StatusBall.swift

import Cocoa

final class AppDelegate: NSObject, NSApplicationDelegate {
    private var statusItem: NSStatusItem!

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        statusItem = NSStatusBar.system.statusItem(withLength: 450)
        let contentView = ContentView()
        contentView.translatesAutoresizingMaskIntoConstraints = false
        if let button = statusItem.button {
            button.addSubview(contentView)
            NSLayoutConstraint.activate([
                contentView.leadingAnchor.constraint(equalTo: button.leadingAnchor),
                contentView.trailingAnchor.constraint(equalTo: button.trailingAnchor),
                contentView.topAnchor.constraint(equalTo: button.topAnchor),
                contentView.bottomAnchor.constraint(equalTo: button.bottomAnchor)
            ])
        }
    }
}

final class ContentView: NSView {
    var ballPosition: CGPoint = .zero
    var ballVelocity: CGPoint = CGPoint(x: 0.5, y: 0.25)
    let ballRadius: CGFloat = 8
    var timer: Timer?

    let leftBorderWidth: CGFloat = 2
    let rightBorderWidth: CGFloat = 2

    override init(frame frameRect: NSRect) {
        super.init(frame: frameRect)
        self.wantsLayer = true
        setupTimer()
    }

    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    private func setupTimer() {
        self.ballPosition = CGPoint(x: self.bounds.width / 2, y: self.bounds.height / 2)
        timer = Timer.scheduledTimer(withTimeInterval: 1/120, repeats: true) { [weak self] _ in
            self?.updateBall()
        }
    }

    private func updateBall() {
        ballPosition.x += ballVelocity.x
        ballPosition.y += ballVelocity.y

        let boundsWidth = self.bounds.width
        let boundsHeight = self.bounds.height

        let leftBoundary = leftBorderWidth + ballRadius - 1.5
        let rightBoundary = boundsWidth - rightBorderWidth + 1.5

        if ballPosition.x <= leftBoundary {
            ballPosition.x = leftBoundary
            ballVelocity.x *= -1
        }

        if ballPosition.x + ballRadius >= rightBoundary {
            ballPosition.x = rightBoundary - ballRadius
            ballVelocity.x *= -1
        }

        if ballPosition.y >= boundsHeight {
            ballPosition.y = boundsHeight
            ballVelocity.y *= -1
        }

        if ballPosition.y <= -0.5 {
            ballPosition.y = -0.5
            ballVelocity.y *= -1
        }

        self.needsDisplay = true
    }

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        if let ctx = NSGraphicsContext.current?.cgContext {
            ctx.clear(dirtyRect)
        }
        NSColor.white.setFill()

        let leftBorderRect = NSRect(
            x: 0,
            y: 0,
            width: leftBorderWidth,
            height: self.bounds.height
        )
        leftBorderRect.fill()

        let rightBorderRect = NSRect(
            x: self.bounds.width - rightBorderWidth,
            y: 0,
            width: rightBorderWidth,
            height: self.bounds.height
        )
        rightBorderRect.fill()

        let ballRect = NSRect(
            x: ballPosition.x - ballRadius,
            y: ballPosition.y - ballRadius,
            width: ballRadius * 2,
            height: ballRadius * 2
        )
        let path = NSBezierPath(ovalIn: ballRect)
        NSColor.black.setFill()
        path.fill()
    }
}