AppDidLaunchScreensaverDidStop.js

var me = Application.currentApplication(); me.includeStandardAdditions = true

ObjC.import('Cocoa')

ObjC.registerSubclass({
  name: 'MainController',
  methods: {
    'appDidLaunch:': {
      types: ['void', ['id']],
      implementation: function(notification) {
        var appName = notification.userInfo.objectForKey('NSApplicationName').js
        me.activate()
        me.displayAlert(`Hello, ${appName}!`, {message: 'Nice to meet you.'})
        Application(appName).activate()
      }
    },
    'screensaverDidStop:': {
      types: ['void', ['id']],
      implementation: function(notification) {
        me.activate()
        me.displayAlert('Goodbye, screensaver!', {message: 'It was nice knowing you.'})
      }
    }
  }
})

var controller = $.MainController.new

$.NSWorkspace.sharedWorkspace.notificationCenter.addObserverSelectorNameObject(controller, 'appDidLaunch:', $.NSWorkspaceDidLaunchApplicationNotification, undefined)

$.NSDistributedNotificationCenter.defaultCenter.addObserverSelectorNameObject(controller, 'screensaverDidStop:', 'com.apple.screensaver.didstop', undefined)