iOS Swift 多线程阻塞情况

Question

I am trying to build an app that downloads some certain images in the background with using grand central dispatch. However even if I use the global queue to get a different thread and download the image it blocks the main thread. My code consists of two swift classes one is DownloadRequestViewController and other is DownloadHandler. Here they are:

class DownloadHandler: NSObject {

    let userDefaults = NSUserDefaults.standardUserDefaults()
    var filePath : String = "Turk Isi Manga"

    override init() {
        super.init()
        NSNotificationCenter.defaultCenter().addObserver(self , selector: "downloadChapter:", name: "downloadListNotification", object: DisplayMangaViewController.self)
        NSNotificationCenter.defaultCenter().addObserver(self, selector: "downloadChapter:", name: "downloadListNotification", object: DownloadRequestListViewController.self)
    }

    func downloadChapter(notification : NSNotification){
        dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
             //Downloading image and save it to memory here
         })
    }

And here is how I call this class and the downloadChapter in DownloadRequestViewController swift class:

@IBAction func downloadBarButton(sender: AnyObject) {
        dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
            var obtainedMangaNameArray = [self.obtainedMangaName]
            let userInfo = ["downloadList" : self.selectedChapters , "mangaName" : obtainedMangaNameArray]
            var sdfs = DownloadHandler()
            let notification = NSNotification(name: "downloadListNotification", object: DownloadRequestListViewController.self, userInfo: userInfo)
            sdfs.downloadChapter(notification)
            //NSNotificationCenter.defaultCenter().postNotification(notification)
        })
    }

What am I doing wrong? Any help would be appreciated, thanks... Does it matter what I do in download chapter method?

Answer 1

Please change this line

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {

to this line

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {

This will execute the code inside the dispatch closure on a thread with background priority, which will not "freeze" the main/UI thread

UPDATE

//Downloader Class

    class DownloadHandler: NSObject {
        class func downloadImageFromURL (url: NSURL) {
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), { () -> Void in
                if let imageData: NSData = NSData(contentsOfURL: url) {
                    imageData.writeToFile("Your local Path", atomically: false)
                }
            })
        }
    }

    //Usage example
    DownloadHandler.downloadImageFromURL(NSURL(string: "Your URL To Image")!)

Answer 2

I think this works:

func downloadChapter(notification : NSNotification){
        //fetch on different thread
        let qos = Int(QOS_CLASS_USER_INITIATED.value)
        dispatch_async(dispatch_get_global_queue(qos, 0), {
            //download here

            //back to main thread
            dispatch_async(dispatch_get_main_queue(), {
                //if there is something more you wanna do or you can just go back
            })
        })
}

@IBAction func downloadBarButton(sender: AnyObject) {
        var obtainedMangaNameArray = [self.obtainedMangaName]
        let userInfo = ["downloadList" : self.selectedChapters , "mangaName" : obtainedMangaNameArray]
        var sdfs = DownloadHandler()
        let notification = NSNotification(name: "downloadListNotification", object: DownloadRequestListViewController.self, userInfo: userInfo)
        sdfs.downloadChapter(notification)
        //NSNotificationCenter.defaultCenter().postNotification(notification)
}