Extension: Quickly return the macOS' current system appearance

NSView+DarkMode.swift

extension NSView {
  var hasDarkAppearance: Bool {
    if #available(OSX 10.14, *) {
      switch effectiveAppearance.name {
      case .darkAqua, .vibrantDark, .accessibilityHighContrastDarkAqua, .accessibilityHighContrastVibrantDark:
        return true
      default:
         return false
      }
    } else {
      switch effectiveAppearance.name {
      case .vibrantDark:
        return true
      default:
        return false
      }
    }
  }
}

Usage Example

if view.hasDarkAppearance {
  // macOS is in dark mode
} else {
  // macOS is in default (light) mode
}

Note: When macOS toggles between modes, there is a slight delay caused by the transition time. Due to this, there is a noticeable interval in which the hierarchy correctly updates NSView's effectiveAppearance value.

One example, that I've found to be particularly annoying, is with observing the system appearance change. Something like this:

DistributedNotificationCenter.default.addObserver(forName: .systemAppearanceDidChange,
                                                          object: nil, queue: OperationQueue.main) {
  [weak weakSelf = self] (notification) in
  weakSelf?.updateAppearance()
}

func updateAppearance() {
  let isDarkMode = view.hasDarkAppearance 
}

The issue here arises when the system appearance changes and updateAppearance() is called:

// view is transitioning from light to dark
isDarkMode = false
// view is transitioning from dark to light
isDarkMode = true

Just be aware of this. The change is not instantaneous and the effectiveAppearance.name has a slight delay before updating to its proper value.