DataImageView.swift

import SwiftUI
import UIKit

struct DataImageView<Placeholder: View>: View {

    
    private final class ViewModel: ObservableObject {

        @Published var image: UIImage?
        private let data: Data

        init(data: Data) {
            self.data = data
        }
        
        func load(size: CGSize) {
            DispatchQueue.global(qos: .userInitiated).async {
                let imageSourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
                let maxDimentionInPixels = max(size.width, size.height) * 2
                let downsampledOptions = [kCGImageSourceCreateThumbnailFromImageAlways: true,
                                          kCGImageSourceShouldCacheImmediately: true,
                                          kCGImageSourceCreateThumbnailWithTransform: true,
                                          kCGImageSourceThumbnailMaxPixelSize: maxDimentionInPixels] as CFDictionary
                
                let imageSource = CGImageSourceCreateWithData(self.data as CFData, imageSourceOptions)!
                let downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampledOptions)!
                
                DispatchQueue.main.async {
                    self.image = UIImage(cgImage: downsampledImage)
                }
            }
        }
        
    }

    @StateObject private var viewModel: ViewModel
    private let placeholder: Placeholder
    
    init(data: Data, @ViewBuilder placeholder: () -> Placeholder) {
        self.placeholder = placeholder()
        self._viewModel = .init(wrappedValue: .init(data: data))
    }

    
    var body: some View {
        GeometryReader { geometry in
            Group {
                if let uiImage = viewModel.image {
                    Image(uiImage: uiImage)
                        .resizable()
                } else {
                    placeholder
                }
            }.onAppear(perform: {
                viewModel.load(size: geometry.size)
            })
        }
        .clipped()
    }
}