objective c - Character index at touch point for UILabel

Question

For a UILabel, I'd like to find out which character index is at specific point received from a touch event. I'd like to solve this problem for iOS 7 using Text Kit.

Since UILabel doesn't provide access to its NSLayoutManager, I created my own based on UILabel's configuration like this:

- (void)textTapped:(UITapGestureRecognizer *)recognizer
{
    if (recognizer.state == UIGestureRecognizerStateEnded) {
        CGPoint location = [recognizer locationInView:self];

        NSTextStorage *textStorage = [[NSTextStorage alloc] initWithAttributedString:self.attributedText];
        NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init];
        [textStorage addLayoutManager:layoutManager];
        NSTextContainer *textContainer = [[NSTextContainer alloc] initWithSize:self.bounds.size];
        [layoutManager addTextContainer:textContainer];

        textContainer.maximumNumberOfLines = self.numberOfLines;
        textContainer.lineBreakMode = self.lineBreakMode;


        NSUInteger characterIndex = [layoutManager characterIndexForPoint:location
                                                          inTextContainer:textContainer
                                 fractionOfDistanceBetweenInsertionPoints:NULL];

        if (characterIndex < textStorage.length) {
            NSRange range = NSMakeRange(characterIndex, 1);
            NSString *value = [self.text substringWithRange:range];
            NSLog(@"%@, %zd, %zd", value, range.location, range.length);
        }
    }
}

The code above is in a UILabel subclass with a UITapGestureRecognizer configured to call textTapped: (Gist).

The resulting character index makes sense (increases when tapping from left to right), but is not correct (the last character is reached at roughly half the width of the label). It looks like maybe the font size or text container size is not configured properly, but can't find the problem.

I'd really like to keep my class a subclass of UILabel instead of using UITextView. Has anyone solved this problem for UILabel?

Update: I spent a DTS ticket on this question and the Apple engineer recommended to override UILabel's drawTextInRect: with an implementation that uses my own layout manager, similar to this code snippet:

- (void)drawTextInRect:(CGRect)rect 
{
    [yourLayoutManager drawGlyphsForGlyphRange:NSMakeRange(0, yourTextStorage.length) atPoint:CGPointMake(0, 0)];
}

I think it would be a lot of work to keep my own layout manager in sync with the label's settings, so I'll probably go with UITextView despite my preference for UILabel.

Update 2: I decided to use UITextView after all. The purpose of all this was to detect taps on links embedded in the text. I tried to use NSLinkAttributeName, but this setup didn't trigger the delegate callback when tapping a link quickly. Instead, you have to press the link for a certain amount of time – very annoying. So I created CCHLinkTextView that doesn't have this problem.

Answer 1

I played around with the solution of Alexey Ishkov. Finally i got a solution! Use this code snippet in your UITapGestureRecognizer selector:

UILabel *textLabel = (UILabel *)recognizer.view;
CGPoint tapLocation = [recognizer locationInView:textLabel];

// init text storage
NSTextStorage *textStorage = [[NSTextStorage alloc] initWithAttributedString:textLabel.attributedText];
NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init];
[textStorage addLayoutManager:layoutManager];

// init text container
NSTextContainer *textContainer = [[NSTextContainer alloc] initWithSize:CGSizeMake(textLabel.frame.size.width, textLabel.frame.size.height+100) ];
textContainer.lineFragmentPadding  = 0;
textContainer.maximumNumberOfLines = textLabel.numberOfLines;
textContainer.lineBreakMode        = textLabel.lineBreakMode;

[layoutManager addTextContainer:textContainer];

NSUInteger characterIndex = [layoutManager characterIndexForPoint:tapLocation
                                inTextContainer:textContainer
                                fractionOfDistanceBetweenInsertionPoints:NULL];

Hope this will help some people out there!