UITextView添加placeHolder

```

UILabel*placeHolderLabel = [[UILabelalloc]init];

    placeHolderLabel.text=@"写笔记...";

    placeHolderLabel.numberOfLines=0;

    placeHolderLabel.textColor= [UIColorlightGrayColor];

    [placeHolderLabelsizeToFit];

    [self.textViewaddSubview:placeHolderLabel];

```