例如:textName -> text_name;

- (NSString *)snakeStyleStringFromHumpStyleString:(NSString *)originString {
    NSArray *separatedArray =  [originString      componentsSeparatedByCharactersInSet:[NSCharacterSet uppercaseLetterCharacterSet]];
NSMutableString *processedString = [[NSMutableString alloc]init];
NSInteger loc = 0;
for (NSInteger i = 0; i < separatedArray.count - 1; i ++) {
    NSString *item = [separatedArray objectAtIndex:i];
    [processedString appendString:item];
    NSString *upperCaseString = [originString substringWithRange:NSMakeRange(item.length + loc, 1)];
    [processedString appendFormat:@"_%@",upperCaseString.lowercaseString];
    loc += item.length;
    loc += 1;
}
if (separatedArray.count) {
    [processedString appendString:separatedArray.lastObject];
}
return processedString;
}