How to detect which word touched with CoreText

Question

I am using CoreText to layout a custom view. The next step for me is to detect which word is tapped on a touch event/gesture event. I have done research on this and found advise on how to custom label a url to receive touches - but nothing generic. Does any one have any idea on how to do this?

UPDATE:

Here is the code within my drawRect: method self.attribString = [aString copy];

CGContextRef context = UIGraphicsGetCurrentContext();

// Flip the coordinate system
CGContextSetTextMatrix(context, CGAffineTransformIdentity);
CGContextTranslateCTM(context, 0, self.bounds.size.height);
CGContextScaleCTM(context, 1.0, -1.0);

CGMutablePathRef path = CGPathCreateMutable(); //1
CGPathAddRect(path, NULL, self.bounds );

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef)aString); //3
CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, [aString length]), path, NULL);

CTFrameDraw(frame, context); //4
UIGraphicsPushContext(context);
frameRef = frame;

CFRelease(path);
CFRelease(framesetter);

Here is where I am trying to handle the touch event

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint point = [touch locationInView:self];
    CGContextRef context = UIGraphicsGetCurrentContext();

    CFArrayRef lines = CTFrameGetLines(frameRef);
    for(CFIndex i = 0; i < CFArrayGetCount(lines); i++)
    {
        CTLineRef line = CFArrayGetValueAtIndex(lines, i);
        CGRect lineBounds = CTLineGetImageBounds(line, context);
        NSLog(@"Line %ld (%f, %f, %f, %f)", i, lineBounds.origin.x, lineBounds.origin.y, lineBounds.size.width, lineBounds.size.height);
        NSLog(@"Point (%f, %f)", point.x, point.y);
        if(CGRectContainsPoint(lineBounds, point))
        {

It seems that CTLineGetImageBounds is returning a wrong origin (the size seems correct) here is one example of the NSLog "Line 0 (0.562500, -0.281250, 279.837891, 17.753906)".

I should add that all of the origins are very similar in value. — xizor

Rob Napier Rob Napier · Accepted Answer · 2012-03-12T15:39:39

There is no "current context" in touchesEnded:withEvent:. You are not drawing at this point. So you can't call CTLineGetImageBounds() meaningfully.

I believe the best solution here is to use CTFrameGetLineOrigins() to find the correct line (by checking the Y origins), and then using CTLineGetStringIndexForPosition() to find the correct character within the line (after subtracting the line origin from point). This works best for simple stacked lines that run the entire view (such as yours).

How to detect which word touched with CoreText

1 Answers