写一个字符串的扩展，实现判断字符串是否为空

- (BOOL) isBlankString {

    if ([self isEqualToString:@"(null)"]) {

        return YES;

    }

    if ([self isEqualToString:@"<null>"]) {

        return YES;

    }

    if (self == nil || self == NULL){

        return YES;

    }

    if ([self isKindOfClass:[NSNull class]]){

        return YES;

    }

    if ([[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0){

        return YES;

    }

    return NO;

}