自动布局TextView NSLayoutConstraint中的Swift未更改，TextView动态的高度更改

我对自动布局一无所知，因为我有一个UITextView，高度为115，宽度为320，如果我在数组中有一些图像，那么我将textview高度设置为50到60，并在下面显示textview的图像，我尝试设置为NSLayoutConstraint，但我无法更改textview的高度。 请给我答案，这是我的代码

首先我试试这个

@IBOutlet var txt_description: UITextView!
     override func viewWillAppear(animated: Bool) {
    self.view.addConstraint(NSLayoutConstraint(item: txt_description, attribute: .Top, relatedBy: .Equal, toItem: view_of_segment, attribute: .Top, multiplier: 1, constant:10))
    }

二是

 override func viewWillAppear(animated: Bool) {
  NSLayoutConstraint.activateConstraints([
            NSLayoutConstraint(item: txt_description, attribute: .Top, relatedBy: .Equal, toItem: txt_title, attribute: .Top, multiplier: 1.0, constant: 24),
            NSLayoutConstraint(item: txt_description, attribute: .Left, relatedBy: .Equal, toItem: txt_title, attribute: .Left, multiplier: 1.0, constant: 0),
            NSLayoutConstraint(item: txt_description, attribute: .Width, relatedBy: .Equal, toItem: txt_title, attribute: .Width, multiplier: 1.0, constant: 0),
            NSLayoutConstraint(item: txt_description, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant:4)
            ])


}

第三，我试试这个

@IBOutlet var EqualHeight_txt_des: NSLayoutConstraint!
@IBOutlet var txt_description: UITextView!

override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated);
 let leftiPadWidth:CGFloat = 50
        txt_description.setTranslatesAutoresizingMaskIntoConstraints(false)
       EqualHeight_txt_des.constant = leftiPadWidth
    }

请给我答案 我无法更改文本视图高度 请分享你的宝贵知识