如何在下载图像之前运行活动指示器？

Question

在图像下载之前，我想启动活动指示器。

这是我使用 url 的图片下载功能

extension UIImageView {

func downloadImage(from url : String){

    let urlRequest = URLRequest(url: URL(string: url)!)
    let task = URLSession.shared.dataTask(with: urlRequest){(data,response,error)in

        if error != nil {

            print("error...")

        }

        DispatchQueue.main.async {
            self.image = UIImage(data:data!)
        }
    }

    task.resume()
}

我的问题是我该怎么做？ （对不起我的英语不好）

Answer 1

您可以在方法中传递一个闭包，该闭包将在下载图像后执行

extension UIImageView {

  func downloadImage(from url : String, completion: ((_ errorMessage: String?) -> Void)?){

    let urlRequest = URLRequest(url: URL(string: url)!)
    let task = URLSession.shared.dataTask(with: urlRequest){ (data,response,error) in

      if error != nil {
        completion?("error...")
      }

      DispatchQueue.main.async {
        self.image = UIImage(data:data!)
        completion?(nil)
      }
    }

    task.resume()
  }
}

在你的 ViewController 中

activityIndicator.startAnimating()
imageView.downloadImage(from: "...") { (err) in
  if err != nil {
    // error handler
  }
  self.activityIndicator.stopAnimating()
}

Answer 2

你可以试试下面的方法

extension UIImageView {

  func downloadImage(from url : String){
    self.activityIndicator.startAnimating()
    let urlRequest = URLRequest(url: URL(string: url)!)
    let task = URLSession.shared.dataTask(with: urlRequest){(data,response,error)in
      if error != nil {
        DispatchQueue.main.async {
          self.activityIndicator.stopAnimating()
        }
        print("error...")
      }
      else {
        DispatchQueue.main.async {
          self.activityIndicator.stopAnimating()
          self.image = UIImage(data:data!)
        }
      }
    }
    task.resume()
  }

  fileprivate var activityIndicator: UIActivityIndicatorView {
    get {
      let activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: .white)
      activityIndicator.hidesWhenStopped = true
      activityIndicator.center = CGPoint(x:self.frame.width/2,
                                     y: self.frame.height/2)
      activityIndicator.stopAnimating()
      self.addSubview(activityIndicator)
      return activityIndicator
    }
  }
}

Answer 3

这是对@Idindu 答案的改进，因为我使用他的答案并发现activityIndicator 存在一些问题。

主要问题是activityIndicator即使图片已经显示也不会停止，我搜索了一下发现是因为每次调用它都会得到一个全新的实例，这就是为什么activityIndicator.stopAnimating()没用。

接下来是activityIndicator的位置并不总是imageView的中心，我找到了更好的方法here。

所以这是我使用的最终版本：

fileprivate var activityIndicator: UIActivityIndicatorView {
get {

    if let indicator = self.subviews.first(where: { $0 is UIActivityIndicatorView }) as? UIActivityIndicatorView {
        print("image view already contains activity indicator")
        return indicator
    }

    let activityIndicator = UIActivityIndicatorView(style: .whiteLarge)
    activityIndicator.hidesWhenStopped = true
    self.addSubview(activityIndicator)
    activityIndicator.translatesAutoresizingMaskIntoConstraints = false

    let centerX = NSLayoutConstraint(item: self,
                                     attribute: .centerX,
                                     relatedBy: .equal,
                                     toItem: activityIndicator,
                                     attribute: .centerX,
                                     multiplier: 1,
                                     constant: 0)

    let centerY = NSLayoutConstraint(item: self,
                                     attribute: .centerY,
                                     relatedBy: .equal,
                                     toItem: activityIndicator,
                                     attribute: .centerY,
                                     multiplier: 1,
                                     constant: 0)

    self.addConstraints([centerX, centerY])
    return activityIndicator
  }
}

Answer 4

这是我加载图像的方式

if(activityList.banner != ""){
    dispatch_async(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), {
    let fileManager = NSFileManager.defaultManager()

    let paths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as String
    let pathSplit = activityList.banner.characters.split{$0 == "/"}.map(String.init)
    let getImagePath = (paths as NSString).stringByAppendingPathComponent("\(pathSplit[pathSplit.count-1])")
    if (fileManager.fileExistsAtPath(getImagePath)){ // if I already download the image before I opened from the folder
        func display_image2(){
            activityImageView.image = UIImage(contentsOfFile: getImagePath)
            activityImageView.image = activityImageView.image?.alpha(0.7)
            activityIndicator.hidden = true
            //iconImageView.hidden = true
        }
        dispatch_async(dispatch_get_main_queue(), display_image2)
    }else{ //in another case I download the image from server
        let url:String = "\(Constants().mainURL)/imagecache/activity_preview/" + activityList.banner
        let imgURL: NSURL = NSURL(string: url)!
        let request: NSURLRequest = NSURLRequest(URL: imgURL)

        let session = NSURLSession.sharedSession()
        let task = session.dataTaskWithRequest(request){
            (data, response, error) -> Void in

            if (error == nil && data != nil){
                func display_image(){
                    activityImageView.image = UIImage(data: data!)

                    if let data = UIImageJPEGRepresentation(UIImage(data: data!)!, 0.8) {
                        data.writeToFile(getImagePath, atomically: true)
                    }
                        activityIndicator.hidden = true
                    //iconImageView.hidden = true
                    activityImageView.image = activityImageView.image?.alpha(0.7)

                }
                dispatch_async(dispatch_get_main_queue(), display_image)
            }

        }

        task.resume()
    }
    })
}