate: delegate,
+    floating: true,
   );
 
   @override

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -1490,6 +1490,60 @@ void main() {
       verifyGeometry(key: appBarKey, paintExtent: 56.0, visible: true);
     });
 
+    testWidgets('snap with pointer signal', (WidgetTester tester) async {
+      final GlobalKey appBarKey = GlobalKey();
+      await tester.pumpWidget(buildFloatTest(
+        floating: true,
+        snap: true,
+        appBarKey: appBarKey,
+      ));
+
+      final Offset scrollEventLocation = tester.getCenter(find.byType(NestedScrollView));
+      final TestPointer testPointer = TestPointer(1, ui.PointerDeviceKind.mouse);
+      // Create a hover event so that |testPointer| has a location when generating the scroll.
+      testPointer.hover(scrollEventLocation);
+
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 56.0, visible: true);
+
+      // Scroll away the outer scroll view and some of the inner scroll view.
+      // We will not scroll back the same amount to indicate that we are
+      // snapping in before reaching the top of the inner scrollable.
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 300.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsNothing);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: appBarKey, paintExtent: 0.0, visible: false);
+
+      // The snap animation should be triggered to expand the app bar
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, -30.0)));
+      await tester.pumpAndSettle();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 56.0, visible: true);
+
+      // Scroll away a bit more to trigger the snap close animation.
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 30.0)));
+      await tester.pumpAndSettle();
+      expect(find.text('Test Title'), findsNothing);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(find.byType(AppBar), findsNothing);
+      verifyGeometry(key: appBarKey, paintExtent: 0.0, visible: false);
+    });
+
     testWidgets('float expanded with pointer signal', (WidgetTester tester) async {
       final GlobalKey appBarKey = GlobalKey();
       await tester.pumpWidget(buildFloatTest(

packages/flutter/test/widgets/slivers_appbar_floating_test.dart

@@ -2,8 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui;
+
+import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
-import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void verifyPaintPosition(GlobalKey key, Offset ideal, bool visible) {
@@ -226,6 +228,230 @@ void main() {
     expect(tester.getTopLeft(find.byType(Container)), Offset.zero);
     expect(tester.getTopLeft(find.text('X')), const Offset(0.0, 250.0));
   });
+
+  group('Pointer scrolled floating', () {
+    Widget buildTest(Widget sliver) {
+      return MaterialApp(
+        home: CustomScrollView(
+          slivers: <Widget>[
+            sliver,
+            SliverFixedExtentList(
+              itemExtent: 50.0,
+              delegate: SliverChildBuilderDelegate(
+                  (BuildContext context, int index) => Text('Item $index'),
+                childCount: 30,
+              )
+            ),
+          ],
+        ),
+      );
+    }
+
+    void verifyGeometry({
+      required GlobalKey key,
+      required bool visible,
+      required double paintExtent
+    }) {
+      final RenderSliver target = key.currentContext!.findRenderObject()! as RenderSliver;
+      final SliverGeometry geometry = target.geometry!;
+      expect(geometry.visible, visible);
+      expect(geometry.paintExtent, paintExtent);
+    }
+
+    testWidgets('SliverAppBar', (WidgetTester tester) async {
+      final GlobalKey appBarKey = GlobalKey();
+      await tester.pumpWidget(buildTest(SliverAppBar(
+        key: appBarKey,
+        floating: true,
+        title: const Text('Test Title'),
+      )));
+
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, visible: true, paintExtent: 56.0);
+
+      // Pointer scroll the app bar away, we will scroll back less to validate the
+      // app bar floats back in.
+      final Offset point1 = tester.getCenter(find.text('Item 5'));
+      final TestPointer testPointer = TestPointer(1, ui.PointerDeviceKind.mouse);
+      testPointer.hover(point1);
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 300.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsNothing);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: appBarKey, paintExtent: 0.0, visible: false);
+
+      // Scroll back to float in appbar
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, -50.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 50.0, visible: true);
+
+      // Float the rest of the way in.
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, -250.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 56.0, visible: true);
+    });
+
+    testWidgets('SliverPersistentHeader', (WidgetTester tester) async {
+      final GlobalKey headerKey = GlobalKey();
+      await tester.pumpWidget(buildTest(SliverPersistentHeader(
+        key: headerKey,
+        floating: true,
+        delegate: HeaderDelegate(),
+      )));
+
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: headerKey, visible: true, paintExtent: 56.0);
+
+      // Pointer scroll the app bar away, we will scroll back less to validate the
+      // app bar floats back in.
+      final Offset point1 = tester.getCenter(find.text('Item 5'));
+      final TestPointer testPointer = TestPointer(1, ui.PointerDeviceKind.mouse);
+      testPointer.hover(point1);
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 300.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsNothing);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: headerKey, paintExtent: 0.0, visible: false);
+
+      // Scroll back to float in appbar
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, -50.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: headerKey, paintExtent: 50.0, visible: true);
+
+      // Float the rest of the way in.
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, -250.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: headerKey, paintExtent: 56.0, visible: true);
+    });
+
+    testWidgets('and snapping SliverAppBar', (WidgetTester tester) async {
+      final GlobalKey appBarKey = GlobalKey();
+      await tester.pumpWidget(buildTest(SliverAppBar(
+        key: appBarKey,
+        floating: true,
+        snap: true,
+        title: const Text('Test Title'),
+      )));
+
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, visible: true, paintExtent: 56.0);
+
+      // Pointer scroll the app bar away, we will scroll back less to validate the
+      // app bar floats back in and then snaps to full size.
+      final Offset point1 = tester.getCenter(find.text('Item 5'));
+      final TestPointer testPointer = TestPointer(1, ui.PointerDeviceKind.mouse);
+      testPointer.hover(point1);
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 300.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsNothing);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      verifyGeometry(key: appBarKey, paintExtent: 0.0, visible: false);
+
+      // Scroll back to float in appbar
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, -30.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 30.0, visible: true);
+      await tester.pumpAndSettle();
+      // The snap animation should have completed and the app bar should be
+      // fully expanded.
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 56.0, visible: true);
+
+
+      // Float back out a bit and trigger snap close animation.
+      await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 50.0)));
+      await tester.pump();
+      expect(find.text('Test Title'), findsOneWidget);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+        56.0,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 6.0, visible: true);
+      await tester.pumpAndSettle();
+      // The snap animation should have completed and the app bar should no
+      // longer be visible.
+      expect(find.text('Test Title'), findsNothing);
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 5'), findsOneWidget);
+      expect(
+        find.byType(AppBar),
+        findsNothing,
+      );
+      verifyGeometry(key: appBarKey, paintExtent: 0.0, visible: false);
+    });
+  });
+}
+
+class HeaderDelegate extends SliverPersistentHeaderDelegate {
+  @override
+  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
+    return Container(
+      height: 56,
+      color: Colors.red,
+      child: const Text('Test Title'),
+    );
+  }
+
+  @override
+  double get maxExtent => 56;
+
+  @override
+  double get minExtent => 56;
+
+  @override
+  bool shouldRebuild(SliverPersistentHeaderDelegate oldDelegate) => false;
 }
 
 class TestDelegate extends SliverPersistentHeaderDelegate {