Swift-使用完成处理程序更新闭包外部的全局变量

Question

我已经搜索过，但我只是无法弄清楚为什么当我调用此函数时我的全局数组变量没有在全局范围内更新。我已经在代码中注释了可以 print() 我想要的结果但无法使其在函数之外可用的位置。我知道这很简单，我不了解闭包和完成处理程序，但我需要其他人看看。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，并且由于它是网络请求，因此需要一些时间才能完成。在您的情况下，在 doURLsessions 完成之前调用 print(arrFromURL) ，因此 arrFromURL 仍然为空。