延遲SwiftUI視圖渲染，直到發布的物件完成加載

我正在嘗試使用 SwiftUI 構建應用程式，并且我創建了一個負責執行 API 請求并將回應解碼為weatherResponse物件的類，并@Published在此類中注釋了該物件。

在我看來，我已經使用@StateObject注釋實體化了這個類。根據這篇文章，這應該是weatherResponse在我的視圖中觀察物件的正確方法，以便它自動更新。

但是，當我的視圖加載時， 的實體WeatherNetworking甚至還沒有被初始化，因此對該類中發布的物件的第一次參考會導致 nil 值訪問。

struct MainWeatherView: View {
    
    @StateObject var weatherNetworking = WeatherNetworking()

    var body: some View {
        VStack {
            Image(systemName: weatherNetworking.getConditionName(weatherID: (weatherNetworking.weatherResponse!.current.weather[0].id))) // weatherResponse is nil
                .resizable()
                .aspectRatio( contentMode: .fit)
                .scaleEffect(0.75)
                .padding()
            
            HStack {
                
                VStack{
                    Text("Temperature")
                        .fontWeight(.bold)
                        .font(.system(size: 24))
                    Text("Humidity")
                        .fontWeight(.bold)
                        .font(.system(size: 24))
                    ...
                }
                
                VStack {
                    Text("\(weatherNetworking.weatherResponse!.current.temp, specifier: "%.2f") °F")
                        .fontWeight(.bold)
                        .font(.system(size: 24))
                    Text("\(weatherNetworking.weatherResponse!.current.humidity, specifier: "%.0f") %")
                        .fontWeight(.bold)
                        .font(.system(size: 24))
                    ...
                }
            }
        }
        .onAppear {
            self.weatherNetworking.getMainWeather()
        }
    }
}

class WeatherNetworking: ObservableObject {
    
    @StateObject var locationManager = LocationManager()
    @Published var weatherResponse: WeatherResponse?
    
    func getMainWeather() {
        print("Location:", locationManager.lastLocation?.coordinate.latitude, locationManager.lastLocation?.coordinate.longitude)
        if let loc = URL(string: "https://api.openweathermap.org/data/2.5/onecall?appid=redacted&exclude=minutely&units=imperial&lat=\(locationManager.lastLocation?.coordinate.latitude ?? 0)&lon=\(locationManager.lastLocation?.coordinate.longitude ?? 0)") {
            let session = URLSession(configuration: .default)
            let task = session.dataTask(with: loc) { data, response, error in
                if error == nil {
                    let decoder = JSONDecoder()
                    if let safeData = data {
                        do {
                            let results = try decoder.decode(WeatherResponse.self, from: safeData)
                            DispatchQueue.main.async {
                                self.weatherResponse = results
                            }
                            print("weatherResponse was succesfully updated")
                        } catch {
                            print(error)
                        }
                    }
                } else {
                    print(error!)
                }
            }
            task.resume()
        }
    } 
}

執行中沒有任何列印陳述句getMainWeather()使我相信視圖正在嘗試在呼叫函式之前分配值。如何在我的視圖中延遲這些值的分配，直到onAppear()方法完成之后？值得注意的是，正如MainWeatherView依賴于實體化和異步呼叫一樣WeatherNetworking，也WeatherNetworking依賴于LocationManager.

class LocationManager: NSObject, ObservableObject, CLLocationManagerDelegate {
    
    private let locationManager = CLLocationManager()
    @Published var locationStatus: CLAuthorizationStatus?
    @Published var lastLocation: CLLocation?

    ...
}

uj5u.com熱心網友回復：

使依賴視圖有條件，例如

VStack {
   if let response = weatherNetworking.weatherResponse {
        Image(systemName: weatherNetworking.getConditionName(weatherID: (response.current.weather[0].id))) // weatherResponse is nil
            .resizable()
            .aspectRatio( contentMode: .fit)
            .scaleEffect(0.75)
            .padding()
   }
   // .. other code

標籤：IOS 迅速 异步 迅捷

上一篇：向上滾動視圖時不出現UINavigationBar大標題

下一篇：SwiftUIChart：如何疊加多個圖形資料？