scrollview动画切断文本（swift3）

Question

我的代码基本上是一个提词器，问题是当代码大约10行时，提词器工作正常，但是当添加20行时，顶行被切断。见图 现在应用此代码动画，前4行被截断。

 extension UITextView {
func numberOfLines() -> Int {
    let layoutManager = self.layoutManager
    let numberOfGlyphs = layoutManager.numberOfGlyphs
    var lineRange: NSRange = NSMakeRange(0, 1)
    var index = 0
    var numberOfLines = 0

    while index < numberOfGlyphs {
        layoutManager.lineFragmentRect(
            forGlyphAt: index, effectiveRange: &lineRange
        )
        index = NSMaxRange(lineRange)
        numberOfLines += 1
    }
    return numberOfLines
}
}
import UIKit
var yPosLabel : CGFloat!
var xPosLabel : CGFloat!



class ViewController: UIViewController {

@IBOutlet var textView: UITextView!



override func viewDidLoad() {
    super.viewDidLoad()
    yPosLabel = self.textView.frame.origin.y
    xPosLabel = self.textView.frame.origin.x






}

func textViewDidBeginEditing(textView: UITextView) {
    if textView.textColor == UIColor.black {
        textView.text = nil
        textView.textColor = UIColor.black
    }
}
func textViewDidEndEditing(textView: UITextView) {
    if textView.text.isEmpty {
        textView.text = ""
        textView.textColor = UIColor.black
    }
}

@IBAction func play(_ sender: Any) {

    let duration = Double(textView.numberOfLines() / 2)

    UIView.animate(withDuration: TimeInterval(duration)) {
        let contentSize = self.textView.contentSize
        let contentInset = self.textView.contentInset
        let textViewBounds = self.textView.bounds
        let newContentOffset = CGPoint(
            x: contentSize.width + contentInset.right - textViewBounds.width,
            y: contentSize.height + contentInset.bottom - textViewBounds.height
        )
        self.textView.contentOffset = newContentOffset
    }




    }}