How to call a function every time user opens application
<p>I want to call the function named update every time user taps my app's icon and open it. How would I do that? Which method fires that I can override whenever the app launches no matter is killed from background or user pressed the home button.</p>

<pre><code> import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var label: UILabel!

    @IBOutlet weak var button: UIButton!

    @IBOutlet weak var labeltwo: UILabel!

    var NSDateDefalt = NSUserDefaults.standardUserDefaults()
    var date : NSDate?

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        date = NSUserDefaults.standardUserDefaults().objectForKey("yourKey") as? NSDate
        label.text = "\(date)"
        update()
    }


    @IBAction func buttona(sender: AnyObject) {
        date = NSDate()
        label.text = "\(date)"

        NSUserDefaults.standardUserDefaults().setObject(NSDate(), forKey:"yourKey")
    }



    func update(){
        let now = NSDate()
        let seconds = now.timeIntervalSinceDate(date!)
        labeltwo.text = "\(seconds))"
    }

}
</code></pre>

<p>Update: I implemented in my App Delegate</p>

<pre><code> func applicationWillEnterForeground(application: UIApplication) {
        ViewController().update()     
    }
</code></pre>