使用JSON ObservedObject的SwiftUI

Question

我目前正在学习SwiftUI，并且在从API打印调用的代码中遇到了问题。

NetworkManager.swift

class NetworkManager: ObservableObject {

    @Published var allCountries = Countries()

    func fetchAllCountries() {
        if let url = URL(string: K.url) {
            print(url)
            let session = URLSession(configuration: .default)
            let task = session.dataTask(with: url) { (data, response, error) in
                if error == nil {
                    let decoder = JSONDecoder()
                    if let countriesData = data {
                        do {
                            let countries = try decoder.decode(Countries.self, from: countriesData)
                            DispatchQueue.main.async {
                                self.allCountries = countries
                            }
                        } catch {
                            print(error)
                        }
                    }
                }
            }
            task.resume()
        }
    }
}

如果我打印allCountries，我可以看到每一项

但是在我的主视图上，当我尝试打印(self.networkManager5.name)时，我得到一个索引超出范围的错误

ContentView.swift

import SwiftUI

struct ContentView: View {

    @ObservedObject var networkManager = NetworkManager()

    var body: some View {
        Text("Hello, World!")
            .onAppear {
                self.networkManager.fetchAllCountries()
                print(self.networkManager.allCountries[5].name)
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

有人能帮我指出我哪里错了吗？

Answer 1

问题出在异步fetchAllCountries上。在尝试使用结果之前，您需要等待它完成。试试这样的东西；

class NetworkManager: ObservableObject {

@Published var allCountries = Countries()

func fetchAllCountries(handler: @escaping () -> Void) {
    if let url = URL(string: K.url) {
        print(url)
        let session = URLSession(configuration: .default)
        let task = session.dataTask(with: url) { (data, response, error) in
            if error == nil {
                let decoder = JSONDecoder()
                if let countriesData = data {
                    do {
                        let countries = try decoder.decode(Countries.self, from: countriesData)
                        DispatchQueue.main.async {
                            self.allCountries = countries
                            return handler()
                        }
                    } catch {
                        print(error)
                    }
                }
            }
            handler()
        }
        task.resume()
    }
}
}

struct ContentView: View {

@ObservedObject var networkManager = NetworkManager()

var body: some View {
    Text("Hello, World!")
        .onAppear {
            self.networkManager.fetchAllCountries() {
                print(self.networkManager.allCountries[5].name)
            }
    }
}
}

Answer 2

您试图过早地访问.allCountries，因为.fetchAllCountries是一个很长的(相对)异步操作。

解决方案是向获取国家添加显式观察者，如下所示

    var body: some View {
        Text("Hello, World!")
            .onAppear {
                self.networkManager.fetchAllCountries()
            }
            .onReceive(networkManager.$allCountries) { countries in
                 print(countries[5].name) // assuming Countries support subscript
            }
    }