Swift: How to increase loading speed of images from documents? - ios

Question

My app saved photo to the local documents folder and I used the UICollectionView to display all the image from that folder. But whenever I try to open the CollectionView it often took several seconds to open. I'm thinking that maybe the image files are too big, each photo is around 10MB. I also tried using thumbnails to display in collectionview but it still too slow. Any idea how to speed that up?
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! SPCell
// Configure the cell
cell.imageView.image = loadImage(fileName: self.fileURLs[indexPath.row])
return cell
}
func loadImagesFromDocuments(){
let fileManager = FileManager.default
let documentsURL = NSHomeDirectory() + "/Documents/Secure/"
do {
fileURLs = try fileManager.contentsOfDirectory(at: URL(string: documentsURL)!, includingPropertiesForKeys: nil)
} catch {
print("Error while enumerating files : \(error.localizedDescription)")
}
}
func loadImage(fileName: URL) -> UIImage? {
do {
let imageData = try Data(contentsOf: fileName)
return UIImage(data: imageData)
} catch {
print("Error loading image : \(error)")
}
return nil
}

Answer 1

Current problem is that you load the image every time the cell appears so Instead of
var fileURLs = [URL]()
Make it
var fileImages = [UIImage]()
Then inside viewDidLoad
fileImages = fileURLs.compactMap { self.loadImage(fileName: $0) }

Answer 2

you are synchronously loading images while returning every cell at indexPath.
cell.imageView.image = loadImage(fileName: self.fileURLs[indexPath.row])
Instead, you can create a custom UICollectionViewCell implementation with a variable in global scope say imageURL.
after cell initialization, do this:
cell.imageURL = self.fileURLs[indexPath.row]
and, in ViewDidLoad() of your custom class, add this line:
self.imageView.image = loadImage(fileName: self.imageURL)
doing so, will lead to images being loaded in each custom implementation of cell unblocking the thread which is invoking the DataSource of your CollectionView.