textView设置placeholder

UILabel *placeHolderLabel = [[UILabel alloc] init];
placeHolderLabel.textColor = [UIColor lightGrayColor];
placeHolderLabel.text = Localized(@"这一刻的想法...");
[_textView addSubview:placeHolderLabel];
[_textView setValue:placeHolderLabel forKey:@"_placeholderLabel"];