带有 JSON ObservedObject 的 SwiftUI答案

【问题标题】：SwiftUI with JSON ObservedObject带有 JSON ObservedObject 的 SwiftUI
【发布时间】：2020-08-01 11:31:30
【问题描述】：

我目前正在学习 SwiftUI，但我无法让我的代码打印来自 API 的调用

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()
        }
    }
}

如果我打印所有国家/地区，我可以看到每个项目

但在我的主视图中，当我尝试打印时（self.networkManager[5].name）我得到一个索引超出范围错误

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()
    }
}

谁能帮忙指出我哪里出错了？

【问题讨论】：

标签： json xcode parsing swiftui observedobject

【解决方案1】：

问题与异步 fetchAllCountries 有关。在尝试使用结果之前，您需要等到它完成。试试这样的；

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)
            }
    }
}
}

【讨论】：

【解决方案2】：

您尝试访问.allCountries 太早，因为.fetchAllCountries 是一个长（相对）异步操作。

解决方案是向获取的国家/地区添加显式观察者，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
            }
    }

【讨论】：