Question

I was getting the NSString from UIWebView which contain iPhone emoji and reversing text excluding emoji. I need to display reverse of NSString again in UIWebView but couldn’t get the reversed string with emoji. I am not identifying the emoji character in string. I have xcode 4.5 with iOS 6.0. here is my code:

 - (NSString *) createReverseStringFromString:(NSString *)inputString {
  if (inputString.length <= 0)
    return inputString;

 NSMutableString *mutableReverseString = [[NSMutableString alloc] 
 initWithCapacity:inputString.length];
 Thfor (NSInteger i = inputString.length -1; i >= 0; i--) {

    NSString *characterString = [inputString substringWithRange:NSMakeRange(i, 1)];
    [mutableReverseString appendString:characterString];

    NSLog(@"mutableReverseString..%@",mutableReverseString);
 }

 NSString *outputString = [mutableReverseString copy];
 [mutableReverseString release];
return [outputString autorelease];
 }

Answer 1

The problem is that many emoji characters are internally stored as a “surrogate pair” of two characters.

For example, “THUMBS UP SIGN” is Unicode point U+1F44D, but if you store it in an NSString and use substringWithRange to get single characters, you will get two characters:

U+D83D, U+DC4D

which is the UTF-16 surrogate pair for U+1F44D.

Of course, if you reverse these two characters, the output is garbage.

The solution is to use rangeOfComposedCharacterSequenceAtIndex to get sequences of characters which “belong together”:

- (NSString *) createReverseStringFromString:(NSString *)inputString {
    if (inputString.length <= 0)
        return inputString;

    NSMutableString *mutableReverseString = [[NSMutableString alloc] 
                         initWithCapacity:inputString.length];

    NSInteger i = inputString.length;
    while (i > 0) {
        NSRange range = [inputString rangeOfComposedCharacterSequenceAtIndex:i-1];
        NSString *characterString = [inputString substringWithRange:range];
        [mutableReverseString appendString:characterString];
        i = range.location;
    }

    NSString *outputString = [mutableReverseString copy];
    return outputString;
}

This function correctly reverses strings containing emoji characters.

(Note: I have omitted all release, autorelease calls because I always compile with ARC. You have to add that again where appropriate.)