Why cache prompt is not shown? - swiftui

Question

I found why the cache prompt will not be shown.
If I use the ImageView directly on the ContentView, the cache prompt will not show.
If I wrap the ImageView with a View, then use the wrapper view on the ContentView, the cache prompt will show.
Here is the working code in the ContentView.swift
struct ContentView: View {
var links =
[NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-0.jpg"),
NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-1.jpg"),
NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-2.jpg"),
NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-3.jpg"),
NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-4.jpg"),
NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-5.jpg"),
NewsItem(urlString: "https://www.jobyme88.com/wp-content/uploads/2020/11/50d0-kj-classroom-6.jpg")]
var body: some View {
List(links) { news in
// working
NewsListItemView(item: news)
// not working
//NewsImageView(urlString: news.urlString)
}
}
}
This is the NewsListItemView which is just a wrapper
struct NewsListItemView: View {
var item: NewsItem
var body: some View {
NewsImageView(urlString: item.urlString)
}
}
This is my cache prompt location.
NewsImageViewModel.swift
class NewsImageViewModel: ObservableObject {
static var placeholder = UIImage(named: "NewsIcon.png")
#Published var image: UIImage?
var urlString: String?
init(urlString: String) {
self.urlString = urlString
loadImage()
}
func loadImage() {
if loadImageFromCache() {
return
}
loadImageFromURL()
}
func loadImageFromCache() -> Bool {
guard let cacheIamge = TemporaryImageCache.getShared()[urlString!] else {
return false
}
print("load from cache")
self.image = cacheIamge
return true
}
func loadImageFromURL() {
print("load from url")
guard let urlString = urlString else {
return
}
let url = URL(string: urlString)!
let task = URLSession.shared.dataTask(with: url, completionHandler: getResponseFromURL(data:response:error:))
task.resume()
}
func getResponseFromURL(data: Data?, response: URLResponse?, error: Error?) {
guard error == nil else {
print("Error \(error!)")
return
}
guard data != nil else {
print("No founded data")
return
}
DispatchQueue.main.async {
guard let loadedImage = UIImage(data: data!) else {
print("Not supported data ")
return
}
self.image = loadedImage
TemporaryImageCache.getShared().cache.setObject(loadedImage, forKey: self.urlString! as NSString)
}
}
}
NewsImageView.swift
import SwiftUI
struct NewsImageView: View {
#ObservedObject var model: NewsImageViewModel
init(urlString: String) {
model = NewsImageViewModel(urlString: urlString)
}
var body: some View {
Image(uiImage: model.image ?? NewsImageViewModel.placeholder!)
.resizable()
.aspectRatio(contentMode: .fit)
.frame(width: 100, height: 100, alignment: .center)
}
}
NewsListItemView.swift
struct NewsListItemView: View {
var item: NewsItem
var body: some View {
NewsImageView(urlString: item.urlString)
}
}
This is ImageCache.swift
protocol ImageCache {
subscript(_ urlString: String) -> UIImage? {get set }
}
struct TemporaryImageCache: ImageCache {
subscript(urlString: String) -> UIImage? {
get {
cache.object(forKey: urlString as NSString)
}
set {
newValue == nil ? cache.removeObject(forKey: urlString as NSString) : cache.setObject(newValue!, forKey: urlString as NSString)
}
}
var cache = NSCache<NSString, UIImage>()
}
extension TemporaryImageCache {
private static var shared = TemporaryImageCache()
static func getShared() -> TemporaryImageCache {
return shared
}
}
This is NewsItem.swift
struct NewsItem: Identifiable {
var id = UUID()
var urlString: String
}