Meet TextKit 2

32:22 - Responding to layout updates: textViewportLayoutControllerWillLayout()

func textViewportLayoutControllerWillLayout(_ controller: NSTextViewportLayoutController) {
    contentLayer.sublayers = nil
    CATransaction.begin()
}

32:47 - Responding to layout updates: textViewportLayoutController(_:configureRenderingSurfaceFor:)

private func animate(_ layer: CALayer, from source: CGPoint, to destination: CGPoint) {
    let animation = CABasicAnimation(keyPath: "position")
    animation.fromValue = source
    animation.toValue = destination
    animation.duration = slowAnimations ? 2.0 : 0.3
    layer.add(animation, forKey: nil)
}

private func findOrCreateLayer(_ textLayoutFragment: NSTextLayoutFragment) -> (TextLayoutFragmentLayer, Bool) {
    if let layer = fragmentLayerMap.object(forKey: textLayoutFragment) as? TextLayoutFragmentLayer {
        return (layer, false)
    } else {
        let layer = TextLayoutFragmentLayer(layoutFragment: textLayoutFragment, padding: padding)
        fragmentLayerMap.setObject(layer, forKey: textLayoutFragment)
        return (layer, true)
    }
}

func textViewportLayoutController(_ controller: NSTextViewportLayoutController,
                                  configureRenderingSurfaceFor textLayoutFragment: NSTextLayoutFragment) {
    let (layer, layerIsNew) = findOrCreateLayer(textLayoutFragment)
    if !layerIsNew {
        let oldPosition = layer.position
        let oldBounds = layer.bounds
        layer.updateGeometry()
        if oldBounds != layer.bounds {
            layer.setNeedsDisplay()
        }
        if oldPosition != layer.position {
            animate(layer, from: oldPosition, to: layer.position)
        }
    }
    if layer.showLayerFrames != showLayerFrames {
        layer.showLayerFrames = showLayerFrames
        layer.setNeedsDisplay()
    }
    
    contentLayer.addSublayer(layer)
}

33:10 - Responding to layout updates: textViewportLayoutControllerDidLayout()

func textViewportLayoutControllerDidLayout(_ controller: NSTextViewportLayoutController) {
    CATransaction.commit()
    updateSelectionHighlights()
    updateContentSizeIfNeeded()
    adjustViewportOffsetIfNeeded()
}

33:47 - Overriding text attributes for comments

func textContentStorage(_ textContentStorage: NSTextContentStorage, textParagraphWith range: NSRange) -> NSTextParagraph? {
    // In this method, we'll inject some attributes for display, without modifying the text storage directly.
    var paragraphWithDisplayAttributes: NSTextParagraph? = nil
    
    // First, get a copy of the paragraph from the original text storage.
    let originalText = textContentStorage.textStorage!.attributedSubstring(from: range)
    if originalText.attribute(.commentDepth, at: 0, effectiveRange: nil) != nil {
        // Use white colored text to make our comments visible against the bright background.
        let displayAttributes: [NSAttributedString.Key: AnyObject] = [.font: commentFont, .foregroundColor: commentColor]
        let textWithDisplayAttributes = NSMutableAttributedString(attributedString: originalText)
        // Use the display attributes for the text of the comment itself, without the reaction.
        // The last character is the newline, second to last is the attachment character for the reaction.
        let rangeForDisplayAttributes = NSRange(location: 0, length: textWithDisplayAttributes.length - 2)
        textWithDisplayAttributes.addAttributes(displayAttributes, range: rangeForDisplayAttributes)
        
        // Create our new paragraph with our display attributes.
        paragraphWithDisplayAttributes = NSTextParagraph(attributedString: textWithDisplayAttributes)
    } else {
        return nil
    }
    // If the original paragraph wasn't a comment, this return value will be nil.
    // The text content storage will use the original paragraph in this case.
    return paragraphWithDisplayAttributes
}

34:06 - Hiding comments

func textContentManager(_ textContentManager: NSTextContentManager,
                        shouldEnumerate textElement: NSTextElement,
                        with options: NSTextElementProviderEnumerationOptions) -> Bool {
    // The text content manager calls this method to determine whether each text element should be enumerated for layout.
    // To hide comments, tell the text content manager not to enumerate this element if it's a comment.
    if !showComments {
        if let paragraph = textElement as? NSTextParagraph {
            let commentDepthValue = paragraph.attributedString.attribute(.commentDepth, at: 0, effectiveRange: nil)
            if commentDepthValue != nil {
                return false
            }
        }
    }
    return true
}

34:28 - Generating special layout fragments for comments

func textLayoutManager(_ textLayoutManager: NSTextLayoutManager,
                       textLayoutFragmentFor location: NSTextLocation,
                       in textElement: NSTextElement) -> NSTextLayoutFragment {
    let index = textLayoutManager.offset(from: textLayoutManager.documentRange.location, to: location)
    // swiftlint:disable force_cast
    let commentDepthValue = textContentStorage!.textStorage!.attribute(.commentDepth, at: index, effectiveRange: nil) as! NSNumber?
    if commentDepthValue != nil {
        let layoutFragment = BubbleLayoutFragment(textElement: textElement, range: textElement.elementRange)
        layoutFragment.commentDepth = commentDepthValue!.uintValue
        return layoutFragment
    } else {
        return NSTextLayoutFragment(textElement: textElement, range: textElement.elementRange)
    }
}

34:58 - Drawing the comment bubble

var commentDepth: UInt = 0

private var tightTextBounds: CGRect {
    var fragmentTextBounds = CGRect.null
    for lineFragment in textLineFragments {
        let lineFragmentBounds = lineFragment.typographicBounds
        if fragmentTextBounds.isNull {
            fragmentTextBounds = lineFragmentBounds
        } else {
            fragmentTextBounds = fragmentTextBounds.union(lineFragmentBounds)
        }
    }
    return fragmentTextBounds
}

// Return the bounding rect of the chat bubble, in the space of the first line fragment.
private var bubbleRect: CGRect { return tightTextBounds.insetBy(dx: -3, dy: -3) }

private var bubbleCornerRadius: CGFloat { return 20 }

private var bubbleColor: Color { return .systemIndigo }

private func createBubblePath(with ctx: CGContext) -> CGPath {
    let bubbleRect = self.bubbleRect
    let rect = min(bubbleCornerRadius, bubbleRect.size.height / 2, bubbleRect.size.width / 2)
    return CGPath(roundedRect: bubbleRect, cornerWidth: rect, cornerHeight: rect, transform: nil)
}

override var renderingSurfaceBounds: CGRect {
    return bubbleRect.union(super.renderingSurfaceBounds)
}

override func draw(at renderingOrigin: CGPoint, in ctx: CGContext) {
    // Draw the bubble and debug outline.
    ctx.saveGState()
    let bubblePath = createBubblePath(with: ctx)
    ctx.addPath(bubblePath)
    ctx.setFillColor(bubbleColor.cgColor)
    ctx.fillPath()
    ctx.restoreGState()
    
    // Draw the text on top.
    super.draw(at: renderingOrigin, in: ctx)
}

37:26 - Opting NSTextView in to TextKit 2

var scrollView: NSScrollView!
var containerSize = CGSize.zero
var textContainer = NSTextContainer()

// Important: Keep a reference to text storage since NSTextView weakly references it.
var textContentStorage = NSTextContentStorage()

override func viewDidLoad() {
    super.viewDidLoad()
    
    scrollView =
        NSScrollView(frame: NSRect(origin: CGPoint(),
                                   size: CGSize(width: view.bounds.width, height: view.bounds.height)))
    scrollView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(scrollView)
    
    NSLayoutConstraint.activate([
        scrollView.leadingAnchor.constraint(equalTo: (view.leadingAnchor)),
        scrollView.trailingAnchor.constraint(equalTo: (view.trailingAnchor)),
        scrollView.topAnchor.constraint(equalTo: (view.topAnchor)),
        scrollView.bottomAnchor.constraint(equalTo: (view.bottomAnchor))
        ])
    
    setUpScrollView(scrollsHorizontally: false)
}

func setUpScrollView(scrollsHorizontally: Bool) {
    scrollView.borderType = .noBorder
    scrollView.hasVerticalScroller = true
    scrollView.hasHorizontalScroller = scrollsHorizontally
    
    setUpTextContainer(scrollsHorizontally: scrollsHorizontally)
    setUpTextView(scrollsHorizontally: scrollsHorizontally)
}

func setUpTextContainer(scrollsHorizontally: Bool) {
    let contentSize = scrollView.contentSize
    if scrollsHorizontally {
        containerSize = NSSize(width: CGFloat.greatestFiniteMagnitude, height: CGFloat.greatestFiniteMagnitude)
        textContainer.containerSize = containerSize
        textContainer.widthTracksTextView = false
    }
    else {
        containerSize = NSSize(width: contentSize.width, height: CGFloat.greatestFiniteMagnitude)
        textContainer.containerSize = containerSize
        textContainer.widthTracksTextView = true
    }
}

func setUpTextView(scrollsHorizontally: Bool) {
    let textLayoutManager = NSTextLayoutManager()
    textLayoutManager.textContainer = textContainer

    textContentStorage.addTextLayoutManager(textLayoutManager)

    // Workaround: Pass textLayoutManager.textContainer to the NSTextView initializer
    let textView = NSTextView(frame: scrollView.contentView.bounds, textContainer: textLayoutManager.textContainer)

    textView.isEditable = true
    textView.isSelectable = true
    textView.minSize = CGSize()
    textView.maxSize = containerSize
    textView.isVerticallyResizable = true
    textView.isHorizontallyResizable = scrollsHorizontally
    textContentStorage.performEditingTransaction {
        textView.textStorage?.append(NSAttributedString(string: "Text content..."))
    }
    scrollView.documentView = textView
}