TextField 当输入密码错误时，再次修改该密码时，会全部清

直接上代码：

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    //得到输入框的内容
    NSString * textfieldContent = [textField.text stringByReplacingCharactersInRange:range withString:string];
    if (textField == self.passWordTF && textField.isSecureTextEntry ) {
        textField.text = textfieldContent;
        return NO;
    }
    return YES;
}