Existing CollectionView Image changes on scroll - ios

Question

I have looked at a lot of questions and I do not believe this is a result of reusing a cell as the new cells image is correct, but and existing cells image is incorrect and used to be correct. I'll post the images first so the issue is easier to understand.
I have a collectionView of image cells (similar to Instagrams user page). I'm fetching all of the data from Firebase. I get the first 12 posts on the initial loading of the screen. However, if you scroll down quickly an EXISTING cells image changes to a newly fetched image. I'm not sure why this is happening... Maybe it's a caching issue? The issue only occurs the first time you load the screen. I've tried setting the images to nil like this:
override func prepareForReuse() {
super.prepareForReuse()
self.imageView.image = UIImage()
}
This didn't help the issue though.
Here is my cellForItemAt:
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "imageCell", for: indexPath) as! ImageCell
cell.indexPath = indexPath
cell.imageView.downloadImage(from: currentTablePosts[indexPath.row].pathToImage)
cell.layer.borderWidth = 1
cell.layer.borderColor = UIColor.black.cgColor
return cell
}
Image downloading and caching:
let imageCache = NSCache<NSString, UIImage>()
extension UIImageView {
func downloadImage(from imgURL: String!) {
let url = URLRequest(url: URL(string: imgURL)!)
// set initial image to nil so it doesn't use the image from a reused cell
image = nil
// check if the image is already in the cache
if let imageToCache = imageCache.object(forKey: imgURL! as NSString) {
self.image = imageToCache
return
}
// download the image asynchronously
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if error != nil {
// user an alert to display the error
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async {
// create UIImage
let imageToCache = UIImage(data: data!)
// add image to cache
imageCache.setObject(imageToCache!, forKey: imgURL! as NSString)
self.image = imageToCache
}
}
task.resume()
}
}
Firebase Queries:
static func getInitialTablesPosts(tableNumber: String) {
tableReference.child(tableNumber).queryLimited(toLast: 12).observeSingleEvent(of: .value, with: { snap in
for child in snap.children {
let child = child as? DataSnapshot
if let post = child?.value as? [String: AnyObject] {
let posst = Post()
if let author = post["author"] as? String, let likes = post["likes"] as? Int, let pathToImage = post["pathToImage"] as? String, let postID = post["postID"] as? String, let postDescription = post["postDescription"] as? String, let timestamp = post["timestamp"] as? Double, let category = post["category"] as? String, let table = post["group"] as? String, let userID = post["userID"] as? String, let numberOfComments = post["numberOfComments"] as? Int, let region = post["region"] as? String {
posst.author = author
posst.likes = likes
posst.pathToImage = pathToImage
posst.postID = postID
posst.userID = userID
posst.fancyPostDescription = Helper.createAttributedString(author: author, postText: postDescription)
posst.postDescription = author + ": " + postDescription
posst.timestamp = timestamp
posst.table = table
posst.region = region
posst.category = category
posst.numberOfComments = numberOfComments
posst.userWhoPostedLabel = Helper.createAttributedPostLabel(username: author, table: table, region: region, category: category)
if let people = post["peopleWhoLike"] as? [String: AnyObject] {
for(_, person) in people {
posst.peopleWhoLike.append(person as! String)
}
}
currentTablePosts.insert(posst, at: 0)
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "reloadTableCollectionView"), object: nil)
} // end of if let
}
}
})
tableReference.removeAllObservers()
}
static func getMoreTablePosts(tableNumber: String, lastVisibleKey: String) {
print("FIRED...")
let currentNumberOfPosts = currentTablePosts.count
print("Number of posts before fetiching ", currentNumberOfPosts)
print("Oldest post key ", oldestTableKeys[tableNumber] ?? "not set yet", "***********")
tableReference.child(tableNumber).queryOrderedByKey().queryEnding(atValue: lastVisibleKey).queryLimited(toLast: 12).observeSingleEvent(of: .value, with: { snap in
for child in snap.children {
let child = child as? DataSnapshot
if let post = child?.value as? [String: AnyObject] {
if let id = post["postID"] as? String {
if id == lastVisibleKey {
return
}
}
let posst = Post()
if let author = post["author"] as? String, let likes = post["likes"] as? Int, let pathToImage = post["pathToImage"] as? String, let postID = post["postID"] as? String, let postDescription = post["postDescription"] as? String, let timestamp = post["timestamp"] as? Double, let category = post["category"] as? String, let table = post["group"] as? String, let userID = post["userID"] as? String, let numberOfComments = post["numberOfComments"] as? Int, let region = post["region"] as? String {
posst.author = author
posst.likes = likes
posst.pathToImage = pathToImage
posst.postID = postID
posst.userID = userID
posst.fancyPostDescription = Helper.createAttributedString(author: author, postText: postDescription)
posst.postDescription = author + ": " + postDescription
posst.timestamp = timestamp
posst.table = table
posst.region = region
posst.category = category
posst.numberOfComments = numberOfComments
posst.userWhoPostedLabel = Helper.createAttributedPostLabel(username: author, table: table, region: region, category: category)
if let people = post["peopleWhoLike"] as? [String: AnyObject] {
for(_, person) in people {
posst.peopleWhoLike.append(person as! String)
}
}
currentTablePosts.insert(posst, at: currentNumberOfPosts)
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "reloadTableCollectionView"), object: nil)
if let oldestTableKey = oldestTableKeys[tableNumber] {
if postID == oldestTableKey {
print("returning")
print("number of posts on return \(currentTablePosts.count)")
return
}
}
} // end if let
}
}
})
tableReference.removeAllObservers()
}

Answer 1

* UPDATE *
The image caching used here still had some issues in my experience. I have since moved on to using Kingfisher which is extremely easy to setup and use.
* OLD SOLUTION *
Found a solution based on the answer suggested in the comments.
I modified the extension I am using to cache my images. Although in the future I think I will subclass UIImageView. Here is the modified version of my code.
import UIKit
let userImageCache = NSCache<NSString, UIImage>()
let imageCache = NSCache<AnyObject, AnyObject>()
var imageURLString: String?
extension UIImageView {
public func imageFromServerURL(urlString: String, collectionView: UICollectionView, indexpath : IndexPath) {
imageURLString = urlString
if let url = URL(string: urlString) {
image = nil
if let imageFromCache = imageCache.object(forKey: urlString as AnyObject) as? UIImage {
self.image = imageFromCache
return
}
URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in
if error != nil{
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async(execute: {
if let imgaeToCache = UIImage(data: data!){
if imageURLString == urlString {
self.image = imgaeToCache
}
imageCache.setObject(imgaeToCache, forKey: urlString as AnyObject)// calls when scrolling
collectionView.reloadItems(at: [indexpath])
}
})
}) .resume()
}
}
func downloadImage(from imgURL: String!) {
let url = URLRequest(url: URL(string: imgURL)!)
// set initial image to nil so it doesn't use the image from a reused cell
image = nil
// check if the image is already in the cache
if let imageToCache = imageCache.object(forKey: imgURL! as AnyObject) as? UIImage {
self.image = imageToCache
return
}
// download the image asynchronously
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if error != nil {
// user an alert to display the error
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async {
let imageToCache = UIImage(data: data!)
imageCache.setObject(imageToCache!, forKey: imgURL! as AnyObject)
self.image = imageToCache
}
}
task.resume()
}
func downloadUserImage(from imgURL: String!) {
let url = URLRequest(url: URL(string: imgURL)!)
// set initial image to nil so it doesn't use the image from a reused cell
image = nil
// check if the image is already in the cache
if let imageToCache = userImageCache.object(forKey: imgURL! as NSString) {
self.image = imageToCache
return
}
// download the image asynchronously
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if error != nil {
// user an alert to display the error
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async {
// create UIImage
let imageToCache = UIImage(data: data!)
// add image to cache
userImageCache.setObject(imageToCache!, forKey: imgURL! as NSString)
self.image = imageToCache
}
}
task.resume()
}
}
I created the first method to cache the images for the collectionView and the other methods are still used for tableViews. The cache was also changed from let imageCache = NSCache<NSString, UIImage>() to let imageCache = NSCache<AnyObject, AnyObject>()