如何仅在更新时刷新collectionview或tableview中的数据？

Question

How can i only refresh or reload newly added data in collectionview or tableview instead of reload all the data? Right now i am using following code but it query the database sever to get all the data then reload everything. For example i upload only one photo on the sever so i just want to get that photo and reload collectionview only for this newly added photo not load everyting. how can i do that? Thank you!

func loadPosts(){

    let query = PFQuery(className: "posts")
    query.whereKey("username", equalTo:PFUser.current()!.username!) 
    query.limit = page
    query.findObjectsInBackground { (objects, error) in

        if error == nil {
            self.picArray.removeAll(keepingCapacity: false)

         if let objects  = objects{ 
            for object in objects{
                self.picArray.append(object.value(forKey: "pic") as! PFFile)
            }
          }
            self.collectionView?.reloadData()
        } else{
            print(error!.localizedDescription)
        }
    }
 }

Collection view function

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return picArray.count
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        //define cell

        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! pictureCell

        picArray[indexPath.row].getDataInBackground { (data, error) in

            if error == nil {
                cell.picImg.image = UIImage(data: data!)
            } else {
                print(error!.localizedDescription)
            }
        }

        return cell
    }

Answer 1

Eventually I would recommend moving the image request out of cellForItem because it will get called a lot depending on user scrolling behavior, but for now that will work.

You will want to check if the image has already been loaded then use reloadItems instead of reloadData so your new section in cellForItem will look something like:

if (cell.picImg.image == nil) {
    picArray[indexPath.row].getDataInBackground { (data, error) in

        if error == nil {
            cell.picImg.image = UIImage(data: data!)
            collectionView.reloadItems(at: indexPath) //reload only this item once the image is retrieved
        } else {
            print(error!.localizedDescription)
        }
    }
}