CollectionView flash when reloadData - ios

Question

I have a collectionView which is populated with around 60 images downloading from parse. These images can be updated depending if any new ones have been uploaded.
But my problem is after I load the view, and I refresh the data using PullToRefresh function, the collection view Flashes white and then displays the images again...
here's a video to show you :
https://www.youtube.com/watch?v=qizaAbUnzYQ&feature=youtu.be
I have been trying to fix this all day & find a solution, but I have had no success..!
Heres how I'm querying the images :
func loadPosts() {
self.activityView.startAnimating()
let followQuery = PFQuery(className: "Follows")
followQuery.whereKey("follower", equalTo: PFUser.currentUser()!.username!)
followQuery.findObjectsInBackgroundWithBlock ({ (objects:[PFObject]?, error:NSError?) -> Void in
if error == nil {
self.followArray.removeAll(keepCapacity: false)
for object in objects! {
self.followArray.append(object.valueForKey("following") as! String)
}
let query = PFQuery(className: "Posts")
query.limit = self.page
query.whereKey("username", notContainedIn: self.followArray)
query.whereKey("username", notEqualTo: PFUser.currentUser()!.username!)
query.findObjectsInBackgroundWithBlock ({ (objects:[PFObject]?, error:NSError?) -> Void in
if error == nil {
self.postImage.removeAll(keepCapacity: false)
self.uuidArray.removeAll(keepCapacity: false)
self.usernameArray.removeAll(keepCapacity: false)
for object in objects! {
self.postImage.append(object.valueForKey("image") as! PFFile)
self.uuidArray.append(object.valueForKey("uuid") as! String)
self.usernameArray.append(object.valueForKey("username") as! String)
}
} else {
print(error!.localizedDescription)
}
self.collectionView.reloadData()
self.refresher.endRefreshing()
self.activityView.stopAnimating()
self.boxView.removeFromSuperview()
})
}
})
}
And here is how I am pulling to refresh:
override func viewDidLoad() {
super.viewDidLoad()
refresher.addTarget(self, action: "reload", forControlEvents: UIControlEvents.ValueChanged)
collectionView.addSubview(refresher)
loadPosts()
}
func reload() {
collectionView.reloadData()
refresher.endRefreshing()
}

Answer 1

I assume that the UUID's are unique for every post, so you can check to see if the count from the previous load is different from the current, then you can see which posts are new, figure out their index path then only reload those index paths. I used sets to determine which id's had been added, which will work assuming you don't want to display the same post twice. There might be a better way of doing it, but in general you need to do something similar to the following:
func loadPosts() {
self.activityView.startAnimating()
let followQuery = PFQuery(className: "Follows")
followQuery.whereKey("follower", equalTo: PFUser.currentUser()!.username!)
followQuery.findObjectsInBackgroundWithBlock ({ (objects:[PFObject]?, error:NSError?) -> Void in
if error == nil {
self.followArray.removeAll(keepCapacity: false)
for object in objects! {
self.followArray.append(object.valueForKey("following") as! String)
}
let query = PFQuery(className: "Posts")
query.limit = self.page
query.whereKey("username", notContainedIn: self.followArray)
query.whereKey("username", notEqualTo: PFUser.currentUser()!.username!)
query.findObjectsInBackgroundWithBlock ({ (objects:[PFObject]?, error:NSError?) -> Void in
if error == nil {
let oldUUIDArray = self.uuidArray
self.postImage.removeAll(keepCapacity: false)
self.uuidArray.removeAll(keepCapacity: false)
self.usernameArray.removeAll(keepCapacity: false)
for object in objects! {
self.postImage.append(object.valueForKey("image") as! PFFile)
self.uuidArray.append(object.valueForKey("uuid") as! String)
self.usernameArray.append(object.valueForKey("username") as! String)
}
let uuidOldSet = Set(oldUUIDArray)
let uuidNewSet = Set(self.uuidArray)
let missingUUIDs = uuidNewSet.subtract(uuidOldSet)
let missingUUIDArray = Array(missingUUIDs)
let missingUUIDIndexPaths = missingUUIDArray.map{NSIndexPath(forItem:self.uuidArray.indexOf($0)!,inSe ction:0)}
let extraUUIDs = uuidOldSet.subtract(uuidNewSet)
let extraUUIDArray = Array(extraUUIDs)
let extraUUIDIndexPaths = extraUUIDArray.map{NSIndexPath(forItem:oldUUIDArray.indexOf($0)!,inSection:0)}
self.collectionView.performBatchUpdates({
if extraUUIDIndexPath != nil {
self.collectionView.deleteItemsAtIndexPaths(extraUUIDIndexPaths)
}
if missingUUIDIndexPaths != nil {self.collectionView.insertItemsAtIndexPaths(missingUUIDIndexPaths)}
}, completion: nil)
} else {
print(error!.localizedDescription)
}
self.refresher.endRefreshing()
self.activityView.stopAnimating()
self.boxView.removeFromSuperview()
})
}
})
}
func reload() {
self.loadPosts()
refresher.endRefreshing()
}