修改TextView的placeholder

修改TextView的placeholder使用runtime 使用kvc

- (void)setupTextView{    // _placeholderLabel    UILabel *placeHolderLabel = [[UILabel alloc] init];    placeHolderLabel.text = @"请输入您宝贵的意见或建议。（200字以内）";    placeHolderLabel.numberOfLines = 0;    placeHolderLabel.textColor = [UIColor lightGrayColor];    placeHolderLabel.font = [UIFont systemFontOfSize:14];    [placeHolderLabel sizeToFit];    [self.contentTextView addSubview:placeHolderLabel];    [self.contentTextView setValue:placeHolderLabel forKey:@"_placeholderLabel"];}