Swift ---UIView addSubView(UItextView),nothing can be displayed
<pre><code>class ViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    //let attstring = NSMutableAttributedString
    let testTextView = UITextView.init(frame: CGRect.init(x: 0, y: 0, width: 100, height: 100))

    let attstring = NSMutableAttributedString.init(string: "You can check flights from the past 4 days, or plan 3 days ahead.\n\nTo find out when and where we fly for the rest of the year, please view our Timetable.", attributes: [NSFontAttributeName:UIFont.boldSystemFont(ofSize: 12),NSForegroundColorAttributeName:UIColor.red])

    attstring.setAttributes([NSForegroundColorAttributeName:UIColor.blue], range: NSRange.init(location: attstring.length-10, length: 9))

    print("test test!")
    self.view.addSubview(testTextView)
}
</code></pre>

<p>There is nothing displayed,what is the problem!?</p>