Smooth and Accelerating ProgressView

Question

I am playing with ProgressView. What I want to achieve is stopping at 33%, 66%, 100% checkpoints on button click. It's working okay if I use progressView.progress = 0.33, but it directly lands to the checkpoint. Instead, having it smooth and accelerating would look so nice.

I thought animateWithDuration would work but unfortunately it doesn't. After some reading, I found out that I can do something with NSTimer, but I couldn't achieve it.

var progressView: UIProgressView?

override func viewDidLoad()
{
    super.viewDidLoad()

    progressView = UIProgressView(progressViewStyle: UIProgressViewStyle.Default)
    progressView?.center = self.view.center

    progressView?.trackTintColor = UIColor.redColor()
    progressView?.progressTintColor = UIColor.greenColor()

    view.addSubview(progressView!)

    progressView!.progress = 0.0
}

Using this answer, I achieved making it move once in every second, but how can I make it go smooth, slow in the beginning and accelerating, so looks nice.

Answer 1

using setProgress method and setting animation to true makes animation work.

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var pb: UIProgressView!
@IBOutlet weak var btn: UIButton!

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view,  typically from a nib.
    pb.progress = 0.00
}
@IBAction func btnPress(sender: UIButton) {

    UIView.animateWithDuration(3, animations: {
        self.pb.setProgress((self.pb.progress + 0.33), animated: true)
        }, completion: nil)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

}