swift - 多线程下载图片

private func fetchBackgroundImageData() {
     backgroundImage = nil
     if let url = self.backgroundURL {
         DispatchQueue.global(qos: .userInitiated).async {
             if let imageData = try? Data(contentsOf: url) {
                 DispatchQueue.main.async {
                     if url == self.backgroundURL {
                         self.backgroundImage = UIImage(data: imageData)
                     }
                 }
             }
         }
     }
 }