Swift-使用完成处理程序更新闭包外的全局变量

Question

我已经搜索了但是当我调用这个函数时，我无法弄清楚为什么我的全局数组变量在全局范围内没有得到更新。我在代码中注释了我可以打印（）我想要的结果但不能在函数外部使用它的地方。我知道这很简单，我不了解闭包和完成处理程序，但我需要别人看一看。谢谢。

import Foundation
import PlaygroundSupport

func pdate(month: String, day: String, year: String) -> String {
    let pdate = month + "/" + day + "/" + year
    return pdate
}

struct Phenomenon {
    var phenom: String
    var date: String
    var time: String
}

struct Apsides: Codable {
    let data: [Apsdata]?
    struct Apsdata: Codable {
        let phenom: String
        let year: String
        let month: String
        let day: String
        let time: String
    }
}

var arrFromURL: [(Phenomenon)] = []  //define a global I can use later- I know, globals are evil

func doURLsessions(completionHandler: @escaping (Phenomenon?)->()) {
    let sessionApsides = URLSession(configuration: URLSessionConfiguration.default)
    if let timeurl = URL(string: "http://api.usno.navy.mil/seasons?year=2018&tz=-8&dst=true") {
        (sessionApsides.dataTask(with: timeurl, completionHandler: { (data, response, error) in
            if let error = error {
                print("Error: \(error)")
            } else if let data = data {
                let jsonDecoder = JSONDecoder()
                let response = try? jsonDecoder.decode(Apsides.self, from: data)
                if (response?.data != nil) {
                    for item in [response?.data] {
                        for p in item! {
                            switch p.phenom {
                            case "Perihelion":
                                let perihelion = Phenomenon.init(
                                    phenom: p.phenom, date: pdate(month: p.month, day: p.day, year: p.year), time: p.time)
                                completionHandler(perihelion)
                            case "Aphelion":
                                let aphelion = Phenomenon.init(
                                    phenom: p.phenom, date: pdate(month: p.month, day: p.day, year: p.year), time: p.time)
                                completionHandler(aphelion)
                            default: ()
                            }
                        }
                    }
                }
            }
            //print(arrFromURL) //This works fine, but not what I want
        })).resume()
    }
}

func chPhenom(phenom: String, date: String, time: String) {
    let p = Phenomenon.init(phenom: phenom, date: date, time: time)
    arrFromURL.append(p)
    //print(arrFromURL) //Also works fine,  but I want it's contents avilable outside this function
}

doURLsessions(completionHandler: {Phenomenon in
    chPhenom(phenom: Phenomenon!.phenom, date: Phenomenon!.date, time: Phenomenon!.time)
})

print(arrFromURL) //This just prints []- why isn't the global var available here? Is it printed before the asynch func finishes?

PlaygroundPage.current.needsIndefiniteExecution = true

Answer 1

函数doURLsessions中的代码是异步的 - 它不会阻止其他代码执行，因为它是一个网络请求，需要一些时间才能完成。在你的情况下，print(arrFromURL)在doURLsessions完成之前被调用，所以arrFromURL仍然是空的。