use layoutIfNeeded

layoutIfNeeded.swift

 // use a button click to show how layoutIfNeeded() works
 // article link: https://medium.com/@abhimuralidharan/ios-swift-setneedslayout-vs-layoutifneeded-vs-layoutsubviews-5a2b486da31c
 @IBAction func heightPressed(_ sender: AnyObject) {
        view.layoutIfNeeded()
        if(self.blueHeight.constant == 25.0)
        {
            self.blueHeight.constant = self.view.bounds.height - 100.0
        }
        else
        {
            self.blueHeight.constant = 25.0
        }
        UIView.animate(withDuration: 2.0, animations: {
            self.view.layoutIfNeeded()
        }) 
    }