optimize layout calculation

Author: grant

Lib/CollectionViewPagingLayout.swift

@@ -43,13 +43,9 @@ public class CollectionViewPagingLayout: UICollectionViewLayout {
     }
 
     private var numberOfItems: Int {
-        var count = 0
-        if let sectionCount = collectionView?.numberOfSections {
-            for i in (0 ..< sectionCount) {
-                count += collectionView?.numberOfItems(inSection: i) ?? 0
-            }
-        }
-        return count
+        return (0..<(collectionView?.numberOfSections ?? 0))
+        .compactMap { collectionView?.numberOfItems(inSection: $0) }
+        .reduce(0, +)
     }
 
 
@@ -94,23 +90,19 @@ public class CollectionViewPagingLayout: UICollectionViewLayout {
         let endIndex = min(numberOfItems, initialEndIndex + startIndexOutOfBounds)
 
         var attributesArray: [UICollectionViewLayoutAttributes] = []
-        let sectionCount = collectionView?.numberOfSections ?? 0
+        var section = 0
+        var numberOfItemsInSection = collectionView?.numberOfItems(inSection: section) ?? 0
+        var numberOfItemsInPrevSections = 0
         for index in startIndex..<endIndex {
-            var section = 0
-            var row = 0
-            var accIndex = 0
-            for i in (0 ..< sectionCount) {
-                if index - accIndex < collectionView!.numberOfItems(inSection: i) {
-                    section = i
-                    row = index - accIndex
-                    break;
-                }
-                else{
-                    accIndex += collectionView!.numberOfItems(inSection: i)
-                }
+            var item = index - numberOfItemsInPrevSections
+            while item >= numberOfItemsInSection {
+                numberOfItemsInPrevSections += numberOfItemsInSection
+                section += 1
+                numberOfItemsInSection = collectionView?.numberOfItems(inSection: section) ?? 0
+                item = index - numberOfItemsInPrevSections
             }
-        
-            let cellAttributes = UICollectionViewLayoutAttributes(forCellWith: IndexPath(row: row, section: section))
+            
+            let cellAttributes = UICollectionViewLayoutAttributes(forCellWith: IndexPath(item: item, section: section))
             let pageIndex = CGFloat(index)
             let progress = pageIndex - currentScrollOffset
             var zIndex = Int(-abs(round(progress)))