Creating NSTableView Gradient Header

Custom

import AppKit

/// Custom drawing of a Table's Header Row by darkening the background to a light gray rather than just being white.
class GradientTableHeaderCell: NSTableHeaderCell {
    
    /// Light Mode Gradient
    var backgroundFillGradient: NSGradient! {
        NSGradient(colorsAndLocations:
                    (NSColor.secondarySystemFill.withAlphaComponent(0.55), 0.0), // Bottom
                    (NSColor.secondarySystemFill.withAlphaComponent(0.075), 1.0)  // Top
        )
    }
    
    /// Dark Mode Gradient
    var darkBackgroundFillGradient: NSGradient! {
        NSGradient(colorsAndLocations:
                    (NSColor.secondarySystemFill.withAlphaComponent(0.05), 0.0), // Bottom
                    (NSColor.secondarySystemFill.withAlphaComponent(0.55), 1.0)  // Top
        )
    }
    
    override func drawInterior(withFrame cellFrame: NSRect, in controlView: NSView) {
        guard
            let context = NSGraphicsContext.current?.cgContext else {
            // If there's an issue, just draw the default and exit
            super.drawInterior(withFrame: cellFrame, in: controlView)
            return
        }
        
        let backgroundFillRect = CGRect(x: cellFrame.minX,
                                        y: controlView.frame.minY,
                                        width: cellFrame.width,
                                        height: controlView.frame.height)
        
        if controlView.window?.isMainWindow == true {
            // Draw a gradient if the window is main
            if controlView.isDarkAqua == true {
                self.darkBackgroundFillGradient.draw(in: backgroundFillRect, angle: -90.0)
            } else {
                self.backgroundFillGradient.draw(in: backgroundFillRect, angle: -90.0)
            }
        } else {
            // Otherwise draw a flat and muted background
            if controlView.isDarkAqua == true {
                NSColor.secondarySystemFill.withAlphaComponent(0.1).setFill()
            } else {
                NSColor.secondarySystemFill.withAlphaComponent(0.3).setFill()
            }
            context.fill(backgroundFillRect)
        }
        
        // Continue with default dividers, text, and sorting indicator drawing
        super.drawInterior(withFrame: cellFrame, in: controlView)
    }
}

extension NSView {
    /// Returns `true` if the current view is being displayed in any variant of Dark Mode.
    var isDarkAqua: Bool {
        return
            self.effectiveAppearance == NSAppearance(named: .darkAqua) ||
            self.effectiveAppearance == NSAppearance(named: .vibrantDark)
    }
}


// USAGE
// When instantiating, populating, and adding your columns to your table, use it like this:

let newColumn = NSTableColumn(identifier: "my_column_identifier")
newColumn.headerCell = GradientTableHeaderCell(textCell: "Column Title")

It should look like the Table Headers in this screenshot: