Revert "fixes sliver list child layout offset calculation (#42703)" (#52923)

This reverts commit 11efa00f.

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

@@ -928,16 +928,13 @@ class SliverLogicalParentData extends ParentData {
   ///
   /// The number of pixels from from the zero scroll offset of the parent sliver
   /// (the line at which its [SliverConstraints.scrollOffset] is zero) to the
-  /// side of the child closest to that offset. A [layoutOffset] can be null
-  /// when it cannot be determined. The value will be set after layout.
+  /// side of the child closest to that offset.
   ///
   /// In a typical list, this does not change as the parent is scrolled.
-  ///
-  /// Defaults to null.
-  double layoutOffset;
+  double layoutOffset = 0.0;
 
   @override
-  String toString() => 'layoutOffset=${layoutOffset == null ? 'None': layoutOffset.toStringAsFixed(1)}';
+  String toString() => 'layoutOffset=${layoutOffset.toStringAsFixed(1)}';
 }
 
 /// Parent data for slivers that have multiple children and that position their

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

@@ -91,28 +91,8 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
     // it's possible for a child to get removed without notice.
     RenderBox leadingChildWithLayout, trailingChildWithLayout;
 
-    RenderBox earliestUsefulChild = firstChild;
-
-    // A firstChild with null layout offset is likely a result of children
-    // reordering.
-    //
-    // We rely on firstChild to have accurate layout offset. In the case of null
-    // layout offset, we have to find the first child that has valid layout
-    // offset.
-    if (childScrollOffset(firstChild) == null) {
-      int leadingChildrenWithoutLayoutOffset = 0;
-      while (childScrollOffset(earliestUsefulChild) == null) {
-        earliestUsefulChild = childAfter(firstChild);
-        leadingChildrenWithoutLayoutOffset += 1;
-      }
-      // We should be able to destroy children with null layout offset safely,
-      // because they are likely outside of viewport
-      collectGarbage(leadingChildrenWithoutLayoutOffset, 0);
-      assert(firstChild != null);
-    }
-
     // Find the last child that is at or before the scrollOffset.
-    earliestUsefulChild = firstChild;
+    RenderBox earliestUsefulChild = firstChild;
     for (double earliestScrollOffset = childScrollOffset(earliestUsefulChild);
         earliestScrollOffset > scrollOffset;
         earliestScrollOffset = childScrollOffset(earliestUsefulChild)) {
@@ -160,15 +140,12 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
           correction += paintExtentOf(firstChild);
           earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
         }
-        earliestUsefulChild = firstChild;
-        if ((correction - earliestScrollOffset).abs() > precisionErrorTolerance) {
-          geometry = SliverGeometry(
-            scrollOffsetCorrection: correction - earliestScrollOffset,
-          );
-          final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData as SliverMultiBoxAdaptorParentData;
-          childParentData.layoutOffset = 0.0;
-          return;
-        }
+        geometry = SliverGeometry(
+          scrollOffsetCorrection: correction - earliestScrollOffset,
+        );
+        final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData as SliverMultiBoxAdaptorParentData;
+        childParentData.layoutOffset = 0.0;
+        return;
       }
 
       final SliverMultiBoxAdaptorParentData childParentData = earliestUsefulChild.parentData as SliverMultiBoxAdaptorParentData;

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

@@ -578,6 +578,7 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
     assert(child != null);
     assert(child.parent == this);
     final SliverMultiBoxAdaptorParentData childParentData = child.parentData as SliverMultiBoxAdaptorParentData;
+    assert(childParentData.layoutOffset != null);
     return childParentData.layoutOffset;
   }
 

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

@@ -1069,7 +1069,6 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
     assert(_currentlyUpdatingChildIndex == null);
     try {
       final SplayTreeMap<int, Element> newChildren = SplayTreeMap<int, Element>();
-      final Map<int, double> indexToLayoutOffset = HashMap<int, double>();
 
       void processElement(int index) {
         _currentlyUpdatingChildIndex = index;
@@ -1081,31 +1080,17 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
         if (newChild != null) {
           _childElements[index] = newChild;
           final SliverMultiBoxAdaptorParentData parentData = newChild.renderObject.parentData as SliverMultiBoxAdaptorParentData;
-          if (index == 0) {
-            parentData.layoutOffset = 0.0;
-          } else if (indexToLayoutOffset.containsKey(index)) {
-            parentData.layoutOffset = indexToLayoutOffset[index];
-          }
           if (!parentData.keptAlive)
             _currentBeforeChild = newChild.renderObject as RenderBox;
         } else {
           _childElements.remove(index);
         }
       }
+
       for (final int index in _childElements.keys.toList()) {
         final Key key = _childElements[index].widget.key;
         final int newIndex = key == null ? null : widget.delegate.findIndexByKey(key);
-        final SliverMultiBoxAdaptorParentData childParentData =
-          _childElements[index].renderObject?.parentData as SliverMultiBoxAdaptorParentData;
-
-        if (childParentData != null && childParentData.layoutOffset != null)
-          indexToLayoutOffset[index] = childParentData.layoutOffset;
-
         if (newIndex != null && newIndex != index) {
-          // The layout offset of the child being moved is no longer accurate.
-          if (childParentData != null)
-            childParentData.layoutOffset = null;
-
           newChildren[newIndex] = _childElements[index];
           // We need to make sure the original index gets processed.
           newChildren.putIfAbsent(index, () => null);
@@ -1324,8 +1309,7 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
           break;
       }
 
-      return parentData.layoutOffset != null &&
-          parentData.layoutOffset < renderObject.constraints.scrollOffset + renderObject.constraints.remainingPaintExtent &&
+      return parentData.layoutOffset < renderObject.constraints.scrollOffset + renderObject.constraints.remainingPaintExtent &&
           parentData.layoutOffset + itemExtent > renderObject.constraints.scrollOffset;
     }).forEach(visitor);
   }

packages/flutter/test/rendering/slivers_block_test.dart

@@ -340,20 +340,18 @@ void main() {
     final SliverMultiBoxAdaptorParentData candidate = SliverMultiBoxAdaptorParentData();
     expect(candidate.keepAlive, isFalse);
     expect(candidate.index, isNull);
-    expect(candidate.toString(), 'index=null; layoutOffset=None');
+    expect(candidate.toString(), 'index=null; layoutOffset=0.0');
     candidate.keepAlive = null;
-    expect(candidate.toString(), 'index=null; layoutOffset=None');
+    expect(candidate.toString(), 'index=null; layoutOffset=0.0');
     candidate.keepAlive = true;
-    expect(candidate.toString(), 'index=null; keepAlive; layoutOffset=None');
+    expect(candidate.toString(), 'index=null; keepAlive; layoutOffset=0.0');
     candidate.keepAlive = false;
-    expect(candidate.toString(), 'index=null; layoutOffset=None');
+    expect(candidate.toString(), 'index=null; layoutOffset=0.0');
     candidate.index = 0;
-    expect(candidate.toString(), 'index=0; layoutOffset=None');
+    expect(candidate.toString(), 'index=0; layoutOffset=0.0');
     candidate.index = 1;
-    expect(candidate.toString(), 'index=1; layoutOffset=None');
+    expect(candidate.toString(), 'index=1; layoutOffset=0.0');
     candidate.index = -1;
-    expect(candidate.toString(), 'index=-1; layoutOffset=None');
-    candidate.layoutOffset = 100.0;
-    expect(candidate.toString(), 'index=-1; layoutOffset=100.0');
+    expect(candidate.toString(), 'index=-1; layoutOffset=0.0');
   });
 }

packages/flutter/test/widgets/sliver_list_test.dart

@@ -163,118 +163,6 @@ void main() {
     expect(find.text('Tile 1'), findsOneWidget);
     expect(find.text('Tile 2'), findsOneWidget);
   });
-
-  testWidgets('SliverList should recalculate inaccurate layout offset case 1', (WidgetTester tester) async {
-    // Regression test for https://github.com/flutter/flutter/issues/42142.
-    final List<int> items = List<int>.generate(20, (int i) => i);
-    final ScrollController controller = ScrollController();
-    await tester.pumpWidget(
-      _buildSliverList(
-        items: List<int>.from(items),
-        controller: controller,
-        itemHeight: 50,
-        viewportHeight: 200,
-      )
-    );
-    await tester.pumpAndSettle();
-
-    await tester.drag(find.text('Tile 2'), const Offset(0.0, -1000.0));
-    await tester.pumpAndSettle();
-
-    // Viewport should be scrolled to the end of list.
-    expect(controller.offset, 800.0);
-    expect(find.text('Tile 15'), findsNothing);
-    expect(find.text('Tile 16'), findsOneWidget);
-    expect(find.text('Tile 17'), findsOneWidget);
-    expect(find.text('Tile 18'), findsOneWidget);
-    expect(find.text('Tile 19'), findsOneWidget);
-
-    // Prepends item to the list.
-    items.insert(0, -1);
-    await tester.pumpWidget(
-      _buildSliverList(
-        items: List<int>.from(items),
-        controller: controller,
-        itemHeight: 50,
-        viewportHeight: 200,
-      )
-    );
-    await tester.pump();
-    // We need second pump to ensure the scheduled animation gets run.
-    await tester.pumpAndSettle();
-    // Scroll offset should stay the same, and the items in viewport should be
-    // shifted by one.
-    expect(controller.offset, 800.0);
-    expect(find.text('Tile 14'), findsNothing);
-    expect(find.text('Tile 15'), findsOneWidget);
-    expect(find.text('Tile 16'), findsOneWidget);
-    expect(find.text('Tile 17'), findsOneWidget);
-    expect(find.text('Tile 18'), findsOneWidget);
-    expect(find.text('Tile 19'), findsNothing);
-
-    // Drags back to beginning and newly added item is visible.
-    await tester.drag(find.text('Tile 16'), const Offset(0.0, 1000.0));
-    await tester.pumpAndSettle();
-    expect(controller.offset, 0.0);
-    expect(find.text('Tile -1'), findsOneWidget);
-    expect(find.text('Tile 0'), findsOneWidget);
-    expect(find.text('Tile 1'), findsOneWidget);
-    expect(find.text('Tile 2'), findsOneWidget);
-    expect(find.text('Tile 3'), findsNothing);
-
-  });
-
-  testWidgets('SliverList should recalculate inaccurate layout offset case 2', (WidgetTester tester) async {
-    // Regression test for https://github.com/flutter/flutter/issues/42142.
-    final List<int> items = List<int>.generate(20, (int i) => i);
-    final ScrollController controller = ScrollController();
-    await tester.pumpWidget(
-      _buildSliverList(
-        items: List<int>.from(items),
-        controller: controller,
-        itemHeight: 50,
-        viewportHeight: 200,
-      )
-    );
-    await tester.pumpAndSettle();
-
-    await tester.drag(find.text('Tile 2'), const Offset(0.0, -1000.0));
-    await tester.pumpAndSettle();
-
-    // Viewport should be scrolled to the end of list.
-    expect(controller.offset, 800.0);
-    expect(find.text('Tile 15'), findsNothing);
-    expect(find.text('Tile 16'), findsOneWidget);
-    expect(find.text('Tile 17'), findsOneWidget);
-    expect(find.text('Tile 18'), findsOneWidget);
-    expect(find.text('Tile 19'), findsOneWidget);
-
-    // Reorders item to the front. This should make item 19 to be first child
-    // with layout offset = null.
-    final int swap = items[19];
-    items[19] = items[3];
-    items[3] = swap;
-
-    await tester.pumpWidget(
-      _buildSliverList(
-        items: List<int>.from(items),
-        controller: controller,
-        itemHeight: 50,
-        viewportHeight: 200,
-      )
-    );
-    await tester.pump();
-    // We need second pump to ensure the scheduled animation gets run.
-    await tester.pumpAndSettle();
-    // Scroll offset should stay the same
-    expect(controller.offset, 800.0);
-    expect(find.text('Tile 14'), findsNothing);
-    expect(find.text('Tile 15'), findsNothing);
-    expect(find.text('Tile 16'), findsOneWidget);
-    expect(find.text('Tile 17'), findsOneWidget);
-    expect(find.text('Tile 18'), findsOneWidget);
-    expect(find.text('Tile 3'), findsOneWidget);
-  });
 }
 
 Widget _buildSliverListRenderWidgetChild(List<String> items) {
@@ -328,11 +216,6 @@ Widget _buildSliverList({
                     child: Text('Tile ${items[i]}'),
                   );
                 },
-                findChildIndexCallback: (Key key) {
-                  final ValueKey<int> valueKey = key as ValueKey<int>;
-                  final int index = items.indexOf(valueKey.value);
-                  return index == -1 ? null : index;
-                },
                 childCount: items.length,
               ),
             ),

packages/flutter/test/widgets/slivers_test.dart

@@ -50,8 +50,7 @@ Future<void> testSliverFixedExtentList(WidgetTester tester, List<String> items)
               findChildIndexCallback: (Key key) {
                 final ValueKey<String> valueKey = key as ValueKey<String>;
                 final String data = valueKey.value;
-                final int index = items.indexOf(data);
-                return index == -1 ? null : index;
+                return items.indexOf(data);
               },
             ),
           ),