Swift 迅速完成及；循环问题

正在尝试从三个URL之一下载PKG文件。该逻辑基本上从每个下载url中查找下载的延迟，并将最终下载url设置为延迟最低的主机

import Cocoa
import Alamofire

// Create my object via Struct
struct Package {
    var latency: Double?
    var name: String?
    var statuscode: Int?
    var download: Bool?
    var downloadUrl: String? 
}
// Download the package from the provided download url and return the object
func getPKG(pkgName: String, dpUrl: String, completion: @escaping (Package) -> (Package)) {
    let url = URL(string: "\(dpUrl)\(pkgName)")
    let parameters: Parameters = ["foo":"bar"]
    Alamofire.download(url, method: .get, parameters: parameters, encoding: JSONEncoding.default, to: destination)
        .downloadProgress(queue: DispatchQueue.global(qos: .utility)) { progress in
            debugPrint("Download Progress...: \(progress.fractionCompleted)")
        }
        .validate(statusCode: 200..<399)
        .response { response in
            debugPrint(response.response!)
            debugPrint(response.response!.statusCode)
            debugPrint(response.timeline.latency)
            let dlObject = Package(latency: response.timeline.latency, name: pkgName, statuscode: response.response?.statusCode, download: true, downloadUrl: dpUrl)
            completion(dlObject)
    }

}

var share_response = [String: Double]()
var package_sources: NSArray! = ["https://www.jss1.com/Share", "https://www.jss2.com/Share", "https://www.jss3.com/Share"]
let package_names: String = ["Dummy1.pkg", "Dummy2.pkg", "Dummy3.pkg"]
// Loop through the package sources and find the one with 
// the lowest latency.
for share_url in package_sources {
    getPKG(pkgName: "Dummy.pkg", dpUrl: share_url, completion: {
        dlObject in
        if dlObject.latency != nil {
            share_response[share_url] = dlObject.latency
        } else {
            debugPrint("nothing yet")
        }
        return dlObject
    })
}

let final_download_url = share_response.min { a, b in a.value < b.value }

//  Here is where it breaks and responds with nil

for package in package_names {
    let download_url = URL(string: final_download_url + package)
    Download commands here...
}

导入可可粉
进口阿拉莫菲尔
//通过Struct创建我的对象
结构包{
var延迟：双倍？
变量名称：字符串？
变量状态码：Int？
var下载：Bool？
var downloadUrl:String？
}
//从提供的下载url下载包并返回对象
func getPKG（pkgName:String，dpUrl:String，completion:@escaping（Package）->（Package））{
让url=url（字符串：“\（dpUrl）\（pkgName）”）
let参数：参数=[“foo”：“bar”]
下载（url，方法：.get，参数：参数，编码：JSONEncoding.default，收件人：目标）
.downloadProgress（队列：DispatchQueue.global（qos:.实用工具））{中的进度
debugPrint（“下载进度…：\（Progress.fractionCompleted）”）
}
.验证（状态代码：200..基于@vadian at的答案

。。。
let group=DispatchGroup（）
var share_response=[String:Double]（）
变量包_来源：NSArray！=[”https://www.jss1.com/Share", "https://www.jss2.com/Share", "https://www.jss3.com/Share"]
让包名为：String=[“Dummy1.pkg”、“Dummy2.pkg”、“Dummy3.pkg”]
//循环浏览包源代码，并找到具有
//最低延迟。
用于包\u源中的共享\u url{
group.enter（）
getPKG（pkgName:Dummy.pkg），dpUrl:share\u url，完成：{
小组请假（）
dlobjectin
如果dlObject.latency！=nil{
共享响应[共享url]=dlObject.latency
}否则{
debugPrint（“还没有”）
}
返回dlObject
})
}
通知组（队列：DispatchQueue.main）{
让final_download_url=share_response.min{a，b在a.value
（包）->（包）
无法工作。您需要
DispatchGroup
来同步循环中的网络请求。顺便说一句，类型注释
：NSArray！
非常糟糕。它既不是
NSArray
也不是可选的。从不注释编译器可以推断出更好的类型。这不是Javascript：变量名应该是camelCased@vadian我很快就把这篇文章整理好了，并且编辑了很多公司特有的东西，所以我不太关心变量名。我想让它尽可能的通用。我真的很感激你的回答/评论，这些都是我将收回的优秀观点。你能举一个例子说明它与Dispa的关系吗tchGroup？我刚刚给@vadian写了一个答案。我已经看了另一篇文章，并开始编辑代码进行测试，但是有一个问题是关于group.notify部分的。我应该将该部分放在我发布的代码中的什么位置？@vadian&ekscrypto，谢谢你的帮助。这就是我所需要的。我现在可以成功地从pro中找到最低延迟查看URL，选择最快的主机，然后相应地从该主机下载软件包。再次感谢！
...

let group = DispatchGroup()
var share_response = [String: Double]()
var package_sources: NSArray! = ["https://www.jss1.com/Share", "https://www.jss2.com/Share", "https://www.jss3.com/Share"]
let package_names: String = ["Dummy1.pkg", "Dummy2.pkg", "Dummy3.pkg"]
// Loop through the package sources and find the one with 
// the lowest latency.
for share_url in package_sources {
    group.enter()
    getPKG(pkgName: "Dummy.pkg", dpUrl: share_url, completion: {
        group.leave()
        dlObject in
        if dlObject.latency != nil {
            share_response[share_url] = dlObject.latency
        } else {
            debugPrint("nothing yet")
        }
        return dlObject
    })
}

group.notify(queue: DispatchQueue.main) {    
    let final_download_url = share_response.min { a, b in a.value < b.value }

    //  Here is where it breaks and responds with nil

    for package in package_names {
        let download_url = URL(string: final_download_url + package)
        Download commands here...
    }
}