gistfile1.m

// Public method to determine the CGRect for the insertion point or selection, used
// when creating or updating our SimpleCaretView instance
- (CGRect)caretRectForIndex:(int)index
{  
    if (_editingFrame) 
    {
        CTFrameRef _frame = (CTFrameRef)[_editingFrame objectForKey:@"frame"];
        NSArray *lines = (NSArray *) CTFrameGetLines(_frame);
        NSString *text = [_editingFrame objectForKey:@"text"];
        
        // Special case, no text
        if (text.length == 0) 
        {
            CGPoint origin = CGPointMake(CGRectGetMinX(self.bounds), CGRectGetMaxY(self.bounds) - 10);
            // Note: using fabs() for typically negative descender from fonts
            return CGRectMake(origin.x, origin.y, 3, 10);
        }    
        
        // Special case, insertion point at final position in text after newline
        if (index == _text.length && [_text characterAtIndex:(index - 1)] == '\n') 
        {
            CTLineRef line = (CTLineRef) [lines lastObject];
            CFRange range = CTLineGetStringRange(line);
            CGFloat xPos = CTLineGetOffsetForStringIndex(line, range.location, NULL);
            CGPoint origin;
            CGFloat ascent, descent;
            CTLineGetTypographicBounds(line, &ascent, &descent, NULL);
            CTFrameGetLineOrigins(_frame, CFRangeMake(lines.count - 1, 0), &origin); 
            // Place point after last line, including any font leading spacing if applicable
            origin.y -= 10;
            return CGRectMake(xPos, origin.y - descent, 3, ascent + descent);        
        }
        
        // Regular case, caret somewhere within our text content range
        for (int i = 0; i < [lines count]; i++) 
        {
            CTLineRef line = (CTLineRef) [lines objectAtIndex:i];
            CFRange range = CTLineGetStringRange(line);
            NSInteger localIndex = index - range.location;
            if (localIndex >= 0 && localIndex <= range.length) 
            {
                // index is in the range for this line
                CGFloat xPos = CTLineGetOffsetForStringIndex(line, index, NULL);
                CGPoint origin;
                CGFloat ascent, descent;
                CTLineGetTypographicBounds(line, &ascent, &descent, NULL);
                CTFrameGetLineOrigins(_frame, CFRangeMake(i, 0), &origin);
                // Make a small "caret" rect at the index position
                NSLog(@"returning");
                NSLog(@"%@", NSStringFromCGRect(CGRectMake(xPos, origin.y - descent, 3, ascent + descent)));
                return CGRectMake(xPos, origin.y - descent, 3, ascent + descent);
            }
        }

    }
  	    
    return CGRectNull;
}

Output

GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Mon Aug 15 16:03:10 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin".sharedlibrary apply-load-rules all
Attaching to process 25246.
2011-12-19 13:26:41.435 CTRenderer[25246:207] returning
2011-12-19 13:26:41.436 CTRenderer[25246:207] {{0, 436.74}, {3, 23.6}}
warning: check_safe_call: could not restore current frame

warning: Unable to restore previously selected frame.
warning: Unable to restore previously selected frame.
Cannot access memory at address 0x42f80000
warning: Unable to restore previously selected frame.
warning: Unable to restore previously selected frame.
warning: Unable to restore previously selected frame.
warning: check_safe_call: could not restore current frame

warning: Unable to restore previously selected frame.
warning: Unable to restore previously selected frame.
warning: Unable to restore previously selected frame.
(gdb) bt
#0  0x42f80000 in ?? ()

This function calls caretRectForIndex: and prints the result.

// Helper method to update caretView when insertion point/selection changes
- (void)selectionChanged
{
	// If not in editing mode, we don't show the caret
    if (!self.editing) 
    {
        [_caretView removeFromSuperview];
        return;
    }
    
    
	// If there is no selection range (always true for this sample), find the
	// insert point rect and create a caretView to draw the caret at this position
    if (self.__selectedTextRange.length == 0) 
    {
        CGRect f = [self caretRectForIndex:self.__selectedTextRange.location];
        
        NSLog(@"%@", NSStringFromCGRect(f));
        
    }
}