Swift: asynchronously loading and displaying photos

Question

I\'m struggling with the problem of displaying photo gallery from iPhone to collectionView.

Everything works fine if someone has 50 photos inside t

Answer 1

What you need is to add a fetchResult property to your collection view controller and fetch your image Assets inside viewDidLoad method.

var fetchResult: PHFetchResult = PHFetchResult()

func fetchAssets() {
    let fetchOptions = PHFetchOptions()
    fetchOptions.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: false)]
    fetchResult = PHAsset.fetchAssets(with: .image, options: fetchOptions)
}

override func viewDidLoad() {
    super.viewDidLoad()
    fetchAssets()
}

Next step is extend UIImageView to request the image data asynchronously setting its image on completion.

extension UIImageView {    
    func fetchImage(asset: PHAsset, contentMode: PHImageContentMode, targetSize: CGSize, version:  PHImageRequestOptionsVersion = .current, deliveryMode: PHImageRequestOptionsDeliveryMode = .opportunistic) {
        let options = PHImageRequestOptions()
        options.version = version
        options.deliveryMode = deliveryMode
        PHImageManager.default().requestImage(for: asset, targetSize: targetSize, contentMode: contentMode, options: options) { image, _ in
            guard let image = image else { return }
            switch contentMode {
            case .aspectFill: self.contentMode = .scaleAspectFill
            case .aspectFit:  self.contentMode = .scaleAspectFit
            @unknown default: fatalError()
            }
            self.image = image
        }
    }
}

Now you can fetch the images one at a time inside collection view cellForItemAt method:

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! CollectionViewCell
    let asset = fetchResult.object(at: indexPath.row)
    cell.imageView.fetchImage(asset: asset, contentMode: .aspectFill, targetSize: cell.imageView.frame.size * UIScreen.main.scale )
    return cell
}

Don't forget to return the fetchResult count for numberOfItemsInSection method.

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

---

class CollectionViewCell: UICollectionViewCell {
    @IBOutlet weak var imageView: UIImageView!
}

---

extension CGSize {
    static func *(lhs: CGSize, rhs: CGFloat) -> CGSize {
        .init(width: lhs.width * rhs, height: lhs.height * rhs)
    }
}