Question

我正在从移动应用程序调用HERE Weather API，需要将当前天气作为对象返回，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https://fluffy.es/return-value-from-a-closure/的异步调用有一个粗略的了解，但是我仍然遇到无法访问在completionHandler闭包之外创建的Weather对象的问题。

我从Weather类中提供了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Weather.Get调用结束时，我应该能够通过其他方法访问testWeather对象。具体来说，一种方法是根据Weather.Get调用返回的当前区域天气来修改速度限制。

Answer 1

您还不了解的是，GET调用中的完成处理程序与原始Weather API调用一样异步。因此，您不能在之后中运行任何依赖于中内容的代码。事件的顺序如下：

// 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

此外，最后一行中的testWeather是什么？ self.testWeather是财产吗？一定会的。但是，即使是这样，您也忽略了为self.testWeather赋值； if let testWeather是一个不同 testWeather（它仅对完成处理程序而言是本地的，不能从中“泄漏”）。但是，即使您这样做了，它 still 也不起作用，因为代码 still 会以错误的顺序运行：

// 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

尽管如此，记住记住写self.testWeather（5）将至少允许 other 代码访问此值，前提是该值运行以后。

斯威夫特||从API调用返回可被其他方法使用的类

1 个答案: