Pfthb

The default layout manager fills in the background color (specified via NSAttributedString .backgroundColor attribute) where there's no text (except for the last line).

I've managed to achieve the effect I want by sublclassing NSLayoutManager and overriding func drawBackground(forGlyphRange glyphsToShow: NSRange, at origin: CGPoint) as follows:

override func drawBackground(forGlyphRange glyphsToShow: NSRange, at origin: CGPoint) 
 guard let textContainer = textContainers.first, let textStorage = textStorage else fatalError() 

 // This just takes the color of the first character assuming the entire container has the same background color.
 // To support ranges of different colours, you'll need to draw each glyph separately, querying the attributed string for the
 // background color attribute for the range of each character.
 guard textStorage.length > 0, let backgroundColor = textStorage.attribute(.backgroundColor, at: 0, effectiveRange: nil) as? UIColor else return 

 var lineRects = [CGRect]()

 // create an array of line rects to be drawn.
 enumerateLineFragments(forGlyphRange: glyphsToShow) (_, usedRect, _, range, _) in
 var usedRect = usedRect
 let locationOfLastGlyphInLine = NSMaxRange(range)-1
 // Remove the space at the end of each line (except last).
 if self.isThereAWhitespace(at: locationOfLastGlyphInLine) 
 let lastGlyphInLineWidth = self.boundingRect(forGlyphRange: NSRange(location: locationOfLastGlyphInLine, length: 1), in: textContainer).width
 usedRect.size.width -= lastGlyphInLineWidth
 
 lineRects.append(usedRect)
 

 lineRects = adjustRectsToContainerHeight(rects: lineRects, containerHeight: textContainer.size.height)

 for (lineNumber, lineRect) in lineRects.enumerated() 
 guard let context = UIGraphicsGetCurrentContext() else return 
 context.saveGState()
 context.setFillColor(backgroundColor.cgColor)
 context.fill(lineRect)
 context.restoreGState()
 


private func isThereAWhitespace(at location: Int) -> Bool 
 return propertyForGlyph(at: location) == NSLayoutManager.GlyphProperty.elastic

However, this doesn't handle the possibility of having multiple colors specified by range in the attributed string. How might I achieve this? I've looked at fillBackgroundRectArray with little success.

Alternatively, you could bypass using the attributes at all, like this:

So first I defined this struct:

struct HighlightBackground 
 let range: NSRange
 let color: NSColor

Then in my NSTextView subclass:

var highlightBackgrounds = [HighlightBackground]()

override func setSelectedRanges(_ ranges: [NSValue], affinity: NSSelectionAffinity, stillSelecting stillSelectingFlag: Bool) 
 if stillSelectingFlag == false 
 return
 

 // remove old ranges first
 highlightBackgrounds = highlightBackgrounds.filter $0.color != .green 

 for value in ranges 
 let range = value.rangeValue

 highlightBackgrounds.append(HighlightBackground(range: range, color: .green))
 

 super.setSelectedRanges(ranges, affinity: affinity, stillSelecting: stillSelectingFlag)

And then call this from your draw(_ rect: NSRect) method:

func showBackgrounds() 
 guard
 let context = NSGraphicsContext.current?.cgContext,
 let lm = self.layoutManager
 else return 

 context.saveGState()
 // context.translateBy(x: origin.x, y: origin.y)

 for bg in highlightBackgrounds 
 bg.color.setFill()

 let glRange = lm.glyphRange(forCharacterRange: bg.range, actualCharacterRange: nil) 
 for rect in lm.rectsForGlyphRange(glRange) 
 let path = NSBezierPath(roundedRect: rect, xRadius: selectedTextCornerRadius, yRadius: selectedTextCornerRadius)
 path.fill()
 
 

 context.restoreGState()

Finally, you'll need this in your NSLayoutManager subclass, although you probably could also put it in the NSTextView subclass:

func rectsForGlyphRange(_ glyphsToShow: NSRange) -> [NSRect] 

 var rects = [NSRect]()
 guard
 let tc = textContainer(forGlyphAt: glyphsToShow.location, effectiveRange: nil)
 else return rects 

 enumerateLineFragments(forGlyphRange: glyphsToShow) _, _, _, effectiveRange, _ in
 let rect = self.boundingRect(forGlyphRange: NSIntersectionRange(glyphsToShow, effectiveRange), in: tc)
 rects.append(rect)
 

 return rects

Hopefully, this works also in your case.