刷新时，UITableView无法滚动

Question

Using Parse I am downloading images, text etc. and using them in table view cells. However, while refreshing (when the indicator is animating in the navigation bar) I cannot scroll or move on the app. It is as if the app has become unresponsive until the table view is done refreshing.

Any help is greatly appreciated.

Answer 1

Like some of the comments already said, it's due to what's called "blocking the main thread". See this great tutorial to learn more about concurrency in Swift. The WithBlock or InBackground methods that you call on your Parse query should call it asynchronously. If for some reason you still have the issue, you should nest your Parse query inside a dispatch_async block. Here's a useful tutorial on how to do this.

Caution: Always do UI updating on your main thread. Bad things will happen if you don't.

Answer 2

Here is a simple example on how to dispatch to a different thread, download the image and get back to main thread when the download is done.

func downloadImageAndUpdateUI(){
    imageView?.image = nil

    if let imageUrl = aValidUrl{
        //get image on another thread
        let qos = Int(QOS_CLASS_BACKGROUND.rawValue)
        dispatch_async(dispatch_get_global_queue(qos, 0), {
            let imageData = NSData(contentsOfURL: imageUrl)
            //get back to main thread
            dispatch_async(dispatch_get_main_queue(), {
                if imageUrl == self.requestedImageUrl { //making sure if this image is still needed (or if user havent moved on to something else)
                    if imageData != nil{
                        self.imageView.image = UIImage(data: imageData!)
                    }
                }
            })
        })
    }
}