bottomToTop scrollOffset preservation (#121)

* bottomToTop scrollOffset preservation

When using the `bottomToTop` layoutDirection, we should preserve scroll position relative to the bottom of the collection view whenever we change bounds.

* fix formating

---------

Co-authored-by: Elfred Pagan <elfred.pagan@airbnb.com>

MagazineLayout/Public/MagazineLayout.swift

@@ -586,11 +586,25 @@ public final class MagazineLayout: UICollectionViewLayout {
   }
 
   override public func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
-    let isSameWidth = collectionView?.bounds.size.width.isEqual(
-      to: newBounds.size.width,
-      threshold: 1 / scale)
-      ?? false
-    return !isSameWidth || hasPinnedHeaderOrFooter
+    // When using the topToBottom layout direction, we only want to invalidate the layout when the
+    // widths differ. When using the bottomToTop layout direction, we want to invalidate on any
+    // size change due to the requirement of  needing to preserve scroll position from the bottom
+    var shouldInvalidateDueToSize = false
+    switch verticalLayoutDirection {
+    case .topToBottom:
+      shouldInvalidateDueToSize = !currentCollectionView.bounds.size.width.isEqual(
+        to: newBounds.size.width,
+        threshold: 1 / scale)
+    case .bottomToTop:
+      shouldInvalidateDueToSize = !(currentCollectionView.bounds.size.width.isEqual(
+        to: newBounds.size.width,
+        threshold: 1 / scale) &&
+      currentCollectionView.bounds.size.height.isEqual(
+        to: newBounds.size.height,
+        threshold: 1 / scale))
+    }
+
+    return shouldInvalidateDueToSize || hasPinnedHeaderOrFooter
   }
 
   override public func invalidationContext(
@@ -605,6 +619,14 @@ public final class MagazineLayout: UICollectionViewLayout {
       height: newBounds.height - currentCollectionView.bounds.height)
     invalidationContext.invalidateLayoutMetrics = false
 
+    // If our layout direction is bottom to top we want to adjust scroll position relative to the
+    // bottom
+    if case .bottomToTop = verticalLayoutDirection {
+      invalidationContext.contentOffsetAdjustment = CGPoint(
+        x: 0.0,
+        y: currentCollectionView.bounds.height - newBounds.height)
+    }
+
     return invalidationContext
   }