ios - SwiftUI ScrollView 不显示从 ObservableObject 获取的结果

Question

我试图通过单击“获取”按钮从 api 获取数据，并通过 ScrollView 中的 ForEach 循环显示它们。

我正在使用 MVVM 模型。提取本身发生在 ObservableObject 类中。

不幸的是，ScrollView 不显示内容。如果我使用的是 List 而不是 ScrollView，它工作正常。

你知道我在这里遗漏了什么吗？

非常感谢您的帮助!

import SwiftUI

struct Photo: Identifiable, Decodable {

    let id = UUID()
    let title: String
}

class ContentViewModel: ObservableObject {

    let api = "https://jsonplaceholder.typicode.com/photos"

    @Published var photos: [Photo] = []

    func fetchData() {
        print("Fetching started")
        guard let url = URL(string: api) else { return }

        URLSession.shared.dataTask(with: url) { data, _, _ in
            DispatchQueue.main.async {
                self.photos = try! JSONDecoder().decode([Photo].self, from: data!)
                print("Fetching successfull. Fetched \(self.photos.count) photos.")
            }
        }.resume()
    }
}

struct ContentView: View {

    @ObservedObject var contentVM = ContentViewModel()

    var body: some View {
        NavigationView {
            ScrollView {
                ForEach(self.contentVM.photos) { photo in
                    Text(photo.title)
                }
            }
            .navigationBarTitle("Home")
            .navigationBarItems(trailing: Button(action: {

                self.contentVM.fetchData()

                }, label: {
                    Text("Fetch")
            }))
        }
    }
}

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

Answer 1

您没有遗漏任何内容，我认为问题出在渲染内容上。即使在您的示例数据显示在真实设备(iPhone 7 iOS 13.1.1)上，但延迟时间很长。尝试使用较少的内容或帮助 ScrollView 对齐。我试过这个 - 在设备上工作得更快，但只在设备上:

class ContentViewModel: ObservableObject {

    let api = "https://jsonplaceholder.typicode.com/photos"

    @Published var photos: [Photo] = []
    @Published var first100Photos: [Photo] = []

    func fetchData() {
        print("Fetching started")
        guard let url = URL(string: api) else { return }

        URLSession.shared.dataTask(with: url) { data, _, _ in
            DispatchQueue.main.async {
                self.photos = try! JSONDecoder().decode([Photo].self, from: data!)
                for index in 0...100 {
                    self.first100Photos.append(self.photos[index])
                }
                print("Fetching successfull. Fetched \(self.photos.count) photos.")
            }
        }.resume()
    }
}

struct ContentView: View {

    @ObservedObject var contentVM = ContentViewModel()

    var body: some View {
        NavigationView {
            ScrollView(.vertical) {
                VStack {
                    ForEach(self.contentVM.first100Photos) { photo in
                        Text(photo.title)
                    }
                }

            }
            .navigationBarTitle("Home")
            .navigationBarItems(trailing: Button(action: {

                self.contentVM.fetchData()

                }, label: {
                    Text("Fetch")
            }))
        }
    }
}