UITextView gradient layer apply not working

Question

I want to apply gradient layer on top 10% and bottom 10% of UITextView. To do this, I place a dummy UIView called container view and make UITextView a subview of it. And then I add the following code:

   if let containerView = textView.superview {
        let gradient = CAGradientLayer(layer: containerView.layer)
        gradient.frame = containerView.bounds
        gradient.colors = [UIColor.clear.cgColor, UIColor.black.cgColor]
        gradient.locations = [0.0, 0.1, 0.9, 1.0]
        containerView.layer.mask = gradient
    } 

But the gradient is only applied to the top, not the bottom. Is there something wrong with the code?

Further, if I resize the container view anytime by modifying it's constraints, do I need to edit the mask layer every time?

Edit: Here is the output from @DonMag answer.

But what I want is something like in this image that text fades at the bottom.

EDIT2:

Here are screenshots after DonMag's revised answer.

Answer 1

@DongMag solution is very complicated. Instead, you just need a mask implemented like:

@IBDesignable
class MaskableLabel: UILabel {
    var maskImageView = UIImageView()

    @IBInspectable
    var maskImage: UIImage? {
        didSet {
            maskImageView.image = maskImage
            updateView()
        }
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        updateView()
    }

    func updateView() {
        if maskImageView.image != nil {
            maskImageView.frame = bounds
            mask = maskImageView
        }
    }
}

Then with a simple gradient mask like this, You can see it even right in the storyboard.

_{Note: You can use this method and replace UILabel with any other view you like to subclass.}

Here is the example project on the GitHub