如何在异步迭代中设置 DispatchGroup？

Question

我正在尝试设置用于下载图像的迭代。整个过程正常，但看看控制台的输出，似乎有问题。

func download() {

    let logos = [Logos]()
    let group = DispatchGroup()

    logos.forEach { logo in

        print("enter")
        group.enter()

        if logo?.data == nil {

            let id = logo?.id as! String

            if let checkedUrl = URL(string: "http://www.apple.com/euro/ios/ios8/a/generic/images/\(id).png") {

                print(checkedUrl)

                LogoRequest.init().downloadImage(url: checkedUrl) { (data)  in

                    logo?.data = data
                    print("stored")

                    group.leave()
                    print("leave")
                }
            }
        }
    }

    print("loop finished")
}

输出：

enter
http://www.apple.com/euro/ios/ios8/a/generic/images/og.png
enter
http://www.apple.com/euro/ios/ios8/a/generic/images/eg.png
enter
http://www.apple.com/euro/ios/ios8/a/generic/images/sd.png
enter
http://www.apple.com/euro/ios/ios8/a/generic/images/hd.png
loop finished
stored
leave
stored
leave
stored
leave
stored
leave

看起来迭代根本不关心进入和离开 DispatchGroup()。网络请求几乎同时被触发。在我看来，输出应该是这样的：

enter
http://www.apple.com/euro/ios/ios8/a/generic/images/og.png
stored
leave
enter
http://www.apple.com/euro/ios/ios8/a/generic/images/eg.png
stored
leave
...
loop finished

我监督了什么？能得到一些想法会很棒。

Answer 1

这个呢：

group.notify(queue: .main) {
print("loop finished")
}

而不是您的正常打印。

编辑：

func download() {

let logos = [Logos]()  // NSManagedObject
let group = DispatchGroup()

logos.forEach { logo in


    if logo?.data == nil {
        let id = logo?.id as! String
        if let checkedUrl = URL(string: "http://www.apple.com/euro/ios/ios8/a/generic/images/\(id).png") {

            print(checkedUrl)
             print("enter")
            group.enter()
            LogoRequest.init().downloadImage(url: checkedUrl) { (data)  in
                //this is async I think

                coin?.logo = data
                print("stored")

                group.leave()
                print("leave")
            }
        }
    }
}

group.notify(queue: .main) {
print("loop finished")
}
}