场景：
我正在玩访问和显示远程图像，以了解Combine的各种通知标记/协议。

一个目标：
访问错误的URL应该立即显示Alert（）。

现实：
警报显示之后，然后显示第二个请求。

这是主（调用）视图：

import Combine
import SwiftUI

struct ContentView: View {
    @EnvironmentObject var settings: MySettings
    @State private var url: String = "https://garbage.com"  // ...purposely set to display alert.
    @State private var image: URLImage = URLImage()
    @State private var angelFish: Image = Image("QueenAngelfish")
    @State private var isPresented = false

    var body: some View {
        ZStack {
            Color.green
            NavigationView {
                VStack {
                    Button(action: {
                        self.url = "garbage.com"
                        self.isPresented = self.image.imageLoader.isPresented
                        self.image.imageLoader.load(url: URL(string: self.url)!)
                    }) {
                        Text("Get An Image")
                    }

                    angelFish
                        .resizable()
                        .frame(width: 300,height: 200,alignment: .center)
                        .padding()
                    image
                        .alert(isPresented: $isPresented,content: { () -> Alert in
                            Alert(title: Text(verbatim: "Unable to Acquire Image."))
                        })

                }.navigationBarTitle(Text(settings.name))
            }
        }
    }
}

这是访问引擎：

import Combine
import SwiftUI

enum ImageURLError: Error {
    case dataIsnotAnImage
}

struct URLImage: View {
    @EnvironmentObject var settings: MySettings
    @ObservedObject var imageLoader: ImageLoader
    var placeholder: Image

    init() {
        self.placeholder = Image(systemName: "photo")
        self.imageLoader = ImageLoader()
    }

    var body: some View {
        VStack {
            imageLoader.image == nil ?
                placeholder : Image(uiImage: imageLoader.image!)
            Button(action: {
                self.settings.name = "Happy Thanks Giving"
                self.settings.isPresented = true
            },label: {
                Text("Touch Me")
            })
        }
    }
}

// =====================================================================================================

class ImageLoader: ObservableObject {
    let id: String = UUID().uuidString
    var didChange = PassthroughSubject<Void,Never>()

    @Published var isPresented = false

    @Published var image: UIImage? {
        didSet {
            DispatchQueue.main.async {
                self.didChange.send()
            }
        }
    }

    // ---------------------------------------------------------------------------

    func load(url: URL) {
        print("Hello Ric: ",#function)

        URLSession.shared.dataTask(with: url) { data,_,error in
            DispatchQueue.main.async {
                if error != nil {
                    self.isPresented = true
                    self.didChange.send()   // ...attempting to activate alert().
                    return
                }

                self.image = UIImage(data: data!)
            }

        }.resume()
    }
}

由于URL错误，此代码有意创建一个错误。
我试图通过将布尔值“ isPresented”标志作为“ @Published”变量传递来立即通知调用例程

预期结果：
显示的警报。

实际结果：
警报将在首次尝试之后显示。

问题：如何执行即时警报显示？