在Swift 3中使用GCD進行多線程

Question

我正在開發一個基於web服務的應用程序，並且由於向web服務發送請求的速度太快而導致崩潰。我簡直無法讓GCD在Swift 3中工作，我正在撓頭。我決定貶低它，只是嘗試加載4個Web圖像到Web視圖按順序。基於我在網上看到的所有內容，下面的代碼應該可以工作，但它仍然凍結用戶界面，直到所有四個圖像加載。我究竟做錯了什麼？

import UIKit 

let imageURLs = ["http://www.planetware.com/photos-large/F/france-paris-eiffel-tower.jpg", "http://adriatic-lines.com/wp-content/uploads/2015/04/canal-of-Venice.jpg", "http://hd-wall-papers.com/images/wallpapers/hi-resolution-pictures/hi-resolution-pictures-5.jpg", "http://hd-wall-papers.com/images/wallpapers/hi-resolution-pictures/hi-resolution-pictures-1.jpg"] 

class Downloader { 

class func downloadImageWithURL(_ url:String) -> UIImage! { 

    let data = try? Data(contentsOf: URL(string: url)!) 
    return UIImage(data: data!) 
    } 
} 

class ViewController: UIViewController { 

@IBOutlet weak var imageView1: UIImageView! 

@IBOutlet weak var imageView2: UIImageView! 

@IBOutlet weak var imageView3: UIImageView! 

@IBOutlet weak var imageView4: UIImageView! 

@IBOutlet weak var sliderValueLabel: UILabel! 

override func viewDidLoad() { 
    super.viewDidLoad() 
    // Do any additional setup after loading the view, typically from a nib. 
} 

override func didReceiveMemoryWarning() { 
    super.didReceiveMemoryWarning() 
    // Dispose of any resources that can be recreated. 
} 

@IBAction func didClickOnStart(_ sender: AnyObject) { 
    let serialQueue = DispatchQueue(label: "syncQueue") 

    serialQueue.sync{ 
     let img1 = Downloader.downloadImageWithURL(imageURLs[0]) 
     DispatchQueue.main.async(execute: { 

      self.imageView1.image = img1 
     }) 
    } 

    serialQueue.sync{ 
     let img2 = Downloader.downloadImageWithURL(imageURLs[1]) 
     DispatchQueue.main.async(execute: { 

      self.imageView2.image = img2 
     }) 
    } 

    serialQueue.sync{ 
     let img3 = Downloader.downloadImageWithURL(imageURLs[2]) 
     DispatchQueue.main.async(execute: { 

      self.imageView3.image = img3 
     }) 
    } 

    serialQueue.sync{ 
     let img4 = Downloader.downloadImageWithURL(imageURLs[3]) 
     DispatchQueue.main.async(execute: { 

      self.imageView4.image = img4 
     }) 
    } 




    } 
    @IBAction func sliderValueChanged(_ sender: UISlider) { 

     self.sliderValueLabel.text = "\(sender.value * 100.0)" 
    } 

} 

Answer 1

凍結UI

因爲你在呼喚serialQueue.sync。你幾乎從來沒有想打電話sync，在這種情況下，你肯定不會。改爲使用async。