ImageDownloading.swift

import Combine
import UIKit

@propertyWrapper 
class ImageDownloading {
    var wrappedValue: UIImageView
    private var cancellable: AnyCancellable?
    
    init(wrappedValue: UIImageView, url: URL?, urlSession: URLSession) {
        self.wrappedValue = wrappedValue
        wrappedValue.image = UIImage(named: "placeholder")
        guard let url = url else { return }
        performFetch(from: url, using: urlSession)
    }
    
    
    func performFetch(from url: URL, using urlSession: URLSession) {
        cancellable = urlSession.dataTaskPublisher(for: url)
        .subscribe(on: DispatchQueue.global(qos: .background)) // Force the background thread
        .map(\.data)
        .map{ UIImage(data: $0) }
        .replaceError(with: UIImage(named: "placeholder"))
        .receive(on: RunLoop.main) // Hop back on the main thread
        .assign(to: \.image, on: wrappedValue)
    }
}

// Usage
@ImageDownloading(url: URL(string: "http://alexanderlogan.co.uk/images/snippit.png"), urlSession: .shared)
var headerImageView: UIImageView = UIImageView()