Sliver Cross Axis Group (#123862)

This widget implements the ability to place slivers side by side in a single ScrollView so that they scroll together. The design document for `SliverCrossAxisGroup` can be found [here](https://docs.google.com/document/d/1e2bdLSYV_Dq2h8aHpF8mda67aOmZocPiMyjCcTTZhTg/edit?resourcekey=0-Xj2X2XA3CAFae22Sv3hAiA).

Fixes #56756.

examples/api/lib/widgets/sliver/sliver_cross_axis_group.0.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+void main() => runApp(const SliverCrossAxisGroupExampleApp());
+
+class SliverCrossAxisGroupExampleApp extends StatelessWidget {
+  const SliverCrossAxisGroupExampleApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(title: const Text('SliverCrossAxisGroup Sample')),
+        body: const SliverCrossAxisGroupExample(),
+      ),
+    );
+  }
+}
+
+class SliverCrossAxisGroupExample extends StatelessWidget {
+  const SliverCrossAxisGroupExample({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return CustomScrollView(
+      slivers: <Widget>[
+        SliverCrossAxisGroup(
+          slivers: <Widget>[
+            SliverList.builder(
+              itemBuilder: (BuildContext context, int index) {
+                return Container(
+                  color: index.isEven ? Colors.amber[300] : Colors.blue[300],
+                  height: 100.0,
+                  child: Center(
+                    child: Text(
+                      'Item $index',
+                      style: const TextStyle(fontSize: 24),
+                    ),
+                  ),
+                );
+              },
+              itemCount: 5,
+            ),
+            SliverConstrainedCrossAxis(
+              maxExtent: 200,
+              sliver: SliverList.builder(
+                itemBuilder: (BuildContext context, int index) {
+                  return Container(
+                    color: index.isEven ? Colors.green[300] : Colors.red[300],
+                    height: 100.0,
+                    child: Center(
+                      child: Text(
+                        'Item ${index + 5}',
+                        style: const TextStyle(fontSize: 24),
+                      ),
+                    ),
+                  );
+                },
+                itemCount: 5,
+              ),
+            ),
+            SliverCrossAxisExpanded(
+              flex: 2,
+              sliver: SliverList.builder(
+                itemBuilder: (BuildContext context, int index) {
+                  return Container(
+                    color: index.isEven ? Colors.purple[300] : Colors.orange[300],
+                    height: 100.0,
+                    child: Center(
+                      child: Text(
+                        'Item ${index + 10}',
+                        style: const TextStyle(fontSize: 24),
+                      ),
+                    ),
+                  );
+                },
+                itemCount: 5,
+              ),
+            ),
+          ],
+        ),
+      ],
+    );
+  }
+}

examples/api/test/widgets/sliver/sliver_cross_axis_group.0_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_api_samples/widgets/sliver/sliver_cross_axis_group.0.dart'
+  as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('SliverCrossAxisGroup example', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.SliverCrossAxisGroupExampleApp(),
+    );
+
+    final RenderSliverCrossAxisGroup renderSliverGroup = tester.renderObject(find.byType(SliverCrossAxisGroup));
+    expect(renderSliverGroup, isNotNull);
+
+    final double crossAxisExtent = renderSliverGroup.constraints.crossAxisExtent;
+
+    final List<RenderSliverList> renderSliverLists = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
+    final RenderSliverList firstList = renderSliverLists[0];
+    final RenderSliverList secondList = renderSliverLists[1];
+    final RenderSliverList thirdList = renderSliverLists[2];
+
+    final double expectedFirstExtent = (crossAxisExtent - 200) / 3;
+    const double expectedSecondExtent = 200;
+    final double expectedThirdExtent = 2 * (crossAxisExtent - 200) / 3;
+    expect(firstList.constraints.crossAxisExtent, equals(expectedFirstExtent));
+    expect(secondList.constraints.crossAxisExtent, equals(expectedSecondExtent));
+    expect(thirdList.constraints.crossAxisExtent, equals(expectedThirdExtent));
+
+    // Also check that the paint offsets are correct.
+    final RenderSliverConstrainedCrossAxis renderConstrained = tester.renderObject<RenderSliverConstrainedCrossAxis>(
+      find.byType(SliverConstrainedCrossAxis)
+    );
+
+    expect((firstList.parentData! as SliverPhysicalParentData).paintOffset.dx, equals(0));
+    expect((renderConstrained.parentData! as SliverPhysicalParentData).paintOffset.dx, equals(expectedFirstExtent));
+    expect((thirdList.parentData! as SliverPhysicalParentData).paintOffset.dx, equals(expectedFirstExtent + expectedSecondExtent));
+  });
+}

packages/flutter/lib/rendering.dart

@@ -61,6 +61,7 @@ export 'src/rendering/sliver.dart';
 export 'src/rendering/sliver_fill.dart';
 export 'src/rendering/sliver_fixed_extent_list.dart';
 export 'src/rendering/sliver_grid.dart';
+export 'src/rendering/sliver_group.dart';
 export 'src/rendering/sliver_list.dart';
 export 'src/rendering/sliver_multi_box_adaptor.dart';
 export 'src/rendering/sliver_padding.dart';

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

@@ -760,8 +760,10 @@ class SliverGeometry with Diagnosticable {
   ///
   /// See also:
   ///
-  ///   * [SliverConstrainedCrossAxis] for an example of a sliver which takes up
-  ///     a smaller cross axis extent than the provided constraint.
+  ///  * [SliverConstrainedCrossAxis] for an example of a sliver which takes up
+  ///    a smaller cross axis extent than the provided constraint.
+  ///  * [SliverCrossAxisGroup] for an example of a sliver which makes use of this
+  ///  [crossAxisExtent] to lay out their children.
   final double? crossAxisExtent;
 
   /// Asserts that this geometry is internally consistent.
@@ -1004,7 +1006,20 @@ class SliverPhysicalParentData extends ParentData {
 
   /// The [crossAxisFlex] factor to use for this sliver child.
   ///
+  /// If used outside of a [SliverCrossAxisGroup] widget, this value has no meaning.
+  ///
   /// If null or zero, the child is inflexible and determines its own size in the cross axis.
+  /// If non-zero, the amount of space the child can occupy in the cross axis is
+  /// determined by dividing the free space (after placing the inflexible children)
+  /// according to the flex factors of the flexible children.
+  ///
+  /// This value is only used by the [SliverCrossAxisGroup] widget to determine
+  /// how to allocate its [SliverConstraints.crossAxisExtent] to its children.
+  ///
+  /// See also:
+  ///
+  ///  * [SliverCrossAxisGroup], which lays out multiple slivers along the
+  ///    cross axis direction.
   int? crossAxisFlex;
 
   /// Apply the [paintOffset] to the given [transform].

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

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:math' as math;
+import 'package:vector_math/vector_math_64.dart';
+
+import 'object.dart';
+import 'sliver.dart';
+
+/// A sliver that places multiple sliver children in a linear array along the cross
+/// axis.
+///
+/// Since the extent of the viewport in the cross axis direction is finite,
+/// this extent will be divided up and allocated to the children slivers.
+///
+/// The algorithm for dividing up the cross axis extent is as follows.
+/// Every widget has a [SliverPhysicalParentData.crossAxisFlex] value associated with them.
+/// First, lay out all of the slivers with flex of 0 or null, in which case the slivers themselves will
+/// figure out how much cross axis extent to take up. For example, [SliverConstrainedCrossAxis]
+/// is an example of a widget which sets its own flex to 0. Then [RenderSliverCrossAxisGroup] will
+/// divide up the remaining space to all the remaining children proportionally
+/// to each child's flex factor. By default, children of [SliverCrossAxisGroup]
+/// are setup to have a flex factor of 1, but a different flex factor can be
+/// specified via the [SliverCrossAxisExpanded] widgets.
+class RenderSliverCrossAxisGroup extends RenderSliver with ContainerRenderObjectMixin<RenderSliver, SliverPhysicalContainerParentData> {
+  @override
+  void setupParentData(RenderObject child) {
+    if (child.parentData is! SliverPhysicalContainerParentData) {
+      child.parentData = SliverPhysicalContainerParentData();
+      (child.parentData! as SliverPhysicalParentData).crossAxisFlex = 1;
+    }
+  }
+
+  @override
+  double childMainAxisPosition(RenderSliver child) => 0.0;
+
+  @override
+  double childCrossAxisPosition(RenderSliver child) {
+    switch (constraints.axisDirection) {
+      case AxisDirection.up:
+      case AxisDirection.down:
+        return (child.parentData! as SliverPhysicalParentData).paintOffset.dx;
+      case AxisDirection.left:
+      case AxisDirection.right:
+        return (child.parentData! as SliverPhysicalParentData).paintOffset.dy;
+    }
+  }
+
+  @override
+  void performLayout() {
+    // Iterate through each sliver.
+    // Get the parent's dimensions.
+    final double crossAxisExtent = constraints.crossAxisExtent;
+    assert(crossAxisExtent.isFinite);
+
+    // First, layout each child with flex == 0 or null.
+    int totalFlex = 0;
+    double remainingExtent = crossAxisExtent;
+    RenderSliver? child = firstChild;
+    while (child != null) {
+      final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+      final int flex = childParentData.crossAxisFlex ?? 0;
+      if (flex == 0) {
+        // If flex is 0 or null, then the child sliver must provide their own crossAxisExtent.
+        assert(_assertOutOfExtent(remainingExtent));
+        child.layout(constraints.copyWith(crossAxisExtent: remainingExtent), parentUsesSize: true);
+        final double? childCrossAxisExtent = child.geometry!.crossAxisExtent;
+        assert(childCrossAxisExtent != null);
+        remainingExtent = math.max(0.0, remainingExtent - childCrossAxisExtent!);
+      } else {
+        totalFlex += flex;
+      }
+      child = childAfter(child);
+    }
+    final double extentPerFlexValue = remainingExtent / totalFlex;
+
+    child = firstChild;
+    double offset = 0.0;
+
+    // At this point, all slivers with constrained cross axis should already be laid out.
+    // Layout the rest and keep track of the child geometry with greatest scrollExtent.
+    geometry = SliverGeometry.zero;
+    while (child != null) {
+      final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+      final int flex = childParentData.crossAxisFlex ?? 0;
+      double childExtent;
+      if (flex != 0) {
+        childExtent = extentPerFlexValue * flex;
+        assert(_assertOutOfExtent(childExtent));
+        child.layout(constraints.copyWith(
+          crossAxisExtent: extentPerFlexValue * flex,
+        ), parentUsesSize: true);
+      } else {
+        childExtent = child.geometry!.crossAxisExtent!;
+      }
+      // Set child parent data.
+      switch (constraints.axis) {
+        case Axis.vertical:
+          childParentData.paintOffset = Offset(offset, 0.0);
+        case Axis.horizontal:
+          childParentData.paintOffset = Offset(0.0, offset);
+      }
+      offset += childExtent;
+      if (geometry!.scrollExtent < child.geometry!.scrollExtent) {
+        geometry = child.geometry;
+      }
+      child = childAfter(child);
+    }
+
+    // Set the geometry with the proper crossAxisExtent.
+    geometry = geometry!.copyWith(crossAxisExtent: constraints.crossAxisExtent);
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    RenderSliver? child = firstChild;
+
+    while (child != null) {
+      final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+      context.paintChild(child, offset + childParentData.paintOffset);
+      child = childAfter(child);
+    }
+  }
+
+  @override
+  void applyPaintTransform(RenderSliver child, Matrix4 transform) {
+    final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+    childParentData.applyPaintTransform(transform);
+  }
+
+  @override
+  bool hitTestChildren(SliverHitTestResult result, {required double mainAxisPosition, required double crossAxisPosition}) {
+    RenderSliver? child = lastChild;
+    while (child != null) {
+      final bool isHit = result.addWithAxisOffset(
+        mainAxisPosition: mainAxisPosition,
+        crossAxisPosition: crossAxisPosition,
+        paintOffset: null,
+        mainAxisOffset: childMainAxisPosition(child),
+        crossAxisOffset: childCrossAxisPosition(child),
+        hitTest: child.hitTest,
+      );
+      if (isHit) {
+        return true;
+      }
+      child = childBefore(child);
+    }
+    return false;
+  }
+}
+
+bool _assertOutOfExtent(double extent) {
+  if(extent <= 0.0) {
+    throw FlutterError.fromParts(<DiagnosticsNode>[
+      ErrorSummary('SliverCrossAxisGroup ran out of extent before child could be laid out.'),
+      ErrorDescription(
+        'SliverCrossAxisGroup lays out any slivers with a constrained cross '
+        'axis before laying out those which expand. In this case, cross axis '
+        'extent was used up before the next sliver could be laid out.'
+      ),
+      ErrorHint(
+        'Make sure that the total amount of extent allocated by constrained '
+        'child slivers does not exceed the cross axis extent that is available '
+        'for the SliverCrossAxisGroup.'
+      ),
+    ]);
+  }
+  return true;
+}

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

@@ -1375,12 +1375,21 @@ class KeepAlive extends ParentDataWidget<KeepAliveParentDataMixin> {
 /// This is useful when you want to apply a custom cross-axis extent constraint
 /// to a sliver child, as slivers typically consume the full cross axis extent.
 ///
+/// This widget also sets its parent data's [SliverPhysicalParentData.crossAxisFlex]
+/// to 0, so that it informs [SliverCrossAxisGroup] that it should not flex
+/// in the cross axis direction.
+///
 /// {@tool dartpad}
 /// In this sample the [SliverConstrainedCrossAxis] sizes its child so that the
 /// cross axis extent takes up less space than the actual viewport.
 ///
 /// ** See code in examples/api/lib/widgets/sliver/sliver_constrained_cross_axis.0.dart **
 /// {@end-tool}
+///
+/// See also:
+///
+///  * [SliverCrossAxisGroup], the widget which makes use of 0 flex factor set by
+///    this widget.
 class SliverConstrainedCrossAxis extends StatelessWidget {
   /// Creates a sliver that constrains the cross axis extent of its sliver child.
   ///
@@ -1436,7 +1445,7 @@ class _SliverZeroFlexParentDataWidget extends ParentDataWidget<SliverPhysicalPar
   }
 
   @override
-  Type get debugTypicalAncestorWidgetClass => Widget;
+  Type get debugTypicalAncestorWidgetClass => SliverCrossAxisGroup;
 }
 
 class _SliverConstrainedCrossAxis extends SingleChildRenderObjectWidget {
@@ -1461,3 +1470,107 @@ class _SliverConstrainedCrossAxis extends SingleChildRenderObjectWidget {
     renderObject.maxExtent = maxExtent;
   }
 }
+
+/// Set a flex factor for allocating space in the cross axis direction.
+///
+/// This is a [ParentDataWidget] to be used in [SliverCrossAxisGroup].
+/// After all slivers with null or zero flex (e.g. [SliverConstrainedCrossAxis])
+/// are laid out (which should determine their own [SliverGeometry.crossAxisExtent]),
+/// the remaining space is laid out among the slivers with nonzero flex
+/// proportionally to their flex value.
+class SliverCrossAxisExpanded extends ParentDataWidget<SliverPhysicalContainerParentData> {
+  /// Creates an object that assigns a [flex] value to the child sliver.
+  ///
+  /// The provided [flex] value must be greater than 0.
+  const SliverCrossAxisExpanded({
+    super.key,
+    required this.flex,
+    required Widget sliver,
+  }): assert(flex > 0 && flex < double.infinity),
+      super(child: sliver);
+
+  /// Flex value for allocating cross axis extent left after laying out the children with
+  /// constrained cross axis. The children with flex values will have the remaining extent
+  /// allocated proportionally to their flex value. This must an integer between
+  /// 0 and infinity, exclusive.
+  final int flex;
+
+  @override
+  void applyParentData(RenderObject renderObject) {
+    assert(renderObject.parentData is SliverPhysicalContainerParentData);
+    assert(renderObject.parent is RenderSliverCrossAxisGroup);
+    final SliverPhysicalParentData parentData = renderObject.parentData! as SliverPhysicalParentData;
+    bool needsLayout = false;
+
+    if (parentData.crossAxisFlex != flex) {
+      parentData.crossAxisFlex = flex;
+      needsLayout = true;
+    }
+
+    if (needsLayout) {
+      final AbstractNode? targetParent = renderObject.parent;
+      if (targetParent is RenderObject) {
+        targetParent.markNeedsLayout();
+      }
+    }
+  }
+
+  @override
+  Type get debugTypicalAncestorWidgetClass => SliverCrossAxisGroup;
+}
+
+
+/// A sliver that places multiple sliver children in a linear array along
+/// the cross axis.
+///
+/// ## Layout algorithm
+///
+/// _This section describes how the framework causes [RenderSliverCrossAxisGroup]
+/// to position its children._
+///
+/// Layout for a [RenderSliverCrossAxisGroup] has four steps:
+///
+/// 1. Layout each child with a null or zero flex factor with cross axis constraint
+///    being whatever cross axis space is remaining after laying out any previous
+///    sliver. Slivers with null or zero flex factor should determine their own
+///    [SliverGeometry.crossAxisExtent]. For example, the [SliverConstrainedCrossAxis]
+///    widget uses either [SliverConstrainedCrossAxis.maxExtent] or
+///    [SliverConstraints.crossAxisExtent], deciding between whichever is smaller.
+/// 2. Divide up the remaining cross axis space among the children with non-zero flex
+///    factors according to their flex factor. For example, a child with a flex
+///    factor of 2.0 will receive twice the amount of cross axis space as a child
+///    with a flex factor 1.0.
+/// 3. Layout each of the remaining children with the cross axis constraint
+///    allocated in the previous step.
+/// 4. Set the geometry to that of whichever child has the longest
+///    [SliverGeometry.scrollExtent] with the [SliverGeometry.crossAxisExtent] adjusted
+///    to [SliverConstraints.crossAxisExtent].
+///
+/// {@tool dartpad}
+/// In this sample the [SliverCrossAxisGroup] sizes its three [children] so that
+/// the first normal [SliverList] has a flex factor of 1, the second [SliverConstrainedCrossAxis]
+/// has a flex factor of 0 and a maximum cross axis extent of 200.0, and the third
+/// [SliverCrossAxisExpanded] has a flex factor of 2.
+///
+/// ** See code in examples/api/lib/widgets/sliver/sliver_cross_axis_group.0.dart **
+/// {@end-tool}
+///
+/// See also:
+///
+///  * [SliverCrossAxisExpanded], which is the [ParentDataWidget] for setting a flex
+///    value to a widget.
+///  * [SliverConstrainedCrossAxis], which is a [RenderObjectWidget] for setting
+///    an extent to constrain the widget to.
+class SliverCrossAxisGroup extends MultiChildRenderObjectWidget {
+  /// Creates a sliver that places sliver children in a linear array along
+  /// the cross axis.
+  const SliverCrossAxisGroup({
+    super.key,
+    required List<Widget> slivers,
+  }): super(children: slivers);
+
+  @override
+  RenderSliverCrossAxisGroup createRenderObject(BuildContext context) {
+    return RenderSliverCrossAxisGroup();
+  }
+}