UITextView 实现 placeholder

class ViewController: UIViewController, UITextViewDelegate {
let defaultLabel: UILabel = UILabel(frame: CGRectMake(6, 8, 0, 0))
let textView: UITextView = UITextView(frame: CGRectMake(10, 44, 300, 200))

func setView() {
// 设置textView
textView.delegate = self
textView.layer.borderWidth = 1
textView.layer.borderColor = UIColor.grayColor().CGColor
textView.tag = 0

// 提示信息Label
defaultLabel.text = "请输入内容"
defaultLabel.textColor = UIColor.grayColor()
defaultLabel.font = UIFont.systemFontOfSize(12)
defaultLabel.sizeToFit()
defaultLabel.numberOfLines = 0
defaultLabel.userInteractionEnabled = true
textView.addSubview(defaultLabel)
view.addSubview(textView)
}

// mark: textView的回调方法
func textViewDidBeginEditing(textView: UITextView) {
self.defaultLabel.hidden = textView.text.characters.count > 0
}
func textViewDidChange(textView: UITextView) {
self.defaultLabel.hidden = textView.text.characters.count > 0
}
func textViewDidEndEditing(textView: UITextView) {
self.defaultLabel.hidden = textView.text.characters.count > 0
}