This is why MonoMac is hot

UserStateObserver.cs

using System;
using System.Collections.Generic;

using MonoMac.Foundation;
using MonoMac.AppKit;
using MonoMac.ObjCRuntime;

using Cadenza;

namespace MySecretApp {

    public class UserStateObserver {
        
        public event EventHandler<UserStateEventArgs> UserStateChanged;  
        
        public UserState State {
            get { return state; }

            private set {  
                if (value != state)
                    UserStateChanged.Raise (this, new UserStateEventArgs (value));
                state = value;
            }
        }
        
        bool ScreenSaverIsOn;
        UserState state;
        List<NSTimer> Timers;
        
        public UserStateObserver ()
        {
            state = UserState.Busy;
            Timers = new List<NSTimer> ();
        }
        
        public void Start ()
        {   
            TransitionTo (UserState.Idle, WheneverComputerSleeps);
            TransitionTo (UserState.Busy, WheneverComputerWakes);
            
            TransitionTo (UserState.Idle, WheneverScreenSaverBegins);
            TransitionTo (UserState.Idle, Every (5.Minutes (), IfScreenSaverIsOn));
            TransitionTo (UserState.Busy, WheneverScreenSaverEnds);
        }
        
        public void Stop ()
        {
            Timers.ForEach (timer => timer.Invalidate ());
        }
        
        void OnWorkspaceNotification (string notification, Action act)
        {
            var center = NSWorkspace.SharedWorkspace.NotificationCenter;
            center.AddObserver (new NSString (notification), note => act ());
        }
        
        void OnDistributedNotification (string notification, Action act)
        {
            // TODO remove extraneous cast when API is typed correctly.
            // TODO how do we remove an observer?
            var center = NSDistributedNotificationCenter.DefaultCenter as NSDistributedNotificationCenter;
            center.AddObserver (new NSString (notification), note => act ());
        }
        
        void TransitionTo (UserState state, Action<Action> whenever)
        {
            whenever (() => State = state);
        }
        
        /// <summary>
        /// If foo is an Action<Action>, and bar is Action, then (Every (period, foo) (bar))
        /// starts a timer which calls foo (bar) every period.
        Action<Action> Every (TimeSpan period, Action<Action> act)
        {
            return transition => { 
                var timer = NSTimer.CreateRepeatingTimer (period, () => act (transition));
                NSRunLoop.Current.AddTimer (timer, NSRunLoop.NSRunLoopCommonModes);
                Timers.Add (timer);
            };
        }
        
        void WheneverComputerSleeps (Action act)
        {
            OnWorkspaceNotification ("NSWorkspaceWillSleepNotification", act);
        }
        
        void WheneverComputerWakes (Action act)
        {
            OnWorkspaceNotification ("NSWorkspaceDidWakeNotification", act);
        }
        
        void WheneverScreenSaverBegins (Action act)
        {
            OnDistributedNotification ("com.apple.screensaver.didstart", () => {
                ScreenSaverIsOn = true;
                act ();
            });
        }
        
        void IfScreenSaverIsOn (Action act)
        {
            if (ScreenSaverIsOn) act ();
        }
        
        void WheneverScreenSaverEnds (Action act)
        {
            OnDistributedNotification ("com.apple.screensaver.didstop", () => {
                ScreenSaverIsOn = false;
                act ();
            });
        }
    }
}

Line 67: Do you want == instead of = ?

No, (() => State = state) is an Action that sets the state.

Oops, sorry for the mistake! Makes sense.