Assign a new image to UIImageView with Swift

Question

I'm using Xcode 6 Beta 4, and I'm trying to retrieve a remote image and assign it to a UIImageView. I'm using an NSURLSession dataTask in order to make this asynchronous. Here is my code:

func dataTask(imageUrl: String, targetImage:UIImageView) {

    let nsURL = NSURL(string: imageUrl)

    let task = NSURLSession.sharedSession().dataTaskWithURL(nsURL) {
        (data, response, error) in
        if !error {
            NSLog("No Error!")                
            var image:UIImage = UIImage(data: data)
            targetImage.image = image
        }
        else {
            NSLog("Error!")
        }
    }

    task.resume()
}

this is how i make the call:

        self.dataTask("http://bicicletaspony.com/img/spn/TeamA.png", targetImage: image)

And this is the error i get :(

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Modifications to the layout engine must not be performed from a background thread.'

It's a very simple image assignment. Why does it crash that way?! any help will be much appreciated!

Answer 1

The closure run at the end of dataTaskWithURL is called on a background thread. UIKit (like most other toolkits) can only be called from the main thread. You'll need to set the image on the main thread with something like this:

if !error {
    NSLog("No Error!")             
    var image:UIImage = UIImage(data: data)
    dispatch_async(dispatch_get_main_queue(), {
        targetImage.image = image
    })
}