How to highlight CoreText with changing highlight colors?

Question

I have a subclass of UIView that implements code to draw CoreText. In the application, that UIView is drawn inside a UIScrollView. Here is the code that I currently use in drawRect: to render an NSAttributedString:

CGContextRef context = UIGraphicsGetCurrentContext();

float viewHeight = self.bounds.size.height;
CGContextTranslateCTM(context, 0, viewHeight);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextSetTextMatrix(context, CGAffineTransformMakeScale(1.0, 1.0));

CGMutablePathRef path = CGPathCreateMutable();
CGRect bounds = CGRectMake(PADDING_LEFT, -PADDING_TOP, self.bounds.size.width-20.0, self.bounds.size.height);
CGPathAddRect(path, NULL, bounds);

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFMutableAttributedStringRef)attrString);
CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), path, NULL);
CFRelease(framesetter);
CFRelease(path);
CTFrameDraw(frame, context);

In my NSAttributedString, I have certain text attributes to color the text.

What I need help with is conceptually, how should this be accomplished? I am confused by all the various methods of drawing frames, layers, etc. I have tried to review several books and other resources, but I feel like I'm missing some basic stuff.

Here are my requirements:

1. Highlights obviously need to be redrawn to the new location on a change of orientation.
2. Highlight color needs to be able to be changed programmatically on an individual highlight basis.
3. Ideally for performance, I don't want to have to re-draw the entire CoreText UIView when I just change the highlight color of one highlight.
4. The individual Rects must be able to be grouped together to form a single highlight since highlights can cover a range that goes beyond a single CTLine.
5. Eventually, I want to implement UIMenuController on CoreText view so I believe that the highlights must be drawn underneath the CoreText UIView?

I would greatly appreciate any assistance.

Here is what I have tried so far:

I added a new UIView as a subview to the UIScrollView to contain the highlights. I then add individual UIViews for the highlights themselves. When I create dummy "test" highlights, they appear correctly under the CoreText view. I can also easily change the highlight color. However, when I go to acquire actual information about the lines from the CoreText view to create real highlights, it returns zero lines. Could this be because drawRect isn't finished drawing yet or something else?

Here are some questions:

1. Should I add a UIView as a subview to the UIScrollView to contain all the highlights with additional subviews for each individual highlight? Then, store each highlight UIView in a nested NSArray?
2. If I change the color of a highlight beneath the CoreText UIView, will that cause the entire UIView to be redrawn? If so, how can I design around this?
3. It is true that if I want the CoreText UIView to receive the input for UIMenuController, that I have to place the highlight objects beneath the CoreText UIView?

I have seen this open source project:

https://github.com/Cocoanetics/NSAttributedString-Additions-for-HTML

This is amazing what it can do, but is far more complex and detailed than I need. The code renders each individual glyph run using CoreText and permits highlighting, which is awesome. However, I am trying to keep my code simple and lightweight. I know very little about CoreGraphics, CoreAnimation or CoreText.

I have also seen this question:

Core Text - Get Pixel Coordinates from NSRange

The answer to the question provides the calls to determine the bounds of a Rect for a given range in a given line. I think I can figure out those calls.

UPDATE

Here is what I have done now. I'm sure this is wrong, but it is the only way I could find to make it work...

1. Create a UIView as a container to add to the UIScrollView as a subview.
2. Pass UIView to CoreTextView. I also pass an array of string ranges to CoreTextView to indicate which strings should be highlighted.
3. Add CoreTextView as a subview of UIScrollView. That way, UIView is underneath CoreTextView.
4. In the drawRect: function of CoreTextView, I draw the CoreText. I then call a function to calculate the highlights.
5. Each NSRange is checked against the lines and the CGRect of each match in the line is calculated.
6. CGRect is used to create a new UIView. That UIView is added as a subView of the UIView container. It is also added to an NSArray so that I can keep track of it and change the highlight color of an individual highlight as needed.

This does not call drawRect each time a highlight is added to the container or color changed. However, I know this isn't an ideal solution. Problems with this solution:

1. When the UIViewController calls setNeedsDisplay on the CoreTextView, there is no way to know when it is done drawing. I still can't find a way to do a callback to tell the ViewController that it is okay to proceed with the next task.
2. By adding UIViews to the other container UIView, drawRect is technically drawing items outside of its own rect, which I know is a no no.
3. I'm sure this isn't the most performant way of doing this, although it does seem to work pretty well.

Answer 1

Your basic idea isn't too bad for highlighting. I don't know if it's really easier than just highlighting the glyphs while you draw, but it's not a bad idea, and makes it easy to turn on and off highlighting, so that's pretty nice.

Rather than creating lots of views, put your Core Text onto a CALayer instead. Then you can create additional highlighting layers to put over the text. This keeps everything inside of a single view. Layers are much cheaper than views. If you're just trying to do "highlighter" style highlighting, then you can just set the backgroundColor and frame for the layers and you're done.