Loading an "overlay" when running long tasks in iOS

The above answers add a loading view but it doesn't block click events on the screen also it does not provides overlay for rest of screen. You can achieve it as follows:

let alert = UIAlertController(title: nil, message: "Please wait...", preferredStyle: .Alert)

alert.view.tintColor = UIColor.blackColor()
let loadingIndicator: UIActivityIndicatorView = UIActivityIndicatorView(frame: CGRectMake(10, 5, 50, 50)) as UIActivityIndicatorView
loadingIndicator.hidesWhenStopped = true
loadingIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.Gray
loadingIndicator.startAnimating();

alert.view.addSubview(loadingIndicator)
presentViewController(alert, animated: true, completion: nil)

Swift 3.0

let alert = UIAlertController(title: nil, message: "Please wait...", preferredStyle: .alert)

let loadingIndicator = UIActivityIndicatorView(frame: CGRect(x: 10, y: 5, width: 50, height: 50))
loadingIndicator.hidesWhenStopped = true
loadingIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray
loadingIndicator.startAnimating();

alert.view.addSubview(loadingIndicator)
present(alert, animated: true, completion: nil)

Swift 4.0 and newer

let alert = UIAlertController(title: nil, message: "Please wait...", preferredStyle: .alert)

let loadingIndicator = UIActivityIndicatorView(frame: CGRect(x: 10, y: 5, width: 50, height: 50))
loadingIndicator.hidesWhenStopped = true
loadingIndicator.style = UIActivityIndicatorView.Style.gray
loadingIndicator.startAnimating();

alert.view.addSubview(loadingIndicator)
present(alert, animated: true, completion: nil)

and you can hide it as follows:

dismiss(animated: false, completion: nil)

It will be shown as follows:

For anyone late like me, I made some modifications to @Sonrobby code. As i understand, @Sonrobby adds the activity to the overlay on every showOverlay call. And some of the configuration can be passed to the init function, letting only the placement on the showOverlay method.

I also change the overlay's background to black, since my app it is mostly white.

here is the code :

public class LoadingOverlay{

    var overlayView : UIView!
    var activityIndicator : UIActivityIndicatorView!

    class var shared: LoadingOverlay {
        struct Static {
            static let instance: LoadingOverlay = LoadingOverlay()
        }
        return Static.instance
    }

    init(){
        self.overlayView = UIView()
        self.activityIndicator = UIActivityIndicatorView()

        overlayView.frame = CGRectMake(0, 0, 80, 80)
        overlayView.backgroundColor = UIColor(white: 0, alpha: 0.7)
        overlayView.clipsToBounds = true
        overlayView.layer.cornerRadius = 10
        overlayView.layer.zPosition = 1

        activityIndicator.frame = CGRectMake(0, 0, 40, 40)
        activityIndicator.center = CGPointMake(overlayView.bounds.width / 2, overlayView.bounds.height / 2)
        activityIndicator.activityIndicatorViewStyle = .WhiteLarge
        overlayView.addSubview(activityIndicator)
    }

    public func showOverlay(view: UIView) {
        overlayView.center = view.center
        view.addSubview(overlayView)
        activityIndicator.startAnimating()
    }

    public func hideOverlayView() {
        activityIndicator.stopAnimating()
        overlayView.removeFromSuperview()
    }
}

Just create yourself an overlay view, which you add to your parent view and remove it once your task is done, e.g. to add it:

var overlay : UIView? // This should be a class variable

[ ... ]

overlay = UIView(frame: view.frame)
overlay!.backgroundColor = UIColor.blackColor()
overlay!.alpha = 0.8

view.addSubview(overlay!)

For removal:

overlay?.removeFromSuperview()

Blur background + Activity Indicator, Swift 5 example

extension UIView {
    func showBlurLoader() {
        let blurLoader = BlurLoader(frame: frame)
        self.addSubview(blurLoader)
    }

    func removeBluerLoader() {
        if let blurLoader = subviews.first(where: { $0 is BlurLoader }) {
            blurLoader.removeFromSuperview()
        }
    }
}


class BlurLoader: UIView {

    var blurEffectView: UIVisualEffectView?

    override init(frame: CGRect) {
        let blurEffect = UIBlurEffect(style: .dark)
        let blurEffectView = UIVisualEffectView(effect: blurEffect)
        blurEffectView.frame = frame
        blurEffectView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        self.blurEffectView = blurEffectView
        super.init(frame: frame)
        addSubview(blurEffectView)
        addLoader()
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    private func addLoader() {
        guard let blurEffectView = blurEffectView else { return }
        let activityIndicator = UIActivityIndicatorView(style: .whiteLarge)
        activityIndicator.frame = CGRect(x: 0, y: 0, width: 50, height: 50)
        blurEffectView.contentView.addSubview(activityIndicator)
        activityIndicator.center = blurEffectView.contentView.center
        activityIndicator.startAnimating()
    }
}