Add a way to reconfigure cell from IndexPaths

ios/chrome/browser/ui/collection_view/collection_view_controller.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/collection_view/collection_view_controller.h"
 
 #include "base/logging.h"
 #include "base/mac/foundation_util.h"
 #import "base/mac/scoped_nsobject.h"
 #import "ios/chrome/browser/ui/collection_view/cells/collection_view_item.h"
@@ skipping 56 matching lines @@
 - (void)loadModel {
   _collectionViewModel.reset([[CollectionViewModel alloc] init]);
 }
 
 - (void)reconfigureCellsForItems:(NSArray*)items
          inSectionWithIdentifier:(NSInteger)sectionIdentifier {
   for (CollectionViewItem* item in items) {
     NSIndexPath* indexPath =
         [self.collectionViewModel indexPathForItem:item
                            inSectionWithIdentifier:sectionIdentifier];
-    MDCCollectionViewCell* cell =
-        base::mac::ObjCCastStrict<MDCCollectionViewCell>(
-            [self.collectionView cellForItemAtIndexPath:indexPath]);
-
-    // |cell| may be nil if the row is not currently on screen.
-    if (cell) {
-      [item configureCell:cell];
-    }
+    [self reconfigureCellAtIndexPath:indexPath withItem:item];
   }
 }
 
+- (void)reconfigureCellsAtIndexPaths:(NSArray*)indexPaths {
+  for (NSIndexPath* indexPath in indexPaths) {
+    CollectionViewItem* item =
+        [self.collectionViewModel itemAtIndexPath:indexPath];
+    [self reconfigureCellAtIndexPath:indexPath withItem:item];
+  }
+}
+
 #pragma mark MDCCollectionViewEditingDelegate
 
 - (void)collectionView:(UICollectionView*)collectionView
     willDeleteItemsAtIndexPaths:(NSArray*)indexPaths {
   // Check that the parent class doesn't implement this method. Otherwise, it
   // would need to be called here.
   DCHECK([self isKindOfClass:[MDCCollectionViewController class]]);
   DCHECK(![MDCCollectionViewController instancesRespondToSelector:_cmd]);
 
   // Sort and enumerate in reverse order to delete the items from the collection
@@ skipping 163 matching lines @@
               targetContentOffset:(inout CGPoint*)targetContentOffset {
   MDCFlexibleHeaderView* headerView =
       self.appBar.headerViewController.headerView;
   if (scrollView == headerView.trackingScrollView) {
     [headerView
         trackingScrollViewWillEndDraggingWithVelocity:velocity
                                   targetContentOffset:targetContentOffset];
   }
 }
 
+#pragma mark - Private
+
+// Reconfigures the cell at |indexPath| by calling |configureCell:| with |item|.
+- (void)reconfigureCellAtIndexPath:(NSIndexPath*)indexPath
+                          withItem:(CollectionViewItem*)item {
+  MDCCollectionViewCell* cell =
+      base::mac::ObjCCastStrict<MDCCollectionViewCell>(
+          [self.collectionView cellForItemAtIndexPath:indexPath]);
+
+  // |cell| may be nil if the row is not currently on screen.
+  if (cell) {
+    [item configureCell:cell];
+  }
+}
+
 @end