Swift ||从 API 调用中返回可由其他方法使用的 class

Question

我正在从移动应用程序调用 HERE Weather API，需要 return 当前天气作为对象，以便其他方法可以在给定时间段内使用该信息（例如 30 分钟更新间隔）。

我对基于此站点的异步调用有粗略的了解 https://fluffy.es/return-value-from-a-closure/ 但我仍然运行我无法访问我在关闭之外创建的 Weather 对象的问题完成处理程序。

我已经从天气 class 中提供了 Get 方法。我还在 AppDelegates 文件中提供了对该 Get 方法的调用。

public static func Get(lat:String, long:String, completionHandler: @escaping (Weather?)  -> Void) {

    //Keys go here
    var request = URLRequest(
        url: URL(string: "https://weather.cit.api.here.com/weather/1.0/report.json?app_code=-fJW5To2WdHdwQyYr_9ExQ&app_id=2m83HBDDcwAqTC2TqdLR&product=observation&latitude="+lat+"&longitude="+long+"&oneobservation=true")!)
    request.httpMethod = "GET"


    URLSession.shared.dataTask(with: request, completionHandler: { dat, response, error in
        do {

            guard let json = try? JSONSerialization.jsonObject(with: dat as! Data, options: []) else {throw JSONParseError.missingJSON}

            //parse json for dictionaries
            guard let response = json as? [String: Any] else {throw JSONParseError.responseNotADictionary}
            guard let observations = response["observations"] as? [String: Any] else {throw JSONParseError.observationsNotADictionary}
            guard let locationJSON = observations["location"] as? [Any] else {throw JSONParseError.locationNotAnArray}
            guard let location = locationJSON.first as? [String: Any] else {throw JSONParseError.locationNotADictionary}
            guard let observationJSON = location["observation"] as? [Any] else {throw JSONParseError.observationNotAnArray}
            guard let observation = observationJSON.first as? [String: Any] else {throw JSONParseError.observationNotADictionary}

            //search dictionaries for values
            guard let feedCreation = response["feedCreation"] as? String else {throw JSONParseError.missingFeedCreationObject}
            guard let city = location["city"] as? String else {throw JSONParseError.missingCityObject}
            guard let state = location["state"] as? String else {throw JSONParseError.missingStateObject}
            guard let temperature = observation["temperature"] as? String else {throw JSONParseError.missingTemperatureObject}
            guard let icon = observation["icon"] as? String else {throw JSONParseError.missingIconObject}
            guard let iconName = observation["iconName"] as? String else {throw JSONParseError.missingIconNameObject}

            //create weather object and return
            guard let currentWeather = Weather(feedCreation: feedCreation,state: state,city: city,temperature: temperature,icon: icon,iconName: iconName) else {throw WeatherObjectCreationError.objectCreationFailed}

            completionHandler(currentWeather)

        } catch {
            print("JSON Serialization error")
        }

    }).resume()
}

Weather.Get(lat:"32.9005", long:"-96.7869", completionHandler: {currentWeather in
            if let testWeather = currentWeather{
//Works fine
print(testWeather.city)
                print("completionHandler")
            }
})
//Error says testWeather is not intialized 
print(testWeather.city)

在我的 Weather.Get 调用结束时，我应该能够从其他方法访问 testWeather 对象。具体来说，一种根据该地区当前天气修改限速的方法，由 Weather.Get 调用 returned。

Answer 1

您没有理解的是，您的 GET 调用中的完成处理程序与原始 Weather API 调用一样异步。因此，您不能运行任何代码 after 它取决于 in 它的内容。活动顺序如下：

// 1
Weather.Get(lat:"32.9005", long:"-96.7869", completionHandler: {currentWeather in
    if let testWeather = currentWeather{ // 3!!!
        print(testWeather.city) // 4!!!!!!
        print("completionHandler")
    }
})
print(testWeather.city) // 2

另外，最后一行的testWeather是什么？是属性、self.testWeather 吗？它需要是。但即使是，你也忽略了给 self.testWeather 赋值； if let testWeather 是一个 different testWeather（它仅在完成处理程序中是本地的，不能 "leak" 超出它）。但是，即使您这样做了，它 still 也不会起作用，因为代码 still 运行的顺序是错误的:

// 1
Weather.Get(lat:"32.9005", long:"-96.7869", completionHandler: {currentWeather in
    if let testWeather = currentWeather{ // 3
        print(testWeather.city) // 4
        print("completionHandler") 
        self.testWeather = testWeather // 5; better but it won't help the _next_ print
    }
})
print(testWeather.city) // 2

不过，记得写入 self.testWeather (5) 至少会允许 other 代码访问此值，前提是运行s 后来.

Swift ||从 API 调用中返回可由其他方法使用的 class

Swift || Returning a class that can be used by other methods from an API call

asynchronous

ios

completionhandler

swift