But, there is another way, using a bit of Expression tree manipulation you can build an ‘Or‘ expression dynamically while staying strongly-typed. The code below achieves this.

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;

public static class ExpressionBuilder
{
  public static Expression<Func<T, bool>> True<T>() { return f => true; }
  public static Expression<Func<T, bool>> False<T>() { return f => false; }

  public static Expression<T> Compose<T>(this Expression<T> first, 
       Expression<T> second, 
       Func<Expression, Expression, Expression> merge)
  {
      // build parameter map (from parameters of second to parameters of first)
      var map = first.Parameters
                   .Select((f, i) => new { f, s = second.Parameters[i] })
                   .ToDictionary(p => p.s, p => p.f);

      // replace parameters in the second lambda expression with parameters from 
      // the first
      var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
      // apply composition of lambda expression bodies to parameters from 
      // the first expression 
      return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
  }

  public static Expression<Func<T, bool>> And<T>(
      this Expression<Func<T, bool>> first,
      Expression<Func<T, bool>> second)
  {
      return first.Compose(second, Expression.And);
  }

  public static Expression<Func<T, bool>> Or<T>(
      this Expression<Func<T, bool>> first,
      Expression<Func<T, bool>> second)
  {
      return first.Compose(second, Expression.Or);
  }

  public class ParameterRebinder : ExpressionVisitor
  {
      private readonly Dictionary<ParameterExpression, ParameterExpression> map;

      public ParameterRebinder(
          Dictionary<ParameterExpression, 
          ParameterExpression> map)
      {
          this.map = map??new Dictionary<ParameterExpression,ParameterExpression>();
      }

      public static Expression ReplaceParameters(
          Dictionary<ParameterExpression, 
          ParameterExpression> map, 
          Expression exp)
      {
          return new ParameterRebinder(map).Visit(exp);
      }

      protected override Expression VisitParameter(ParameterExpression p)
      {
          ParameterExpression replacement;
          if (map.TryGetValue(p, out replacement))
          {
              p = replacement;
          }
          return base.VisitParameter(p);
      }
  }
}

NB Some of the ideas in this case from other blog posts, I can’t find them right now but if part of this was your idea I’d be happy to add a link to your blog.

The post Dynamically building ‘Or’ Expressions in LINQ appeared first on Ian Mercer.

Yet, such a variable would be extremely useful when writing a Home Automation System because you often need to make comparisons between a current value and some historical average, or between two ranges (e.g. was the kitchen more or less occupied than yesterday). Now, normally you wouldn’t want to mix persistence up with the representation of a value in your code (see ‘Separation of Concerns’), but in this case I decided that it was worth mixing the two concepts because the benefits of doing so were so great.

So I created a class called VariableWithHistory<T> which is the abstract base class for IntegerWithHistory, DoubleWithHistory, BoolWithHistory, StringWithHistory and a number of others.  The first property worth noting on these classes is the .Current property.  This always gives you the latest value that has been set.  Setting the .Current value stores both the value and the DateTime (Utc of course) at which the value became current.  A history of all past values is maintained in MongoDB up to some suitable limit per variable (each variable can have its own adjustable history size in bytes by using MongoDB’s capped collections).  If the new value is the same as the old one no update is made, the implicit behavior being that the value changed and stayed there until it changes again, so if you want to know what the value is now it is the same as the last change recorded.

With this new variable type in place any object in the house can have any number of persistent fields on it (bool occupied, double temperature, string triggeredBy, …).  Updating these values is as simple as assigning to their .Current property.  When the system loads, each value comes back with the value it had when the system was shut down.  To accomplish this every VariableWithHistory is given a unique id (based on the unique id of it’s parent, e.g. a room).

So far so good, shut down, restart and the house doesn’t need to query a device to know if it’s on or off and all the long running Sequential Logic Blocks I use for rules (e.g. .Delay(days:2)) carry on running as if nothing happened.  This is particularly useful since I typically deploy a new version almost every day and some logic blocks have long delays built into them.

But besides providing simple recovery from a reboot, these persistent variables allow me to do some much more interesting things.

int CountTransitions(DateTimeRange range, T direction);
Counts how many transitions there have been to the value T in a given time range, e.g. how many times did the driveway alarm go ‘true’ this evening?

Dictionary<T, double> Fractional(DateTimeRange range);
Builds a histogram of all the values seen in the time range, e.g. 50% hot, 20% cold, 30% warm for a string variable that tracks temperature

DateTimeOffset LastChangedState
e.g. when was this sensor last triggered?

TimedValue<T> ValueAtTime(DateTimeOffset dt)
What was the value at a given time in the past, e.g. what was the temperature at the same time yesterday?

Each specific type of VariableWithHistory<T> may also have additional methods relevant to the type T.  For example, on DoubleWithHistory there is a method double Average(DateTimeOffset minValue, DateTimeOffset maxValue) which gets the average value over the specified time range.  On BoolWithHistory there is a method double PercentageTrue(DateTimeRange range) which you could use to find the average occupancy for a room yesterday.

My initial implementation waited for the database to write each update before allowing any queries but now I simply cache the Current value and assume that queries will probably get executed after updates and that the average temperature yesterday is close enough with or without the last 100ms of updates.  I did try to keep this class isolated from MongoDB but in the end the benefit of some of the atomic update capabilities in MongoDB made it easier to just take the dependency.

My previous implementation of this feature used my own in-memory database, MongoDB has slowed it down a bit but I’ve gained the ability to archive terabytes of sensor data which should prove useful for my next project which is to add some machine learning to the system.

The post VariableWithHistory – making persistence invisible, making history visible appeared first on Ian Mercer.

The most interesting thing about Neo4j is the Cypher query language used to construct graph queries that follow relationships, evaluate conditions on properties on relationships and nodes. Neo4j shows much promise in terms of being able to represent data in a very natural way and to query it using Cypher in ways that would bring SQL to its knees with join-upon-join-upon-join.

In an earlier blog post I lamented the lack of a single database solution that was the best of all worlds: relational + document + graph + semantic web. Tonight that feeling was compounded: Neo4j is a graph database but it’s missing several key features that could make it much more.

We were privileged to get a first hand explanation as to how Neo4j worked internally but what we saw looked like a work in progress: an unfinished implementation of something that could be so much better. Here’s some of the things Neo4j needs to fix before I’ll give it a go:-

1) Stealing bits from one value to give to another to create odd word lengths like 23 bits is so 1980′s. I cannot believe this is a worthwhile optimization to make in 2012. Neo should bite the bullet, upgrade their few existing customers and move to a more modern byte aligned, 64-bit address space. I was equally amazed at the implementation of compression schemes for text on disk but the omission of other obvious space-saving opportunities like declaring some relationships to be one-way only (no reverse queries, thus no need to store the back link). It’s 2012: disk space is essentially limitless; I should never have to hit a file-size limit because someone decided to use 23, 28 or some other random number of bits instead of 64.

2) The extremely limited set of data types. If you want to store json you’d better support at least all the common Javascript options including Dates. Frankly I don’t care if your database is written in Java, it exposes a web api using json so that’s what it should support. Also odd was the choice of a linked list, meandering its way through the file, as the way to store properties for a node. IMHO Neo4j should just switch to Bson and put a document size limit on nodes like MongoDB instead of carrying on down this bit-packing, linked-list approach to properties with a partial implementation of types.

3) The lack of file splitting at 2GB/4GB boundaries.

4) Putting nodes and relationships into separate files. Sure this simplifies the access pattern but it’s not going to give good locality to data on disk. An alignment based on disk block sizes with nodes and relationships packed into blocks seems likely to be a much better approach to minimizing disk seeks and reads.

3) Reliance on Lucene to provide indexing. Much as I appreciate Lucene, Neo4j needs built-in indexes; without them it’s impossible to optimize query plans across the graph and the indexes. MongoDB has a good selection of indexing options including 2D geo-spatial indexing; IMHO Neo4j should adopt the same set of options and offer queries that are both good relational database queries and good graph queries not force their users to pick one or the other whilst handling the interop between two different systems.

In fact, in my ideal world Neo4j and MongoDB would just become one database: a document database that also has great graph-querying capabilities!

I’ll keep monitoring Neo4j but in the meantime it’s full speed ahead with my own implementation of a graph database in MongoDB with the added twist that in my implementation, relationships are all modeled as triples (just like in a semantic web triple-store). My graph-query language isn’t likely to be as powerful as Cypher any time soon but I have indexes, the ability to query by relationships easily and a robust implementation of properties on each node with support for all common data-types and through my interface-based approach to storing objects with multiple-inheritance I get strongly-typed result sets in C#.

The post Neo4j Meetup in Seattle – some observations appeared first on Ian Mercer.

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The post A Quantified House – My Talk to the Seattle Quantified Self Meetup appeared first on Ian Mercer.

One breaking change in this version is that the state machine is now specified using three Type parameters instead of two:

public class OccupancyStateMachine : 
          StateMachine<OccupancyStateMachine, Event, BuildingArea>

The third type parameter, TContext, is a context object that can be passed in with every event occurrence or tick. This means that you don’t need to store any extraneous data in the state machine itself and can keep it as a pure representation of the state of the system.

In the example above I have an OccupancyStateMachine and the context is a BuildingArea. Each call to EventHappens now takes the event that happened and a BuildingArea object.

When you define your state machine you will need to include 4 parameters in each lambda expression.

Here, for example, is the current state machine for a BuildingArea in my home automation. It uses a hierarchy of states with two base states: Not Occupied and Occupied. It has timers for activity within a room or for occupancy within rooms that are contained by a floor. Note how it also exposes an IObservable<State> so that other objects can subscribe to state machine changes. I didn’t want to take the Rx dependency in the state machine class itself but you can see how easy it is to hook it up.

Of interest also is the way I represent occupancy as three distinct states, the extra one ‘Asleep’ represents a room that is not-occupied in the sense that there is no motion there now but there was at some point during the evening before.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Abodit.StateMachine;
using log4net;
using Abodit.Units;
using AboditUnits.Units;
using System.Reactive.Subjects;
using System.Reactive.Linq;

namespace Abodit
{
    /// <summary>
    /// An Occupancy State machine handles not occupied, occupied, asleep
    /// </summary>
    [Serializable]
    public class OccupancyStateMachine : StateMachine<OccupancyStateMachine, Event, BuildingArea>
    {
        private readonly Subject<State> watch = new Subject<State>();
        public IObservable<State> Watch { get { return watch.AsObservable(); } }

        public override void OnStateChanging(StateMachine<OccupancyStateMachine, Event, BuildingArea>.State newState, BuildingArea context)
        {
            watch.OnNext(newState);
        }

        public static readonly State Starting = AddState("Starting");

        public static readonly State NotOccupied = AddState("Not occupied",
                (m, e, s, c) => { 
                                m.CancelScheduledEvent(eTick);          // Stop the clock
                                m.IsTimerRunning = false;
                                m.IsRecentlyOccupied = false;
                                m.IsHeavilyOccupied = false;
                                m.After(new TimeSpan(hours:0, minutes:5, seconds:0), e5MinutesSinceOccupied);
                                m.After(new TimeSpan(hours:24, minutes:0, seconds:0), e24hoursSinceOccupied);
                                m.After(new TimeSpan(hours:48, minutes:0, seconds:0), e48hoursSinceOccupied);
                             },
                (m, e, s, c) => { });

        public static readonly State NotOccupiedIn5Minutes = AddState("Not occupied in over 5 minutes",
                (m, e, s, c) => { },
                (m, e, s, c) => { }, NotOccupied);

        public static readonly State NotOccupiedInOver24Hours = AddState("Not occupied in over 24 hours",
                (m, e, s, c) => { },
                (m, e, s, c) => { }, NotOccupiedIn5Minutes);

        public static readonly State NotOccupiedInOver48Hours = AddState("Not occupied in over 48 hours",
                (m, e, s, c) => { },
                (m, e, s, c) => { }, NotOccupiedInOver24Hours);

        public static readonly State NotOccupiedInOver1Week = AddState("Not occupied in over 1 week",
                (m, e, s, c) => { },
                (m, e, s, c) => { }, NotOccupiedInOver48Hours);

        public static readonly State Asleep = AddState("Asleep",
                (m, e, s, c) =>
                {
                    // Set a timer going for morning
                    var now = TimeProvider.Current.Now.LocalDateTime;
                    var morning = now.Hour < 8 ? now.AddHours(-now.Hour + 8) : now.AddHours(24 - now.Hour + 8);
                    m.At(morning.ToUniversalTime(), eMorning);
                },
                (m, e, s, c) => { },
                parent:NotOccupied);

        public static readonly State Occupied = AddState("Occupied",
                (m, e, s, c) =>
                {
                    m.IsRecentlyOccupied = true;
                    // Add a timer that runs while we are occupied
                    m.Every(new TimeSpan(hours:0, minutes:0, seconds:10), eTick);
                    // And set a timer going to mark 5 minutes since occupied
                    m.After(new TimeSpan(hours:0, minutes:5, seconds:0), e5MinutesAfterBecomingOccupied);
                    m.CancelScheduledEvent(e5MinutesSinceOccupied);
                    m.CancelScheduledEvent(e24hoursSinceOccupied);
                    m.CancelScheduledEvent(e48hoursSinceOccupied);
                },
                (m, e, s, c) => { });

        public static readonly State HeavilyOccupied = AddState("Heavily occupied",
                (m, e, s, c) => { },
                (m, e, s, c) => { },
                parent:Occupied);

        private static readonly Event eStart = new Event("Starts");
        private static readonly Event eUserActivity = new Event("User activity");
        private static readonly Event eTick = new Event("Tick");
        private static readonly Event eTimeout = new Event("Timeout");
        private static readonly Event eMorning = new Event("Morning");
        private static readonly Event e5MinutesAfterBecomingOccupied = new Event("5 minutes after becoming occupied");
        private static readonly Event e5MinutesSinceOccupied = new Event("5 minutes since occupied");
        private static readonly Event e24hoursSinceOccupied = new Event("24 hours since occupied");
        private static readonly Event e48hoursSinceOccupied = new Event("48 hours since occupied");

        private static readonly Event eAllChildrenNotOccupied = new Event("No child occupied");
        private static readonly Event eAtLeastOneChildOccupied = new Event("At least one child occupied");

        private double decliningActivity = 0.0;         // Up 1000 every UserInput, down x0.9 every n seconds
        private const int ActivityPerUserInput = 1000;
        private const double rateOfDecline = 0.92;

        public bool IsTimerRunning { get; set; }
        public bool IsRecentlyOccupied { get; set; }
        public bool IsHeavilyOccupied { get; set; }

        static OccupancyStateMachine()
        {
            // On startup we transition immediately to starting
            // but we want an event call to do this so we aren't doing any work
            // in the constructor, and so the initialization only happens when it's
            // a true 'cold start' not a 'warm start' from some database state
            Starting
                .When(eStart, (m, s, e, c) => { return NotOccupied; });

            // Note: This is a hierarchical state machine so NotOccupied includes Asleep
            NotOccupied
                .When(eAtLeastOneChildOccupied, (m, s, e, c) => 
                {
                    return Occupied;
                })
                .When(e5MinutesSinceOccupied, (m, s, e, c) =>
                {
                    // Could signal something??
                    return s;
                })
                .When(e24hoursSinceOccupied, (m, s, e, c) =>
                {
                    // Could signal something??
                    return s;
                })
                .When(e48hoursSinceOccupied, (m, s, e, c) =>
                {
                    // Could signal something??
                    return s;
                })
                .When(eUserActivity, (m, s, e, c) =>
                {
                    m.After(c.OccupancyTimeout, eTimeout);                // start a new timeout
                    m.IsTimerRunning = true;
                    return Occupied;
                });

            // Asleep is a substate of not occupied so no need for more logic on becoming occupied ...
            Asleep
                .When(eMorning, (m, s, e, c) =>
                {
                    // Eliminate Asleep if appropriate
                    return NotOccupied;
                });

            // Occupied includes recently occupied and heavily occupied ...
            Occupied
                .When(e5MinutesAfterBecomingOccupied, (m, s, e, c) => 
                {
                    m.IsRecentlyOccupied = false;
                    return s;
                })
                .When(eUserActivity, (m, s, e, c) =>
                {
                    // Accumulate activity ...
                    m.decliningActivity += ActivityPerUserInput;

                    m.CancelScheduledEvent(eTimeout);               // cancel the old timeout

                    m.After(c.OccupancyTimeout, eTimeout);                // start a new timeout
                    m.IsTimerRunning = true;

                    if (m.decliningActivity > 20 * ActivityPerUserInput)
                        return HeavilyOccupied;
                    else
                        return s;
                })
                .When(eAllChildrenNotOccupied, (m, s, e, c) =>
                    {
                        if (m.IsTimerRunning)
                        {
                            // If the timer is running ... wait until it runs out
                            return s;
                        }
                        else
                        {
                            DateTime nowLocal = TimeProvider.Current.Now.LocalDateTime;
                            if (nowLocal.Hour > 17)
                                return Asleep;
                            else
                                return NotOccupied;
                        }
                    })
                .When(eTick, (m, s, e, c) =>
                    {
                        m.decliningActivity *= rateOfDecline;
                        return s;
                    })
                .When(eTimeout, (m, s, e, c) =>
                    {
                        DateTime nowLocal = TimeProvider.Current.Now.LocalDateTime;
                        if (nowLocal.Hour > 17)
                            return Asleep;
                        else
                            return NotOccupied;
                    });

            HeavilyOccupied.When(eTick, (m, s, e, c) =>
            {
                // Same code as Occupied but this one will override if we are in HeavilyOccupied mode
                m.decliningActivity *= rateOfDecline;
                // Fall back to just occupied when ...
                if (m.decliningActivity < 0.2 * ActivityPerUserInput)
                    return Occupied;
                else
                    return s;

            });


        }

        public OccupancyStateMachine()
            : base(Starting)
        {
        }

        public OccupancyStateMachine(State initialState)
            : base(initialState)
        {
        }

        public override void Start()
        {
            this.EventHappens(eStart, null);
        }

        public void UserActivity(BuildingArea ba)
        {
            this.EventHappens(eUserActivity, ba);
        }

        public void AllChildrenNotOccupied(BuildingArea ba)
        {
            this.EventHappens(eAllChildrenNotOccupied, ba);
        }

        public void AtLeastOneChildOccupied(BuildingArea ba)
        {
            this.EventHappens(eAtLeastOneChildOccupied, ba);
        }
    }
}

The post Updated Release of the Abodit State Machine appeared first on Ian Mercer.

If it sees a call from someone not already in the database it will ask (via chat) for you to enter their full name. It then updates its database replacing the caller ID name (which is often useless, especially for mobile numbers). You can also query it using the natural language interface to ask about any calls you might have missed, or to lookup a number by time of day or by a fragment of their name. You can even ask complex queries like “who called last year on a friday after 5pm” and it will construct an efficient SQL query to get the results.

It also synchronizes all these contact records with Google Contacts.

But until recently my mobile phone hasn’t been part of the home automation system. Yes, I can use it as an input device for Google Talk, and yes, the house still notices when it comes and goes because the house tracks every device that ever gets an IP address on the local network, but other than that it really doesn’t ‘understand’ much about my cell phone.

But that’s about to change. Recently I installed Tasker on my Android phone and using that app I can now set up a whole variety of triggers that can report back to the home automation system information such as phone calls made or received, GPS location, wifi-located position, phone unlocks, shakes and more.

So I’ve extended the web interface on the Home Automation web server to accept POSTs from Tasker with updates from my cell phone. These are placed into a PubSubHub implementation that uses SignalR to distribute messages to any connected clients. The home automation server is itself a client of this service (it publishes information about every device change in the house and the PubSubHub shares those updates with any connected Web client) so you get real-time updates for what’s happening in the house on the house’s web page. Extending that architecture to include messages from remote devices like the Android phone was easy and I plan to use it in the future for other remote devices, such as a Netduino with a collection of environment and HVAC sensors on it (more about that later).

As to precisely what I’ll do with this new capability I have a long list of features to implement now:

1) Logging all cell phone calls to the same database, automatically building my contacts list
2) Tracking how long it takes to get to work by each of the various ways I can go, correlating that with the traffic flow information and automatically figuring out which route I should take for future trips
3) Shake cellphone to change music in the house
4) Adjusting the heating at home based on how far away we are (and thus the soonest we could get back)
5) Finishing up my semantic, location-aware shopping list (knows which store you are in and what you need there and presents it in order by aisle)
6) Automatically delivering notifications by the best possible means (talking on the speakers at home, XMPP, or by email if I’m in a different time zone)
etc.

The post Integrating an Android phone into my home automation system appeared first on Ian Mercer.

… allows us to gain instant access to information which may actually be stored on the other side of the world.

It allows you to prepare ‘pages’ and to connect these, one with another, in any way you like.

… you don’t have to be a computer expert to set up your own information system.

You can link one system to another, so that you can, in effect, create one large system.

You can set up a massive information system accessible from anywhere on the network!

It has an extremely powerful facility called an ‘execution frame’ … which allows you to ‘call up’ a computer program …

This came from a manual that I wrote in 1985 for a program called BEEBTel which was a networked, page-based viewer and editor for linked pages and executable scripts. Of course TimBL’s version was somewhat more successful, despite coming four years later

The post Before there was the web there was BeebTel appeared first on Ian Mercer.

BEGIN INTEGER A,B,C,D,E,F,G,ANS'
READ A,B,C,D,E,F,G'
ANS:=(A+B+C+D+E+F+G)/7'
PRINT ANS'
END'

Can you identify the language and the computer it ran on?

The post My first programme [sic] appeared first on Ian Mercer.

There are several state machine implementations for .NET out there but, sadly, none of them met all of the requirements I have for a state machine. These are:-

1) Well written using encapsulation and other good practices
2) Able to be easily serialized to disk
3) Able to handle temporal events easily (After … At … Every …)
4) Disk serialized form must expose a property saying when it next needs to be fetched from disk to run
5) Implements hierarchical states with entry and exit actions

So I built one, and have made the source code available on Nuget so you can add it to any project easily without any extra DLLs.

Look for “AboditStateMachine” on Nuget to download it. The download includes a sample state machine documented to show off some of its capabilities.

Defining states is easy, just give them a name and specify their parent state if any:-

        public static readonly State UnVerified = AddState("UnVerified");

        public static readonly State Verified = AddState("Verified");

        // States are hierarchical.  If you are in state VerifiedRecently you are also in is parent state Verified.

        public static readonly State VerifiedRecently = AddState("Verified recently", parent: Verified);
        public static readonly State VerifiedAWhileAgo = AddState("Verified a while ago", parent: Verified);

You can use any other type that’s IEquatable as an Event type or you can use the provided Event class:

        private static Event eUserVerifiedEmail = new Event("User verified email");
        private static Event eScheduledCheck = new Event("Scheduled Check");
        private static Event eBeenHereAWhile = new Event("Been here a while");

The state machine itself is specified in a static constructor so it runs just once no matter how many instances of the state machine you create. Each method is provided with an instance of the state machine ‘m’ as well as the state ‘s’ and the event ‘e’ as appropriate:

        static DemoStatemachine()
        {
            UnVerified
                    .OnEnter((m, s, e) =>
                        {
                            // States can execute code when they are entered or when they are left
                            // In this case we start a timer to bug the user until they confirm their email
                            m.Every(new TimeSpan(hours: 10, minutes:0, seconds:0), eScheduledCheck);

                            // You can also set a reminder to happen at a specific time, or after a given interval just once
                            m.At(new DateTime(DateTime.Now.Year+1, 1, 1), eScheduledCheck);
                            m.After(new TimeSpan(hours: 24, minutes: 0, seconds: 0), eScheduledCheck);

                            // All necessary timing information is serialized with the state machine
                            // The serialized state machine also exposes a property showing when it next needs to be woken up
                            // External code will need to call the Tick(utc) method at that time to trigger the next temporal event
                        })
                    .When(eScheduledCheck, (m, s, e) =>
                    {
                        Trace.WriteLine("Here is where we would send a message to the user asking them to verify their email");
                        // We return the current state 's' rather than 'UnVerified' in case we are in a child state of 'Unverified'
                        // This makes it easy to handle hierarchical states and to either change to a different state or stay in the same state
                        return s;
                    })
                    .When(eUserVerifiedEmail, (m, s, e) =>
                    {
                        Trace.WriteLine("The user has verified their email address, we are done (almost)");
                        // Kill the scheduled check event, we no longer need it
                        m.CancelScheduledEvent(eScheduledCheck);
                        // Start a timer for one last transition
                        m.After(new TimeSpan(hours:24, minutes:0, seconds:0), eBeenHereAWhile);
                        return VerifiedRecently;
                    });

            VerifiedRecently
                    .When(eBeenHereAWhile, (m, s, e) =>
                    {
                        Trace.WriteLine("User has now been a member for over 24 hours - give them additional priviledges for example");
                        // No need to cancel the eBeenHereAWhile event because it wasn't auto-repeating
                        //m.CancelScheduledEvent(eBeenHereAWhile);
                        return VerifiedAWhileAgo;
                    });

            Verified.OnEnter((m, s, e) => 
                {
                    Trace.WriteLine("The user is now fully verified");
                });

            VerifiedAWhileAgo.OnEnter((m, s, e) =>
                {
                    Trace.WriteLine("The user has been verified for over 24 hours");
                });

        }

With your state machine defined you can now create instances of it, trigger events on them, serialize them to disk, fetch them back, carry on eventing on them, …

            DemoStatemachine demoStateMachine = new DemoStatemachine(DemoStatemachine.UnVerified);

            // At the time specified in demoStateMachine.NextTimedEventAt you reload the state machine from disk and call
            demoStateMachine.Tick(DateTime.UtcNow);

            // When the user verifies their email address you call ...
            demoStateMachine.VerifiesEmail();

            // At any other time you can examine the current state, act on the state changed event, ...

I hope you find this new state machine implementation useful, and if you have any feedback, do please send it my way.

The post Building a better .NET State Machine appeared first on Ian Mercer.