Optimizing Image Downloading for Better App Performance in iOS

Rahul Goel
2 min readSep 3, 2023

Enhancing App Responsiveness Through Image Management

“Image Downloading Made Fast”

Improving app performance in iOS for image downloading involves writing efficient Swift code that handles image download and rendering effectively. Here’s a step-by-step guide to help you achieve this:

1. Use a Dedicated Library/Framework:
— Consider using a library like Alamofire, SDWebImage, Kingfisher, or URLSession combined with caching for efficient image downloading. Each library has its own set of features and optimisations.

2. Asynchronous Image Downloads:
— Perform image downloads asynchronously to avoid blocking the UI. You can use closures or completion handlers for this purpose. Here’s a simple example using URLSession:

let imageUrl = URL(string: "https://example.com/image.jpg")!
URLSession.shared.dataTask(with: imageUrl) { data, response, error in
if let data = data, let image = UIImage(data: data) {
// Update UI with the downloaded image on the main thread
DispatchQueue.main.async {
imageView.image = image
}
} else {
// Handle errors
print("Error downloading image: \(error?.localizedDescription ?? "Unknown Error")")
}
}.resume()

3. Image Caching:
— Implement image caching to store downloaded images locally for quick retrieval. Libraries like Kingfisher and SDWebImage offer built-in caching mechanisms. For manual caching, you can use NSCache or FileManager to save and retrieve images efficiently.

4. Image Resizing:
— Resize images on the client side to match the display size. This reduces memory usage and speeds up rendering. Here’s a basic example of resizing an image:

func resizeImage(image: UIImage, targetSize: CGSize) -> UIImage {
let renderer = UIGraphicsImageRenderer(size: targetSize)
return renderer.image { (context) in
image.draw(in: CGRect(origin: .zero, size: targetSize))
}
}

5. Progressive Image Loading:
— Implement progressive image loading to display lower-quality versions initially and improve the quality as more data arrives. This can be achieved by downloading the image data in chunks and rendering it progressively.

6. Prefetching:
— Implement image prefetching to download images that are likely to be displayed soon based on user behavior and navigation patterns. You can use URLSession’s `URLSessionDataTask` to initiate these prefetch requests in advance.

9. Background Fetching:
— Use background fetching when appropriate to download images even when the app is not in the foreground. Be cautious of battery and data usage.

Remember that the specific optimisations needed may vary depending on your app’s requirements and architecture. It’s crucial to continuously monitor and improve performance as your app evolves.

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Rahul Goel

Computer Science Enthusiast | 11+ Year of Software Evolution | @Sharechat, Groupon, Paytm, Myntra https://www.linkedin.com/in/therahulgoel/