Question

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Чтоможносделатьчтобывначалевывелоданнуюстрокуprint（“Lat：（lat！）lon :( lon！）temp：（temp！）city :( name！）”）апослепродолжиловыполнениекода？

Answer 1

如果您想在getWeather完成后执行某些操作，请添加完成处理程序。就个人而言，我通过创建一个传递捕获信息的结构来简化这一过程：

struct WeatherReport {
    let latitude: Double
    let longitude: Double
    let temperature: Double
    let name: String
    let desc: String
}

func getWeather(city: String, completion: @escaping (WeatherReport?, Error?) -> Void) {

    ...

    let task = session.dataTask(with: url!) { data, _, error in
        ...
        guard successful else {
            completion(nil, error)
            return
        }

        let weatherReport = WeatherReport(...)
        completion(weatherReport, nil) 
    }

    task.resume()
}

然后

override func viewDidLoad() {
    super.viewDidLoad()

    getWeather(city: "Minsk") { weatherReport, error in
        guard let weatherReport = weatherReport, error == nil else {
            print(error?.localizedDescription ?? "Unknown error")
            return
        }

        DispatchQueue.main.async {
            // do something with `weatherReport` here
        }
    }
}

注意，dataTask已经（a）以异步方式运行;并且（b）在后台线程上运行其完成处理程序，因此viewDidLoad必须显式地将它对模型对象和/或UI做的任何事情分派到getWeather完成处理程序中的主队列，如图所示上方。

顺便说一句，如果您使用的是Swift 4，我建议删除第三方JSON解析库并使用JSONDecoder和Decodable的模型结构，例如：

struct Coordinate: Decodable {
    let latitude: Double
    let longitude: Double

    enum CodingKeys: String, CodingKey {
        case latitude = "lat"
        case longitude = "lon"
    }
}

struct WeatherReportDetails: Decodable {
    let temperature: Double
    let pressure: Double
    let humidity: Double
    let temperatureMin: Double
    let temperatureMax: Double

    enum CodingKeys: String, CodingKey {
        case pressure, humidity
        case temperature = "temp"
        case temperatureMin = "temp_min"
        case temperatureMax = "temp_max"
    }
}

struct WeatherReport: Decodable {
    let coordinate: Coordinate
    let details: WeatherReportDetails
    let name: String

    enum CodingKeys: String, CodingKey {
        case name
        case coordinate = "coord"
        case details = "main"
    }
}

然后

func getWeather(city: String, completion: @escaping (WeatherReport?, Error?) -> Void) {
    let cityEscaped = city.addingPercentEncoding(withAllowedCharacters: CharacterSet.urlHostAllowed)!
    let urlString = "http://samples.openweathermap.org/data/2.5/weather?q=" + cityEscaped + "&appid=..." // appid removed for privacy’s sake
    let url = URL(string: urlString)!

    let task = URLSession.shared.dataTask(with: url) { data, _, error in
        guard let data = data, error == nil else {
            completion(nil, error)
            return
        }

        do {
            let weatherReport = try JSONDecoder().decode(WeatherReport.self, from: data)
            completion(weatherReport, nil)
        } catch {
            completion(nil, error)
        }
    }

    task.resume()
}

DispatchQueue.main.sync swift

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