ios - 为什么 dequeueReusableCell(withReuseIdentifier :for:) is nil outside the cellForItemAtIndexPath function scope?

Question

您好，我试图在 NSFetchedResultsController 的委托(delegate)函数中修改 UICollectionView 的单元格

controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?)

通过 dequeueReusableCell(withReuseIdentifier:for:) 函数。但它总是返回零。所以我不得不使用 cellForItemAtIndexPath 函数。但是为什么 dequeueReusableCell 返回 nil。当然，我之前已经注册过类(class)。

 collectionView?.register(MessageCell.self, forCellWithReuseIdentifier: cellId)

代码如下:

   if type == .update {
        print("UPDATE")
        blockOperations.append(BlockOperation(block: {              
           let cell = self.collectionView?.cellForItem(at: indexPath!)
             as! MessageCell          
 //   let cell = self.collectionView.dequeueReusableCell(withReuseIdentifier: self.cellId, for: indexPath!)  returns nil. why?
            let friend = self.fetchedResultsController.object(at: indexPath!) as! Friend               
            cell.message = friend.lastMessage

        }))
    }

Answer 1

首先你确实需要 cellForItem ，其次它会返回一个当前可见的单元格

   collec.cellForItem(at: NSIndexPath.init(row: 0, section: 0  ) as IndexPath)