[New feature] Allowing the ListView slivers to have different extents while still having scrolling performance (#131393)

Fixes flutter/flutter#113431

Currently we only support specifying all slivers to have the same extent.
This patch introduces an `itemExtentBuilder` property for `ListView`, allowing the slivers to have different extents while still having scrolling performance, especially when the scroll position changes drastically(such as scrolling by the scrollbar or controller.jumpTo()).

@Piinks Hi, Any thoughts about this?  :)

Author: xu-baolin

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

@@ -5,6 +5,7 @@
 import 'dart:ui' show lerpDouble;
 
 import 'package:flutter/gestures.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'debug.dart';
@@ -76,6 +77,7 @@ class ReorderableListView extends StatefulWidget {
     this.onReorderStart,
     this.onReorderEnd,
     this.itemExtent,
+    this.itemExtentBuilder,
     this.prototypeItem,
     this.proxyDecorator,
     this.buildDefaultDragHandles = true,
@@ -96,8 +98,10 @@ class ReorderableListView extends StatefulWidget {
     this.clipBehavior = Clip.hardEdge,
     this.autoScrollerVelocityScalar,
   }) : assert(
-         itemExtent == null || prototypeItem == null,
-         'You can only pass itemExtent or prototypeItem, not both',
+        (itemExtent == null && prototypeItem == null) ||
+        (itemExtent == null && itemExtentBuilder == null) ||
+        (prototypeItem == null && itemExtentBuilder == null),
+        'You can only pass one of itemExtent, prototypeItem and itemExtentBuilder.',
        ),
        assert(
          children.every((Widget w) => w.key != null),
@@ -142,6 +146,7 @@ class ReorderableListView extends StatefulWidget {
     this.onReorderStart,
     this.onReorderEnd,
     this.itemExtent,
+    this.itemExtentBuilder,
     this.prototypeItem,
     this.proxyDecorator,
     this.buildDefaultDragHandles = true,
@@ -163,8 +168,10 @@ class ReorderableListView extends StatefulWidget {
     this.autoScrollerVelocityScalar,
   }) : assert(itemCount >= 0),
        assert(
-         itemExtent == null || prototypeItem == null,
-         'You can only pass itemExtent or prototypeItem, not both',
+         (itemExtent == null && prototypeItem == null) ||
+         (itemExtent == null && itemExtentBuilder == null) ||
+         (prototypeItem == null && itemExtentBuilder == null),
+         'You can only pass one of itemExtent, prototypeItem and itemExtentBuilder.',
        );
 
   /// {@macro flutter.widgets.reorderable_list.itemBuilder}
@@ -269,6 +276,9 @@ class ReorderableListView extends StatefulWidget {
   /// {@macro flutter.widgets.list_view.itemExtent}
   final double? itemExtent;
 
+  /// {@macro flutter.widgets.list_view.itemExtentBuilder}
+  final ItemExtentBuilder? itemExtentBuilder;
+
   /// {@macro flutter.widgets.list_view.prototypeItem}
   final Widget? prototypeItem;
 
@@ -440,6 +450,7 @@ class _ReorderableListViewState extends State<ReorderableListView> {
           sliver: SliverReorderableList(
             itemBuilder: _itemBuilder,
             itemExtent: widget.itemExtent,
+            itemExtentBuilder: widget.itemExtentBuilder,
             prototypeItem: widget.prototypeItem,
             itemCount: widget.itemCount,
             onReorder: widget.onReorder,

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

@@ -16,6 +16,71 @@ import 'viewport_offset.dart';
 // CORE TYPES FOR SLIVERS
 // The RenderSliver base class and its helper types.
 
+/// Called to get the item extent by the index of item.
+///
+/// Used by [ListView.itemExtentBuilder] and [SliverVariedExtentList.itemExtentBuilder].
+typedef ItemExtentBuilder = double Function(int index, SliverLayoutDimensions dimensions);
+
+/// Relates the dimensions of the [RenderSliver] during layout.
+///
+/// Used by [ListView.itemExtentBuilder] and [SliverVariedExtentList.itemExtentBuilder].
+@immutable
+class SliverLayoutDimensions {
+  /// Constructs a [SliverLayoutDimensions] with the specified parameters.
+  const SliverLayoutDimensions({
+    required this.scrollOffset,
+    required this.precedingScrollExtent,
+    required this.viewportMainAxisExtent,
+    required this.crossAxisExtent
+  });
+
+  /// {@macro flutter.rendering.SliverConstraints.scrollOffset}
+  final double scrollOffset;
+
+  /// {@macro flutter.rendering.SliverConstraints.precedingScrollExtent}
+  final double precedingScrollExtent;
+
+  /// The number of pixels the viewport can display in the main axis.
+  ///
+  /// For a vertical list, this is the height of the viewport.
+  final double viewportMainAxisExtent;
+
+  /// The number of pixels in the cross-axis.
+  ///
+  /// For a vertical list, this is the width of the sliver.
+  final double crossAxisExtent;
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) {
+      return true;
+    }
+    if (other is! SliverLayoutDimensions) {
+      return false;
+    }
+    return other.scrollOffset == scrollOffset &&
+      other.precedingScrollExtent == precedingScrollExtent &&
+      other.viewportMainAxisExtent == viewportMainAxisExtent &&
+      other.crossAxisExtent == crossAxisExtent;
+  }
+
+  @override
+  String toString() {
+    return 'scrollOffset: $scrollOffset'
+      ' precedingScrollExtent: $precedingScrollExtent'
+      ' viewportMainAxisExtent: $viewportMainAxisExtent'
+      ' crossAxisExtent: $crossAxisExtent';
+  }
+
+  @override
+  int get hashCode => Object.hash(
+    scrollOffset,
+    precedingScrollExtent,
+    viewportMainAxisExtent,
+    viewportMainAxisExtent
+  );
+}
+
 /// The direction in which a sliver's contents are ordered, relative to the
 /// scroll offset axis.
 ///
@@ -224,12 +289,13 @@ class SliverConstraints extends Constraints {
   /// {@macro flutter.rendering.ScrollDirection.sample}
   final ScrollDirection userScrollDirection;
 
+  /// {@template flutter.rendering.SliverConstraints.scrollOffset}
   /// The scroll offset, in this sliver's coordinate system, that corresponds to
   /// the earliest visible part of this sliver in the [AxisDirection] if
-  /// [growthDirection] is [GrowthDirection.forward] or in the opposite
-  /// [AxisDirection] direction if [growthDirection] is [GrowthDirection.reverse].
+  /// [SliverConstraints.growthDirection] is [GrowthDirection.forward] or in the opposite
+  /// [AxisDirection] direction if [SliverConstraints.growthDirection] is [GrowthDirection.reverse].
   ///
-  /// For example, if [AxisDirection] is [AxisDirection.down] and [growthDirection]
+  /// For example, if [AxisDirection] is [AxisDirection.down] and [SliverConstraints.growthDirection]
   /// is [GrowthDirection.forward], then scroll offset is the amount the top of
   /// the sliver has been scrolled past the top of the viewport.
   ///
@@ -240,7 +306,7 @@ class SliverConstraints extends Constraints {
   ///
   /// For slivers whose top is not past the top of the viewport, the
   /// [scrollOffset] is `0` when [AxisDirection] is [AxisDirection.down] and
-  /// [growthDirection] is [GrowthDirection.forward]. The set of slivers with
+  /// [SliverConstraints.growthDirection] is [GrowthDirection.forward]. The set of slivers with
   /// [scrollOffset] `0` includes all the slivers that are below the bottom of the
   /// viewport.
   ///
@@ -249,9 +315,11 @@ class SliverConstraints extends Constraints {
   /// partially 'protrude in' from the bottom of the viewport.
   ///
   /// Whether this corresponds to the beginning or the end of the sliver's
-  /// contents depends on the [growthDirection].
+  /// contents depends on the [SliverConstraints.growthDirection].
+  /// {@endtemplate}
   final double scrollOffset;
 
+  /// {@template flutter.rendering.SliverConstraints.precedingScrollExtent}
   /// The scroll distance that has been consumed by all [RenderSliver]s that
   /// came before this [RenderSliver].
   ///
@@ -273,6 +341,7 @@ class SliverConstraints extends Constraints {
   /// content forever without reaching the end. For any [RenderSliver]s that
   /// appear after the infinite [RenderSliver], the [precedingScrollExtent] will
   /// be [double.infinity].
+  /// {@endtemplate}
   final double precedingScrollExtent;
 
   /// The number of pixels from where the pixels corresponding to the