Question

What I’d like to do is to create a text-container component that is able to indicate what is the nearest word when there’s is a touch on it (i.e. the word “behind” the touched point).

First, I created a UILabel subclass and overrode the touchesEnded:withEvent: method to determine the touched CGPoint. I also wrote a method that compute the corresponding “frame” (CGRect) of each word of the text using sizeWithFont:forWidth:lineBreakMode:. With the touched CGPoint and this frame, I can determine which word is actually being touched. But the method that compute the frame only works with mono-line text.

So now I need to know which part of the text is on a given line (i.e. how the text has been splited), so that I can figure the correct left-margin and top-margin of each word.

Any idea how I can obtain this?
Or maybe have you a more straightforward solution to achieve this? This post was unfortunately not very helpful……

Answer 1

(There is a link to a sample code project in your linked post that does contain some useful sample code, but I will outline the process for you here too.)

In short, you are going to need to use Core Text, which is Apple’s advanced C-based text handling framework that backs all sophisticated text layout in iOS and OS X.

The full code is going to be somewhat involved, but key methods you are going to want to look at are:

CTFramesetterCreateWithAttributedString() – use this, in conjunction with an NSAttributedString that you will get from your label’s text – to create the framesetter

CTFramesetterCreateFrame() – use this to get a CTFrameRef for your text from the above framesetter. You will need to create a CGPathRef using your label bounds to do this.

CTFrameGetLines(), CTFrameGetLineOrigins() – use these to get CTLineRefs corresponding to the typeset lines, and the coordinates of the line origins, respectively, then use CTLineGetStringIndexForPosition() to find the character index at a touch location.

You can then use this character index (in the line’s reference frame) to work backward and find the actual character/word/etc within your full string.

Don’t forget that matters are complicated by a couple issues:

1. If you use UILabel’s native drawing you will have to take care to perfectly match your typesetting metrics, which can be cumbersome since most of the objects (e.g. CTFontRef) are not toll-free bridged with their UIKit counterparts. Implementing your own drawing may, sometimes, be easier, which will guarantee metric matching.

2. Core Text uses an inverted coordinate system with respect to the normal iOS drawing system. If you are getting wacky results, and especially if you do your own drawing, this is something to take a look at.

Not the easiest task in the world, but far from impossible. Good luck!