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Fetching an Image with URLSession

Fetching an image from the internet is a common task in iOS development. This snippet shows how to do it cleanly using URLSession in Swift, with best practices like error handling and separation of concerns.

Details

URL: 🔗 https://developer.apple.com/documentation/foundation/urlsession

Source: 🔗 Book: Swift Programming

Author: [Author Name]

Tags:
Networking, URLSession, Swift, iOS, Clean Code

Platforms Supported: iOS, macOS

Swift Version: 5.x

Code​

import Foundation
import UIKit

class ImageDownloader {
    static func downloadImage(from urlString: String, completion: @escaping (Result<UIImage, Error>) -> Void) {
        guard let url = URL(string: urlString) else {
            completion(.failure(NetworkError.invalidURL))
            return
        }
        
        let session = URLSession(configuration: .default)
        let request = URLRequest(url: url)
        
        let task = session.dataTask(with: request) { data, response, error in
            if let error = error {
                completion(.failure(error))
                return
            }
            
            guard let data = data, let image = UIImage(data: data) else {
                completion(.failure(NetworkError.noData))
                return
            }
            
            completion(.success(image))
        }
        
        task.resume()
    }
    
    enum NetworkError: Error {
        case invalidURL
        case noData
    }
}

Usage​

class ImageViewController: UIViewController {
    let imageView = UIImageView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.addSubview(imageView)
        imageView.frame = view.bounds
        
        ImageDownloader.downloadImage(from: "https://imgs.xkcd.com/comics/api.png") { [weak self] result in
            DispatchQueue.main.async {
                switch result {
                case .success(let image):
                    self?.imageView.image = image
                case .failure(let error):
                    print("Failed to download image: \(error)")
                    // Handle error (e.g., show an alert to the user)
                }
            }
        }
    }
}

Discussion​

This refactored snippet demonstrates how to fetch an image using URLSession while adhering to clean code principles. By separating concerns, the ImageDownloader class handles the networking logic, and the ImageViewController focuses on UI updates. This approach improves code reusability, maintainability, and testability.

Key Points​

  • Error Handling: Uses the Result type for clear success and failure cases.
  • Code Reusability: Networking logic is encapsulated in a separate class.
  • Avoiding Force Unwrapping: Optional values are safely unwrapped using guard.
  • Separation of Concerns: The view controller is responsible only for UI logic.

Notes​

  • Flexibility: This pattern can be extended to fetch other types of data by adjusting the parsing logic in the ImageDownloader class.
  • Compatibility: Suitable for iOS and macOS with Swift 5.x.
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