如何在异步迭代中设置DispatchGroup？ [英] How to set up DispatchGroup in asynchronous iteration?

问题描述

我正在尝试设置下载图像的迭代。整个过程有效，但看看控制台的输出，似乎是错误的。

I´m trying to set up an iteration for downloading images. The whole process works, but taking a look in the console´s output, something seems to be wrong.

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

看起来迭代不关心关于进入和离开DispatchGroup（）的问题。 webrequests几乎同时被解雇。在我看来，输出应如下所示：

It looks like the iteration does not care about entering and leaving the DispatchGroup() at all. The webrequests are fired almost at the same time. In my opinion the output should look like this:

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

我是否监督过某事？获得一些想法会很棒。

Did I oversee something? Would be awesome to get some ideas.

推荐答案

这个怎么样：

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")
}
}

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