Return value from completion handler - Swift

问题:

I'm using a loadImage method in a Utilities class and am having some trouble with returning images via closures. Basically because my code could return either an image or an error, assigning it to an image property when the method is called will not work.

Is the approach I'm using wrong in the method declaration of the class, or should I be calling the method differently to anticipate potentially differing results? Thanks

public class UtilitiesService: NSObject {     public class func loadImage(urlString:String)     {      var imgURL: NSURL = NSURL(string: urlString)!     let request: NSURLRequest = NSURLRequest(URL: imgURL)     NSURLConnection.sendAsynchronousRequest(         request, queue: NSOperationQueue.mainQueue(),         completionHandler: {(response: NSURLResponse!,data: NSData!,error: NSError!) -> Void in             if error == nil {                 self.returnImage(data)             }     }) }  public class func returnImage(imageData: NSData) -> UIImage {      return UIImage(data: imageData)!  } }  //// view controller class someView: UIViewController { var image.image = loadImage(url) ///will throw a return type error  } 

回答1:

Add a handler to your loadImage func:

Swift 3

 func loadImage(_ urlString: String, handler:@escaping (_ image:UIImage?)-> Void)     {          let imageURL: URL = URL(string: urlString)!          URLSession.shared.dataTask(with: imageURL) { (data, _, _) in             if let data = data{                 handler(UIImage(data: data))             }         }.resume()     } 

Call func like this:

loadImage("SomeURL") { (image) -> Void in             if let image = image{                 DispatchQueue.main.async {                     self.imageView.image = image                 }             }         } 

Swift 2.3

func loadImage(urlString: String, handler: (image:UIImage?)-> Void)     {          let imageURL: NSURL = NSURL(string: urlString)!          NSURLSession.sharedSession().dataTaskWithURL(imageURL) { (data, _, _) in             if let data = data{                 handler(image: UIImage(data: data))             }             }.resume()     } 

Call func like this:

  loadImage("someURL") { (image) -> Void in             if let image = image{                 dispatch_async(dispatch_get_main_queue()) {                     self.imageView.image = image                 }             }         } 

回答2:

I would do it this way:

public class UtilitiesService: NSObject {     public class func loadImage(urlString:String, completion:(resultImage:UIImage) -> Void)     {      var imgURL: NSURL = NSURL(string: urlString)!     let request: NSURLRequest = NSURLRequest(URL: imgURL)     NSURLConnection.sendAsynchronousRequest(         request, queue: NSOperationQueue.mainQueue(),         completionHandler: {(response: NSURLResponse!,data: NSData!,error: NSError!) -> Void in             if error == nil {                 completion(resultImage: UIImage(data: data))             }     }) } }  //// view controller class someView: UIViewController { var image: UIImageView()? //Create the variable the way you need loadImage(url, completion: { (resultImage) -> Void in     image = resultImage //Assign the result to the variable })  } 

I think this will work, if it doesn't, tell me and I'll fix it

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