ios - Resizing textview by content

I am trying to resize height of my textview by content. I am using following code in viewDidLoad:

var textField : UITextView!
textField = UITextView(frame: CGRect(x: 0, y: self.view.bounds.height/2 - 50, width: self.view.bounds.width, height: 36))
self.view.addSubview(textField)
textField.editable=true
textField.font = UIFont (name: "Helvetica Neue", size: 15)
textField.scrollEnabled=false
var amountOfLinesToBeShown:CGFloat = 6
var maxHeight:CGFloat = textField.font.lineHeight * amountOfLinesToBeShown
textField.sizeThatFits(CGSizeMake(textField.frame.size.width, maxHeight))

But it is not working. It looks fixed height.

How can I fix it?