macOS detecting contrast changes using notifications (NSWorkspaceAccessibilityDisplayOptionsDidChangeNotification, NSWorkspace.accessibilityDisplayOptionsDidChangeNotification)

README.swift

// NSWorkspace.accessibilityDisplayOptionsDidChangeNotification fires when option changes. 
// The only thing is, it fires in NSWorkspace's notification center. Not in the default one.

///// Using a selector

NSWorkspace.shared.notificationCenter.addObserver(
   self, 
   selector: #selector(accessibilityDisplayOptionsDidChange(_:)),
   name: NSWorkspace.accessibilityDisplayOptionsDidChangeNotification, 
   object: NSWorkspace.shared)

// ...
 
@objc func accessibilityDisplayOptionsDidChange(_ notification: Notification) {
    // Do whatever you need when the contrast settings change
}
  
///// Using a block

NSWorkspace.shared.notificationCenter.addObserver(
    forName: NSWorkspace.accessibilityDisplayOptionsDidChangeNotification,
    object: NSWorkspace.shared,
    queue: nil) { (_) in
       // Do whatever you need when the contrast settings change
    }
  
  
// Then you have to use NSWorkspace.shared.accessibilityDisplayShouldIncreaseContrast property 
// in your code to alter the appearance of your UI when necessary.

// https://stackoverflow.com/a/49530299/146361