设置plachHoder的占位符

NSMutableAttributedString *placeholder = [[NSMutableAttributedString alloc] initWithString:placeholderString];
    [placeholder addAttribute:NSForegroundColorAttributeName
                        value:UIColorFromRGB(0x999999)
                        range:NSMakeRange(0, placeholderString.length)];
    [placeholder addAttribute:NSFontAttributeName
                        value:[UIFont systemFontOfSize:14*SCALE]
                        range:NSMakeRange(0, placeholderString.length)];
    self.transTextField.attributedPlaceholder = placeholder;