SwiftUI @ObservedObject 不會在異步獲取圖像時更新

Question

我有一個應用程序從遠程 API 獲取帶有圖像 URL 的項目列表，然后它必須從該項目的 Z4757FE07FD492A8BE0EA6A760EZ68 中的給定 url 中獲取每個項目的圖像。 問題是，當向上和向下滾動並因此從視圖中刪除列表項並將它們移回視圖時，它們確實會顯示出來。 然而，在初始加載時，它們永遠處於“加載”狀態，直到移出和移入。我的代碼：

import SwiftUI

struct ContentView: View {

    @EnvironmentObject var artObjectStore: ArtObjectStore
    @State private var pageCount = 1
    @State private var tappedLink: String? = nil
    @Environment(\.imageCache) var cache: ImageCache

    var body: some View {
        NavigationView {
            Form {

                Section(header: Text("Art")) {

                    List {
                        ForEach(artObjectStore.artObjects, id: \.self) { artObject in
                            self.link(for: artObject)

                        }
                        Button(action: loadMore) {
                            Text("")
                        }
                        .onAppear {
                            DispatchQueue.global(qos: .background).asyncAfter(deadline: DispatchTime(uptimeNanoseconds: 10)) {
                                self.loadMore()
                            }
                        }
                    }
                }
            }
            .navigationBarTitle("Art objects")
        }
        .onAppear(perform: loadMore)

    }

    func loadMore() {
        pageCount += 1
        artObjectStore.loadMore(pageCount)
    }

    private func link(for artObject: ArtObject) -> some View {
        let selection = Binding(get: { self.tappedLink },
            set: {
                UIApplication.shared.endEditing()
                self.tappedLink = $0
        })

        return NavigationLink(destination: DetailView(artObject: artObject, cache: self.cache),
                              tag: artObject.id,
                              selection: selection) {
            HStack(alignment: .center) {
                VStack(alignment: .leading){
                    Text("\(artObject.title)").font(.system(size: 12))
                    Text("\(artObject.principalOrFirstMaker)").font(.system(size: 9)).foregroundColor(.gray)
                }
                Spacer()
                AsyncImage(
                    url: URL(string: artObject.headerImage.url)!,
                    cache: self.cache,
                    width: 200,
                    height: 50
                )
            }
        }
    }
}


extension UIApplication {
    func endEditing() {
        sendAction(#selector(UIResponder.resignFirstResponder), to: nil, from: nil, for: nil)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

圖像容器：

import SwiftUI
import Combine
import Foundation

class ImageLoader: ObservableObject {

    let objectWillChange = ObservableObjectPublisher()

    private var cancellable: AnyCancellable?
    @Published var image: UIImage? {
        willSet {
            objectWillChange.send()
        }
    }


    private let url: URL

    private var cache: ImageCache?

    init(url: URL, cache: ImageCache? = nil) {
        self.url = url
        self.cache = cache
    }

    deinit {
        cancellable?.cancel()
    }

    private func cache(_ image: UIImage?) {
        image.map { cache?[url] = $0 }
    }

    func load() {
        if let image = cache?[url] {
            self.image = image
            return
        }

        cancellable = URLSession.shared.dataTaskPublisher(for: url)
            .map { UIImage(data: $0.data) }
            .replaceError(with: nil)
            .handleEvents(receiveOutput: { [weak self] in self?.cache($0) })
            .receive(on: DispatchQueue.main)
            .assign(to: \.image, on: self)


    }

    func cancel() {
        cancellable?.cancel()
    }
}


struct AsyncImage: View {
    @ObservedObject private var loader: ImageLoader
    private let width: CGFloat?
    private let height: CGFloat?
    @State var spin = false

    init(url: URL, cache: ImageCache? = nil, width: CGFloat? = nil, height: CGFloat? = nil) {
        loader = ImageLoader(url: url, cache: cache)
        self.width = width
        self.height = height
    }

    var body: some View {
        image
            .onAppear(perform: loader.load)
            .onDisappear(perform: loader.cancel)
    }

    private var image: some View {
        Group {
            if loader.image != nil {
                Image(uiImage: loader.image!)
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: width, height: height)
            } else {
                Image("loadingCircle")
                    .resizable()
                    .frame(width: 20, height: 20)
                    .rotationEffect(.degrees(spin ? 360 : 0))
                    .animation(Animation.linear(duration: 0.8).repeatForever(autoreverses: false))
                    .onAppear() {
                        self.spin.toggle()
                }
            }
        }
    }
}

protocol ImageCache {
    subscript(_ url: URL) -> UIImage? { get set }
}

struct TemporaryImageCache: ImageCache {
    private let cache = NSCache<NSURL, UIImage>()

    subscript(_ key: URL) -> UIImage? {
        get { cache.object(forKey: key as NSURL) }
        set { newValue == nil ? cache.removeObject(forKey: key as NSURL) : cache.setObject(newValue!, forKey: key as NSURL) }
    }
}


struct ImageCacheKey: EnvironmentKey {
    static let defaultValue: ImageCache = TemporaryImageCache()
}

extension EnvironmentValues {
    var imageCache: ImageCache {
        get { self[ImageCacheKey.self] }
        set { self[ImageCacheKey.self] = newValue }
    }
}

我確實嘗試在圖像上添加 willSet，這似乎不起作用。 你能幫助我嗎？

Answer 1

以下所有測試均使用 Xcode 11.4 / iOS 13.4

修改工作AsyncImage

我將Group更改為VStack並開始更新

    private var image: some View {
        VStack {                        // << here !!
            if loader.image != nil {

修改后的工作圖像加載器

class ImageLoader: ObservableObject {

    @Published var image: UIImage?

    private var cancellable: AnyCancellable?
    private let url: URL
    private var cache: ImageCache?

    init(url: URL, cache: ImageCache? = nil) {
        self.url = url
        self.cache = cache
    }

    deinit {
        cancellable?.cancel()
    }

    private func cache(_ image: UIImage?) {
        self.image = image
        image.map { cache?[url] = $0 }
    }

    func load() {
        if let image = cache?[url] {
            self.image = image
            return
        }

        cancellable = URLSession.shared.dataTaskPublisher(for: url)
            .map { UIImage(data: $0.data) }
            .receive(on: DispatchQueue.main)
            .replaceError(with: nil)
            .sink(receiveValue: { [weak self] in self?.cache($0) })
    }

    func cancel() {
        cancellable?.cancel()
    }
}