Re-use methods to calculate leading and trailing garbage in RenderSliverMultiBoxAdaptor (#143884)

My RenderSliverMultiBoxAdaptor/RenderSliverFixedExtentList/RenderSliverVariedExtentList yak shave continues.

I've been subclassing RenderSliverVariedExtentList for SliverTree and have found some opportunities for clean up.
There is a larger clean up I'd like to do, but this week SliverTree comes first. 

I noticed these methods were getting repeated 🔁, and I was about to repeat them again 🔁 for the tree, so I figured bumping them up to the base class was better than continuing to copy-paste the same methods.

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

@@ -236,26 +236,6 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     }
   }
 
-  int _calculateLeadingGarbage(int firstIndex) {
-    RenderBox? walker = firstChild;
-    int leadingGarbage = 0;
-    while (walker != null && indexOf(walker) < firstIndex) {
-      leadingGarbage += 1;
-      walker = childAfter(walker);
-    }
-    return leadingGarbage;
-  }
-
-  int _calculateTrailingGarbage(int targetLastIndex) {
-    RenderBox? walker = lastChild;
-    int trailingGarbage = 0;
-    while (walker != null && indexOf(walker) > targetLastIndex) {
-      trailingGarbage += 1;
-      walker = childBefore(walker);
-    }
-    return trailingGarbage;
-  }
-
   int _getChildIndexForScrollOffset(double scrollOffset, ItemExtentBuilder callback) {
     if (scrollOffset == 0.0) {
       return 0;
@@ -323,8 +303,8 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
         getMaxChildIndexForScrollOffset(targetEndScrollOffset, deprecatedExtraItemExtent) : null;
 
     if (firstChild != null) {
-      final int leadingGarbage = _calculateLeadingGarbage(firstIndex);
-      final int trailingGarbage = targetLastIndex != null ? _calculateTrailingGarbage(targetLastIndex) : 0;
+      final int leadingGarbage = calculateLeadingGarbage(firstIndex: firstIndex);
+      final int trailingGarbage = targetLastIndex != null ? calculateTrailingGarbage(lastIndex: targetLastIndex) : 0;
       collectGarbage(leadingGarbage, trailingGarbage);
     } else {
       collectGarbage(0, 0);

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

@@ -597,8 +597,8 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
     final int? targetLastIndex = targetEndScrollOffset.isFinite ?
       layout.getMaxChildIndexForScrollOffset(targetEndScrollOffset) : null;
     if (firstChild != null) {
-      final int leadingGarbage = _calculateLeadingGarbage(firstIndex);
-      final int trailingGarbage = targetLastIndex != null ? _calculateTrailingGarbage(targetLastIndex) : 0;
+      final int leadingGarbage = calculateLeadingGarbage(firstIndex: firstIndex);
+      final int trailingGarbage = targetLastIndex != null ? calculateTrailingGarbage(lastIndex: targetLastIndex) : 0;
       collectGarbage(leadingGarbage, trailingGarbage);
     } else {
       collectGarbage(0, 0);
@@ -708,24 +708,4 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
     }
     childManager.didFinishLayout();
   }
-
-  int _calculateLeadingGarbage(int firstIndex) {
-    RenderBox? walker = firstChild;
-    int leadingGarbage = 0;
-    while (walker != null && indexOf(walker) < firstIndex) {
-      leadingGarbage += 1;
-      walker = childAfter(walker);
-    }
-    return leadingGarbage;
-  }
-
-  int _calculateTrailingGarbage(int targetLastIndex) {
-    RenderBox? walker = lastChild;
-    int trailingGarbage = 0;
-    while (walker != null && indexOf(walker) > targetLastIndex) {
-      trailingGarbage += 1;
-      walker = childBefore(walker);
-    }
-    return trailingGarbage;
-  }
 }

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

@@ -504,6 +504,48 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
     return null;
   }
 
+  /// Returns the number of children preceding the `firstIndex` that need to be
+  /// garbage collected.
+  ///
+  /// See also:
+  ///
+  ///   * [collectGarbage], which takes the leading and trailing number of
+  ///     children to be garbage collected.
+  ///   * [calculateTrailingGarbage], which similarly returns the number of
+  ///     trailing children to be garbage collected.
+  @visibleForTesting
+  @protected
+  int calculateLeadingGarbage({required int firstIndex}) {
+    RenderBox? walker = firstChild;
+    int leadingGarbage = 0;
+    while (walker != null && indexOf(walker) < firstIndex) {
+      leadingGarbage += 1;
+      walker = childAfter(walker);
+    }
+    return leadingGarbage;
+  }
+
+  /// Returns the number of children following the `lastIndex` that need to be
+  /// garbage collected.
+  ///
+  /// See also:
+  ///
+  ///   * [collectGarbage], which takes the leading and trailing number of
+  ///     children to be garbage collected.
+  ///   * [calculateLeadingGarbage], which similarly returns the number of
+  ///     leading children to be garbage collected.
+  @visibleForTesting
+  @protected
+  int calculateTrailingGarbage({required int lastIndex}) {
+    RenderBox? walker = lastChild;
+    int trailingGarbage = 0;
+    while (walker != null && indexOf(walker) > lastIndex) {
+      trailingGarbage += 1;
+      walker = childBefore(walker);
+    }
+    return trailingGarbage;
+  }
+
   /// Called after layout with the number of children that can be garbage
   /// collected at the head and tail of the child list.
   ///
@@ -513,6 +555,13 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
   /// This method also collects any children that were previously kept alive but
   /// are now no longer necessary. As such, it should be called every time
   /// [performLayout] is run, even if the arguments are both zero.
+  ///
+  /// See also:
+  ///
+  ///   * [calculateLeadingGarbage], which can be used to determine
+  ///     `leadingGarbage` here.
+  ///   * [calculateTrailingGarbage], which can be used to determine
+  ///     `trailingGarbage` here.
   @protected
   void collectGarbage(int leadingGarbage, int trailingGarbage) {
     assert(_debugAssertChildListLocked());

packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart

@@ -346,6 +346,29 @@ void main() {
     );
     expect(maxScrollOffset, 90.0);
   });
+
+  test('RenderSliverMultiBoxAdaptor has calculate leading and trailing garbage', () {
+    final List<RenderBox> children = <RenderBox>[
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+    ];
+    final TestRenderSliverBoxChildManager childManager = TestRenderSliverBoxChildManager(
+      children: children,
+    );
+    final RenderSliverFixedExtentList sliver = childManager.createRenderSliverFixedExtentList(30.0);
+    final RenderViewport root = RenderViewport(
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.zero(),
+      cacheExtent: 100,
+      children: <RenderSliver>[ sliver ],
+    );
+    layout(root);
+    // There are 3 children. If I want to garbage collect based on keeping only
+    // the middle child, then I should get 1 for leading and 1 for trailing.
+    expect(sliver.calculateLeadingGarbage(firstIndex: 1), 1);
+    expect(sliver.calculateTrailingGarbage(lastIndex: 1), 1);
+  });
 }
 
 int testGetMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {