Adding new UIImage to SubView takes very long - ios

Question

In my first swift project I face some problems by adding a new UIImage to the SubView after downloading the content of this image.
It looks like the download is done pretty fast but the App needs another 5-15 seconds to update the view. I have no clue why.
Here is what I have done:
override func viewDidLoad() {
super.viewDidLoad()
//...
loadPic(PrevPic)
}
func loadPic(prevImage: UIImageView){
//... get URL; result in object["data"]
let fileUrl = NSURL(string: object["data"] as! String)
let session = NSURLSession.sharedSession()
let request = NSMutableURLRequest(URL: fileUrl!)
let task = session.dataTaskWithRequest(request){
(data, response, error) -> Void in
if error != nil {
print(error!.localizedDescription)
} else {
var imagePic: UIImageView?
let imageData = NSData(data: data!)
prevImage.removeFromSuperview()
let image: UIImage = UIImage(data: imageData)!
imagePic = UIImageView(image: image)
imagePic?.contentMode = .ScaleAspectFit
self.view.addSubview(imagePic!)
//... alignment
}
}
task.resume()
//...
}
An ideas why?
Thanks for the support.

Answer 1

You dont have to remove UIImageView from your view and add it back. That is taking a lot of time. You should replace imageView's old image with new one.You can use following code.
if let newImage = UIImage(data: imageData){
imageView.image = newImage
}

Answer 2

When call for the web service then the task execute on main thread thats why the UI become freeze and none responsive. To get rid of this problem call the main queue and the update the UI like this:
func loadPic(prevImage: UIImageView){
//... get URL; result in object["data"]
let fileUrl = NSURL(string: object["data"] as! String)
let session = NSURLSession.sharedSession()
let request = NSMutableURLRequest(URL: fileUrl!)
let task = session.dataTaskWithRequest(request){
(data, response, error) -> Void in
if error != nil {
print(error!.localizedDescription)
} else {
var imagePic: UIImageView?
let imageData = NSData(data: data!)
dispatch_async(dispatch_get_main_queue(), {
prevImage.removeFromSuperview()
let image: UIImage = UIImage(data: imageData)!
imagePic = UIImageView(image: image)
imagePic?.contentMode = .ScaleAspectFit
self.view.addSubview(imagePic!)
//... alignment
})
}
}
task.resume()
//...
}
The basic idea is
dispatch_async(dispatch_get_main_queue(), {
// update UI
})