avoid unneeded offset update

Author: m-sugawara

Example/Sources/View Controllers/AnnouncementExampleViewController.swift

@@ -31,6 +31,12 @@ class AnnouncementExampleViewController: MessagesViewController {
         conversationMessages = ConversationMessageMockFactory.getConversations()
     }
 
+    override func viewDidAppear(_ animated: Bool) {
+        super.viewDidAppear(animated)
+
+        messagesCollectionView.reloadDataAndKeepOffset()
+    }
+
     func configureMessageCollectionView() {
 
         if let layout = messagesCollectionView.collectionViewLayout as? MessagesCollectionViewFlowLayout {

Sources/Views/MessagesCollectionView.swift

@@ -126,6 +126,10 @@ open class MessagesCollectionView: UICollectionView {
         let newOffset = CGPoint(
             x: contentOffset.x + (afterContentSize.width - beforeContentSize.width),
             y: contentOffset.y + (afterContentSize.height - beforeContentSize.height))
+        // if offset.y is smaller than contentSize, no need to update contentOffset
+        guard newOffset.y >= contentSize.height else {
+            return
+        }
         setContentOffset(newOffset, animated: false)
     }