NSView subclass that applies an effect when the mouse hovers over it.

HoverView.swift

import AppKit

class HoverView: NSView {

    override var wantsUpdateLayer: Bool {
        return true
    }

    private var isMouseOver = false {
        didSet {
            needsDisplay = true
        }
    }

    override func awakeFromNib() {
        super.awakeFromNib()
        wantsLayer = true
        layerContentsRedrawPolicy = .onSetNeedsDisplay

        let trackingArea = NSTrackingArea(
            rect: bounds,
            options: [.activeInKeyWindow, .mouseEnteredAndExited],
            owner: self,
            userInfo: nil
        )

        addTrackingArea(trackingArea)
    }

    override func updateLayer() {
        super.updateLayer()

        if isMouseOver {
            // Apply hover style.
        } else {
            // Apply normal style.
        }
    }

    override func mouseEntered(with event: NSEvent) {
        super.mouseEntered(with: event)
        isMouseOver = true
    }

    override func mouseExited(with event: NSEvent) {
        super.mouseExited(with: event)
        isMouseOver = false
    }
}