How to set a custom prefetching time for LazyVStack in SwiftUI?

Question

TL;DR: Is there some parameter or way to set the offset at which LazyVStack initialises views?

LazyVStack initialises the views lazily, so when I scroll, the next (few?) views are initialised. I am loading an image once a view is drawn, using SDWebImage Package in swift. This takes a view milliseconds, and since I am using a LazyVStack, if one scrolls fast (even within reasonable limits), the placeholder is visible for a short moment, because the view has just been created a (too) short moment ago. If I scroll very slowly, the image loads just before the view appears, so no placeholder is visible.

If I could make the LazyVStack initialise the views just a few milliseconds earlier my problem would be gone...

Once would think this is a pretty common problem, timing this initialisation just right so as not to load too early or too late.. but nothing at all in the docs about this

Answer 1

Quick answer to the question: no

---

That being said, in this case there is still a solution: Since I was using SDWebImageSwiftUI before, simply calling the following already before the view starts to initialise solved my problem:

SDWebImagePrefetcher.shared.prefetchURLs(urls) { finishedCount, skippedCount in
     print("preloading complete")
}

then in my LazyVStack I use:

LazyVStack {
     ForEach(items, id: \.self) { item in
             ItemView(item: item)
                   .onAppear {
                        // calling function to prefetch next x-items by their url
                  }
            }
     }
}

Answer 2

this process is called as prefetching -because you're prefetching them so it will look smooth-

And sorry, but there's no way to access prefetching of LazyVStack in SwiftUI right now. Also, keep in mind that both SwiftUI's Grid And LazyH/VStack is not performant as UIKit's UICollectionView. So what you could do here is you can use UICollectionView's UICollectionViewDataSourcePrefetching protocol in your collection view's data.

I used SDWebImage Library to Fetch Images from internet (one of the most popular libraries for UIKit)

I tried to explain everything as comments in the code so give your attention to them, here's what it looks like:

here's the code:

import SwiftUI
import SDWebImage

struct CollectionView: UIViewRepresentable {
    let items: [String]
    
    func makeUIView(context: Context) -> UICollectionView {
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: UICollectionViewFlowLayout())
        collectionView.register(ImageCell.self, forCellWithReuseIdentifier: "ImageCell")
        collectionView.delegate = context.coordinator
        collectionView.dataSource = context.coordinator
        return collectionView
    }
    
    func updateUIView(_ uiView: UICollectionView, context: Context) {
        // Reload the collection view data if the items array changes
        uiView.reloadData()
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
    
    class Coordinator: NSObject, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout, UICollectionViewDataSourcePrefetching {
        let parent: CollectionView
        
        init(_ collectionView: CollectionView) {
            self.parent = collectionView
        }
        
        func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
            return parent.items.count
        }
        
        func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
            let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "ImageCell", for: indexPath) as! ImageCell
            let item = parent.items[indexPath.item]
            
            // Set the progress of the progress view as the image is being downloaded
            cell.progressView.progress = 0.0
            SDWebImageDownloader.shared.downloadImage(with: URL(string: item), options: .highPriority, progress: { (receivedSize, expectedSize, url) in
                DispatchQueue.main.async {
                    cell.progressView.progress = Float(receivedSize) / Float(expectedSize)
                }
            }) { (image, data, error, finished) in
                DispatchQueue.main.async {
                    cell.imageView.sd_setImage(with: URL(string: item))
                    cell.progressView.isHidden = true
                }
            }
            return cell
        }
        
        func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
            return CGSize(width: 100, height: 100)
        }
        
        // MARK: - UICollectionViewDataSourcePrefetching
        
        func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
            // Filter the index paths to only include the ones that are within the desired range, trick relies on here
            // In our example, i'm fetching 6 items beforehand which equals 2 rows, so i'm prefetching 2 rows beforehand. you can increase that amount if you w ant to
            let prefetchIndexPaths = indexPaths.filter { $0.item < collectionView.numberOfItems(inSection: $0.section) - 6 }
            let urls = prefetchIndexPaths.compactMap { URL(string: self.parent.items[$0.item])! }
            SDWebImagePrefetcher.shared.prefetchURLs(urls)
        }
        
        func collectionView(_ collectionView: UICollectionView, cancelPrefetchingForItemsAt indexPaths: [IndexPath]) {
            // Cancel the prefetching for the given index paths, this is not required but i wanted to add it
            let urls = indexPaths.map { URL(string: self.parent.items[$0.item]) }
        }
    }
}

class ImageCell: UICollectionViewCell {
    let imageView = UIImageView()
    let progressView = UIProgressView()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        addSubview(imageView)
        addSubview(progressView)
        // if you're not familiar with uikit this is just a disgusting uikit code to make proper layouts :(
        progressView.translatesAutoresizingMaskIntoConstraints = false
        progressView.topAnchor.constraint(equalTo: topAnchor).isActive = true
        progressView.leadingAnchor.constraint(equalTo: leadingAnchor).isActive = true
        progressView.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
        
        imageView.translatesAutoresizingMaskIntoConstraints = false
        imageView.topAnchor.constraint(equalTo: progressView.bottomAnchor).isActive = true
        imageView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
        imageView.leadingAnchor.constraint(equalTo: leadingAnchor).isActive = true
        imageView.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

and Here's how you can implement it to swiftui:

struct ContentView: View {
    var items : [String] {
        var i = 0
        var _items = [String]()
        while (i < 900) {
            _items.append("https://picsum.photos/\(Int.random(in: 300..<600))/\(Int.random(in: 300..<600))")
            i = i + 1
        }
        return _items
    }
    
    var body: some View {
        CollectionView(items: items)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

I used lorem picsum which is a website for generating random images and that's why you see images reloading randomly in my sample(that white ones), in your case, this shouldn't be a problem

Answer 3

You can try adding extra space to the ScrollView and removing it using .padding:

private enum Constant {
    static let topInset: CGFloat = UIScreen.main.bounds.height * 0.4
    static let bottomOffset: CGFloat = UIScreen.main.bounds.height * 0.4
}

struct PrefetchedScrollView<Content: View>: View {
    let axes: Axis.Set
    let showsIndicators: Bool
    let content: () -> Content

   var body: some View {
        ScrollView(axes, showsIndicators: showsIndicators) {
            Spacer(minLength: Constant.topInset)
            content()
            Spacer(minLength: Constant.bottomOffset)
        }
        .padding(top: -Constant.topInset, bottom: -Constant.bottomOffset)
    }
}

PS: This method adds a bug with pull to refresh, but this bug is easy to fix, using a custom pull to refresh implementation