- (BOOL)checkUsername{    NSString * regex = @"^[A-Za-z0-9]{1,15}$";
    //正则表达式    NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", regex];
    //Cocoa框架中的NSPredicate用于查询，原理和用法都类似于SQL中的where，作用相当于数据库的过滤取    BOOL isMatch = [pred evaluateWithObject:userNameField.text];
    //判读userNameField的值是否吻合

return isMatch; }

- (BOOL)checkPassword{    NSLog(@"passsowrd:%@",passwordField.text);    if (passwordField.text.length > 0) {        return YES;    }    return NO;}