test3.swift

@objc func touchUpStartButtonAction() {
        print("start")
        
        for url in imageUrls {
            DispatchQueue.global(qos: .userInteractive).async {
                let image = UIImage(contentsOfFile: url!.relativePath)
                DispatchQueue.main.async {
                    print(url!.relativePath)
                    self.loadingView!.imageView.image = image
                    self.loadingView!.setNeedsDisplay()
                }
                
                 var t = 0
                 for i in 0 ... 10000000 {
                     t += i
                 }
                 print(t)
            }
           
        }
        
        print("end")
    }