ios - A lighter way of discovering text writing direction

Question

I want to determine the writing direction of a string so that I can render Right-to-Left languages such as Arabic correctly in a CALayer.

so I have this method

+(UITextAlignment)alignmentForString:(NSString *)astring
{
    UITextView *text = [[UITextView alloc] initWithFrame:CGRectZero];
    text.text = astring;

    if ([text baseWritingDirectionForPosition:[text beginningOfDocument] inDirection:UITextStorageDirectionForward] == UITextWritingDirectionRightToLeft) {
        return UITextAlignmentRight;
    }

    return UITextAlignmentLeft;

}

Works fine but feels a little heavy just for the purpose of discovering which way to align my text especially as its been called in drawInContext (although relatively infrequently).

Is there a lighter way of determining the writing direction for a given string or should I just stick with this under the basis of premature optimisation. And its got to be iOS 5 friendly.

Answer 1

The code in the question although functional is brutally expensive. Run it through a profiler and you will find that it spends close to 80% of the time in UITextView.setText when used in the drawInContext method for a layer.

Most of the answer is here in Detect Language of NSString

a better form is thus...

+(UITextAlignment)alignmentForString:(NSString *)astring
{

    if (astring.length) {

        NSArray *rightLeftLanguages = @[@"ar",@"he"];

        NSString *lang = CFBridgingRelease(CFStringTokenizerCopyBestStringLanguage((CFStringRef)astring,CFRangeMake(0,[astring length])));

        if ([rightLeftLanguages containsObject:lang]) {

            return UITextAlignmentRight;

        }
    }

    return UITextAlignmentLeft;

}

As Arabic and Hebrew are the only Right-to-Left languages detectable by CFStringTokenizerCopyBestStringLanguage and should also cover Persian, Urdu and Yiddish though I havent tested that.

see also http://en.wikipedia.org/wiki/Right-to-left