✨ Emphasis API for Highlighting Text Ranges (#62)

* Add `cgPathFallback` property to `NSBezierPath` for macOS versions < 14 to support conversion to `CGPath`

* Add convenience initializer to `NSColor` for creating color from hex value

* Add `smoothPath ` method to `NSBezierPath` for smooth path creation

* Add EmphasizeAPI class to manage text range emphasis with dynamic highlighting

Sources/CodeEditTextView/EmphasizeAPI/EmphasizeAPI.swift

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+//
+//  EmphasizeAPI.swift
+//  CodeEditTextView
+//
+//  Created by Tom Ludwig on 05.11.24.
+//
+
+import AppKit
+
+/// Emphasizes text ranges within a given text view.
+public class EmphasizeAPI {
+    // MARK: - Properties
+
+    private var highlightedRanges: [EmphasizedRange] = []
+    private var emphasizedRangeIndex: Int?
+    private let activeColor: NSColor = NSColor(hex: 0xFFFB00, alpha: 1)
+    private let inactiveColor: NSColor = NSColor.lightGray.withAlphaComponent(0.4)
+
+    weak var textView: TextView?
+
+    init(textView: TextView) {
+        self.textView = textView
+    }
+
+    // MARK: - Structs
+    private struct EmphasizedRange {
+        var range: NSRange
+        var layer: CAShapeLayer
+    }
+
+    // MARK: - Public Methods
+
+    /// Emphasises multiple ranges, with one optionally marked as active (highlighted usually in yellow).
+    ///
+    /// - Parameters:
+    ///   - ranges: An array of ranges to highlight.
+    ///   - activeIndex: The index of the range to highlight in yellow. Defaults to `nil`.
+    ///   - clearPrevious: Removes previous emphasised  ranges. Defaults to `true`.
+    public func emphasizeRanges(ranges: [NSRange], activeIndex: Int? = nil, clearPrevious: Bool = true) {
+        if clearPrevious {
+            removeEmphasizeLayers() // Clear all existing highlights
+        }
+
+        ranges.enumerated().forEach { index, range in
+            let isActive = (index == activeIndex)
+            emphasizeRange(range: range, active: isActive)
+
+            if isActive {
+                emphasizedRangeIndex = activeIndex
+            }
+        }
+    }
+
+    /// Emphasises a single range.
+    /// - Parameters:
+    ///   - range: The text range to highlight.
+    ///   - active: Whether the range should be highlighted as active (usually in yellow). Defaults to `false`.
+    public func emphasizeRange(range: NSRange, active: Bool = false) {
+        guard let shapePath = textView?.layoutManager?.roundedPathForRange(range) else { return }
+
+        let layer = createEmphasizeLayer(shapePath: shapePath, active: active)
+        textView?.layer?.insertSublayer(layer, at: 1)
+
+        highlightedRanges.append(EmphasizedRange(range: range, layer: layer))
+    }
+
+    /// Removes the highlight for a specific range.
+    /// - Parameter range: The range to remove.
+    public func removeHighlightForRange(_ range: NSRange) {
+        guard let index = highlightedRanges.firstIndex(where: { $0.range == range }) else { return }
+
+        let removedLayer = highlightedRanges[index].layer
+        removedLayer.removeFromSuperlayer()
+
+        highlightedRanges.remove(at: index)
+
+        // Adjust the active highlight index
+        if let currentIndex = emphasizedRangeIndex {
+            if currentIndex == index {
+                // TODO: What is the desired behaviour here?
+                emphasizedRangeIndex = nil // Reset if the active highlight is removed
+            } else if currentIndex > index {
+                emphasizedRangeIndex = currentIndex - 1 // Shift if the removed index was before the active index
+            }
+        }
+    }
+
+    /// Highlights the previous emphasised range (usually in yellow).
+    ///
+    /// - Returns: An optional `NSRange` representing the newly active emphasized range.
+    ///            Returns `nil` if there are no prior ranges to highlight.
+    @discardableResult
+    public func highlightPrevious() -> NSRange? {
+        return shiftActiveHighlight(amount: -1)
+    }
+
+    /// Highlights the next emphasised range (usually in yellow).
+    ///
+    /// - Returns: An optional `NSRange` representing the newly active emphasized range.
+    ///            Returns `nil` if there are no subsequent ranges to highlight.
+    @discardableResult
+    public func highlightNext() -> NSRange? {
+        return shiftActiveHighlight(amount: 1)
+    }
+
+    /// Removes all emphasised ranges.
+    public func removeEmphasizeLayers() {
+        highlightedRanges.forEach { $0.layer.removeFromSuperlayer() }
+        highlightedRanges.removeAll()
+        emphasizedRangeIndex = nil
+    }
+
+    // MARK: - Private Methods
+
+    private func createEmphasizeLayer(shapePath: NSBezierPath, active: Bool) -> CAShapeLayer {
+        let layer = CAShapeLayer()
+        layer.cornerRadius = 3.0
+        layer.fillColor = (active ? activeColor : inactiveColor).cgColor
+        layer.shadowColor = .black
+        layer.shadowOpacity = active ? 0.3 : 0.0
+        layer.shadowOffset = CGSize(width: 0, height: 1)
+        layer.shadowRadius = 3.0
+        layer.opacity = 1.0
+
+        if #available(macOS 14.0, *) {
+            layer.path = shapePath.cgPath
+        } else {
+            layer.path = shapePath.cgPathFallback
+        }
+
+        // Set bounds of the layer; needed for the scale animation
+        if let cgPath = layer.path {
+            let boundingBox = cgPath.boundingBox
+            layer.bounds = boundingBox
+            layer.position = CGPoint(x: boundingBox.midX, y: boundingBox.midY)
+        }
+
+        return layer
+    }
+
+    /// Shifts the active highlight to a different emphasized range based on the specified offset.
+    ///
+    /// - Parameter amount: The offset to shift the active highlight.
+    ///                     - A positive value moves to subsequent ranges.
+    ///                     - A negative value moves to prior ranges.
+    ///
+    /// - Returns: An optional `NSRange` representing the newly active highlight, colored in the active color.
+    ///            Returns `nil` if no change occurred (e.g., if there are no highlighted ranges).
+    private func shiftActiveHighlight(amount: Int) -> NSRange? {
+        guard !highlightedRanges.isEmpty else { return nil }
+
+        var currentIndex = emphasizedRangeIndex ?? -1
+        currentIndex = (currentIndex + amount + highlightedRanges.count) % highlightedRanges.count
+
+        guard currentIndex < highlightedRanges.count else { return nil }
+
+        // Reset the previously active layer
+        if let currentIndex = emphasizedRangeIndex {
+            let previousLayer = highlightedRanges[currentIndex].layer
+            previousLayer.fillColor = inactiveColor.cgColor
+            previousLayer.shadowOpacity = 0.0
+        }
+
+        // Set the new active layer
+        let newLayer = highlightedRanges[currentIndex].layer
+        newLayer.fillColor = activeColor.cgColor
+        newLayer.shadowOpacity = 0.3
+
+        applyPopAnimation(to: newLayer)
+        emphasizedRangeIndex = currentIndex
+
+        return highlightedRanges[currentIndex].range
+    }
+
+    private func applyPopAnimation(to layer: CALayer) {
+        let scaleAnimation = CAKeyframeAnimation(keyPath: "transform.scale")
+        scaleAnimation.values = [1.0, 1.5, 1.0]
+        scaleAnimation.keyTimes = [0, 0.3, 1]
+        scaleAnimation.duration = 0.2
+        scaleAnimation.timingFunctions = [CAMediaTimingFunction(name: .easeOut)]
+
+        layer.add(scaleAnimation, forKey: "popAnimation")
+    }
+}

Sources/CodeEditTextView/Extensions/NSBezierPath+CGPathFallback.swift

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+//
+//  NSBezierPath+CGPathFallback.swift
+//  CodeEditTextView
+//
+//  Created by Tom Ludwig on 27.11.24.
+//
+
+import AppKit
+
+extension NSBezierPath {
+    /// Converts the `NSBezierPath` instance into a `CGPath`, providing a fallback method for compatibility(macOS < 14).
+    public var cgPathFallback: CGPath {
+        let path = CGMutablePath()
+        var points = [CGPoint](repeating: .zero, count: 3)
+
+        for index in 0 ..< elementCount {
+            let type = element(at: index, associatedPoints: &points)
+            switch type {
+            case .moveTo:
+                path.move(to: points[0])
+            case .lineTo:
+                path.addLine(to: points[0])
+            case .curveTo:
+                path.addCurve(to: points[2], control1: points[0], control2: points[1])
+            case .closePath:
+                path.closeSubpath()
+            @unknown default:
+                continue
+            }
+        }
+
+        return path
+    }
+}

Sources/CodeEditTextView/Extensions/NSBezierPath+SmoothPath.swift

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+//
+//  NSBezierPath+SmoothPath.swift
+//  CodeEditSourceEditor
+//
+//  Created by Tom Ludwig on 12.11.24.
+//
+
+import AppKit
+import SwiftUI
+
+extension NSBezierPath {
+    private func quadCurve(to endPoint: CGPoint, controlPoint: CGPoint) {
+        guard pointIsValid(endPoint) && pointIsValid(controlPoint) else { return }
+
+        let startPoint = self.currentPoint
+        let controlPoint1 = CGPoint(x: (startPoint.x + (controlPoint.x - startPoint.x) * 2.0 / 3.0),
+                                    y: (startPoint.y + (controlPoint.y - startPoint.y) * 2.0 / 3.0))
+        let controlPoint2 = CGPoint(x: (endPoint.x + (controlPoint.x - endPoint.x) * 2.0 / 3.0),
+                                    y: (endPoint.y + (controlPoint.y - endPoint.y) * 2.0 / 3.0))
+
+        curve(to: endPoint, controlPoint1: controlPoint1, controlPoint2: controlPoint2)
+    }
+
+    private func pointIsValid(_ point: CGPoint) -> Bool {
+        return !point.x.isNaN && !point.y.isNaN
+    }
+
+    // swiftlint:disable:next function_body_length
+    static func smoothPath(_ points: [NSPoint], radius cornerRadius: CGFloat) -> NSBezierPath {
+        // Normalizing radius to compensate for the quadraticCurve
+        let radius = cornerRadius * 1.15
+
+        let path = NSBezierPath()
+
+        guard points.count > 1 else { return path }
+
+        // Calculate the initial corner start based on the first two points
+        let initialVector = NSPoint(x: points[1].x - points[0].x, y: points[1].y - points[0].y)
+        let initialDistance = sqrt(initialVector.x * initialVector.x + initialVector.y * initialVector.y)
+
+        let initialUnitVector = NSPoint(x: initialVector.x / initialDistance, y: initialVector.y / initialDistance)
+        let initialCornerStart = NSPoint(
+            x: points[0].x + initialUnitVector.x * radius,
+            y: points[0].y + initialUnitVector.y * radius
+        )
+
+        // Start path at the initial corner start
+        path.move(to: points.first == points.last ? initialCornerStart : points[0])
+
+        for index in 1..<points.count - 1 {
+            let p0 = points[index - 1]
+            let p1 = points[index]
+            let p2 = points[index + 1]
+
+            // Calculate vectors
+            let vector1 = NSPoint(x: p1.x - p0.x, y: p1.y - p0.y)
+            let vector2 = NSPoint(x: p2.x - p1.x, y: p2.y - p1.y)
+
+            // Calculate unit vectors and distances
+            let distance1 = sqrt(vector1.x * vector1.x + vector1.y * vector1.y)
+            let distance2 = sqrt(vector2.x * vector2.x + vector2.y * vector2.y)
+
+            // TODO: Check if .zero should get used or just skipped
+            if distance1.isZero || distance2.isZero { continue }
+            let unitVector1 = distance1 > 0 ? NSPoint(x: vector1.x / distance1, y: vector1.y / distance1) : NSPoint.zero
+            let unitVector2 = distance2 > 0 ? NSPoint(x: vector2.x / distance2, y: vector2.y / distance2) : NSPoint.zero
+
+            // This uses the dot product formula: cos(θ) = (u1 • u2),
+            // where u1 and u2 are unit vectors. The result will range from -1 to 1:
+            let angleCosine = unitVector1.x * unitVector2.x + unitVector1.y * unitVector2.y
+
+            // If the cosine of the angle is less than 0.5 (i.e., angle > ~60 degrees),
+            // the radius is reduced to half to avoid overlapping or excessive smoothing.
+            let clampedRadius = angleCosine < 0.5 ? radius /** 0.5 */: radius  // Adjust for sharp angles
+
+            // Calculate the corner start and end
+            let cornerStart = NSPoint(x: p1.x - unitVector1.x * radius, y: p1.y - unitVector1.y * radius)
+            let cornerEnd = NSPoint(x: p1.x + unitVector2.x * radius, y: p1.y + unitVector2.y * radius)
+
+            // Check if this segment is a straight line or a curve
+            if unitVector1 != unitVector2 {  // There's a change in direction, add a curve
+                path.line(to: cornerStart)
+                path.quadCurve(to: cornerEnd, controlPoint: p1)
+            } else {  // Straight line, just add a line
+                path.line(to: p1)
+            }
+        }
+
+        // Handle the final segment if the path is closed
+        if points.first == points.last, points.count > 2 {
+            // Closing path by rounding back to the initial point
+            let lastPoint = points[points.count - 2]
+            let firstPoint = points[0]
+
+            // Calculate the vectors and unit vectors
+            let finalVector = NSPoint(x: firstPoint.x - lastPoint.x, y: firstPoint.y - lastPoint.y)
+            let distance = sqrt(finalVector.x * finalVector.x + finalVector.y * finalVector.y)
+            let unitVector = NSPoint(x: finalVector.x / distance, y: finalVector.y / distance)
+
+            // Calculate the final corner start and initial corner end
+            let finalCornerStart = NSPoint(
+                x: firstPoint.x - unitVector.x * radius,
+                y: firstPoint.y - unitVector.y * radius
+            )
+
+            let initialCornerEnd = NSPoint(
+                x: points[0].x + initialUnitVector.x * radius,
+                y: points[0].y + initialUnitVector.y * radius
+            )
+
+            path.line(to: finalCornerStart)
+            path.quadCurve(to: initialCornerEnd, controlPoint: firstPoint)
+            path.close()
+
+        } else if let lastPoint = points.last {  // For open paths, just connect to the last point
+            path.line(to: lastPoint)
+        }
+
+        return path
+    }
+}

Sources/CodeEditTextView/Extensions/NSColor+Hex.swift

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+//
+//  NSColor+Hex.swift
+//  CodeEditTextView
+//
+//  Created by Tom Ludwig on 27.11.24.
+//
+
+import AppKit
+
+extension NSColor {
+    convenience init(hex: Int, alpha: Double = 1.0) {
+        let red = (hex >> 16) & 0xFF
+        let green = (hex >> 8) & 0xFF
+        let blue = hex & 0xFF
+        self.init(srgbRed: Double(red) / 255, green: Double(green) / 255, blue: Double(blue) / 255, alpha: alpha)
+    }
+}