2

我有一个视图，它在滚动视图中显示从 API 加载的几张照片。我想推迟获取图像，直到显示视图。我的观点，简化看起来像这样：

struct DetailView : View {
    @ObservedObject var viewModel: DetailViewModel

    init(viewModel: DetailViewModel) {
        self.viewModel = viewModel
    }

    var body: some View {
        GeometryReader { geometry in
            ZStack {
                Color("peachLight").edgesIgnoringSafeArea(.all)
                if self.viewModel.errorMessage != nil {
                    ErrorView(error: self.viewModel.errorMessage!)
                } else if self.viewModel.imageUrls.count == 0 {
                    VStack {
                        Text("Loading").foregroundColor(Color("blueDark"))
                        Text("\(self.viewModel.imageUrls.count)").foregroundColor(Color("blueDark"))
                    }
                } else {
                    VStack {
                        UIScrollViewWrapper {
                            HStack {
                                ForEach(self.viewModel.imageUrls, id: \.self) { imageUrl in
                                    LoadableImage(url: imageUrl)
                                        .scaledToFill()
                                }.frame(width: geometry.size.width, height: self.scrollViewHeight)
                            }.edgesIgnoringSafeArea(.all)
                        }.frame(width: geometry.size.width, height: self.scrollViewHeight)
                        Spacer()
                    }
                }
            }
        }.onAppear(perform: { self.viewModel.fetchDetails() })
            .onReceive(viewModel.objectWillChange, perform: {
                print("Received new value from view model")
                print("\(self.viewModel.imageUrls)")
            })
    }
}

我的视图模型如下所示：

import Foundation
import Combine

class DetailViewModel : ObservableObject {
    @Published var imageUrls: [String] = []
    @Published var errorMessage : String?

    private var fetcher: Fetchable
    private var resourceId : String


    init(fetcher: Fetchable, resource: Resource) {
        self.resourceId = resource.id
        // self.fetchDetails() <-- uncommenting this line results in onReceive being called + a view update
    }


    // this is a stubbed version of my data fetch that performs the same way as my actual
    // data call in regards to ObservableObject updates

    // MARK - Data Fetching Stub
    func fetchDetails() {
        if let path = Bundle.main.path(forResource: "detail", ofType: "json") {
            do {
                let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .mappedIfSafe)
                let parsedData = try JSONDecoder().decode(DetailResponse.self, from: data)
                self.imageUrls = parsedData.photos // <-- this doesn't trigger a change, and even manually calling self.objectWillChange.send() here doesn't trigger onReceive/view update
                print("setting image urls to \(parsedData.photos)")

            } catch  {
                print("error decoding")
            }
        }
    }
}

如果我在init视图模型的方法中获取数据，则在设置onReceive@PublishedimageUrls属性时会调用视图上的块。但是，当我fetch从init方法中删除并从视图中调用时，使用：

 .onAppear(perform: { self.viewModel.fetchDetails() })

即使数据已更新，也不会调用for onReceive。viewModel.objectWillChange我不知道为什么会这样，并且非常感谢这里的任何帮助。

4

2 回答 2

2

改为使用

.onReceive(viewModel.$imageUrls, perform: { newUrls in
    print("Received new value from view model")
    print("\(newUrls)")
})
于 2020-03-25T13:07:22.303 回答
0

我测试了这个，因为我发现了同样的问题，似乎只有值类型可以与 onReceive 一起使用

使用枚举、字符串等。

它不适用于引用类型，因为我猜从技术上讲，引用类型不会更改引用位置，而只是在更改时指向其他地方？idk哈哈但是你

作为一种解决方案，您可以设置一个类似于状态枚举的 viewModel @published 属性，当您有新数据时对其进行更改，然后在接收时可以访问它...希望这是有道理的，如果没有，请告诉我

于 2021-12-20T03:05:32.953 回答