How to set up DispatchGroup in asynchronous iteration?

问题

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")
}

Output:

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

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

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

回答1:

What about this:

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

Instead of your normal print.

edit:

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")
}
}

来源：https://stackoverflow.com/questions/44912000/how-to-set-up-dispatchgroup-in-asynchronous-iteration

标签

swift

cocoa

asynchronous

iteration

dispatch