Get display ID from NSScreen for use with CGDirectDisplay API

NSScreen+CGDirectDisplayID.swift

import Cocoa
import CoreGraphics

extension NSScreen {
  var displayID: CGDirectDisplayID {
    return deviceDescription["NSScreenNumber"] as? CGDirectDisplayID ?? 0
  }
}

NSScreen+CGDirectDisplayID_Alt.swift

import Cocoa
import CoreGraphics

extension NSScreen {
  var displayID: CGDirectDisplayID? {
    return deviceDescription["NSScreenNumber"] as? CGDirectDisplayID
  }
}

The latter option is the better way to go, IMHO.

now Swift 5 requires this:

extension NSScreen {
    var displayID: CGDirectDisplayID? {
        return deviceDescription[NSDeviceDescriptionKey(rawValue: "NSScreenNumber")] as? CGDirectDisplayID
    }
}

nice