Fades the last line of a `UITextView` horizontally, instead of truncating it.

UITextView+FadeLast.swift

extension UITextView {
    var lineFrames: [CGRect] {
        let numberOfGlyphs = layoutManager.numberOfGlyphs
        var numberOfLines = 0
        var index = 0
        var lineRange = NSRange()
        let maxNumberOfLines = textContainer.maximumNumberOfLines
        var lineRects = [CGRect]()

        while (index < numberOfGlyphs) {
            let frame = layoutManager.lineFragmentRect(forGlyphAt: index, effectiveRange: &lineRange)
            lineRects.append(frame)
            index = NSMaxRange(lineRange);
            numberOfLines = numberOfLines + 1

            if maxNumberOfLines != 0 && numberOfLines == maxNumberOfLines {
                break
            }
        }

        return lineRects
    }

    func fadeLastVisibleLine() {
        let mask = CALayer()
        mask.frame = layer.bounds
        let topInset = self.textContainerInset.top
        zip(0..<lineFrames.count, lineFrames).forEach { args in
            let (index, frame) = args
            if index == lineFrames.count - 1 {
                let gradientMask = CAGradientLayer()
                gradientMask.colors = [UIColor.white.cgColor, UIColor.clear.cgColor]
                gradientMask.startPoint = CGPoint(x: 0.7, y: 0)
                gradientMask.endPoint = CGPoint(x: 0.95, y: 0)
                gradientMask.frame = frame.offsetBy(dx: 0, dy: topInset)
                mask.addSublayer(gradientMask)
            } else {
                let solidMask = CALayer()
                solidMask.backgroundColor = UIColor.white.cgColor
                solidMask.opacity = 1
                solidMask.frame = frame.offsetBy(dx: 0, dy: topInset)
                mask.addSublayer(solidMask)
            }
        }

        self.layer.mask = mask
    }
}