pmilet.DomainEvents 1.3.0

Purpose

The purppose of this library is to simplify the development of applications that use the Domain Event design pattern. This library is based on an article from Jimmy Boggard A better domain events pattern..

Domain Events

Martin Fowlers defines the domainEvent DDD pattern as: Domain Events captures the memory of something interesting which affects the domain. The essence of a Domain Event is that you use it to capture important things that happens into the domain and that can produce a change into the state of the application you are developing. Another good source of information is this chapter about Domain Events from the Microsoft ebook: .NET Microservices. Architecture for Containerized .NET Applications

1. Model your domain event

To create your DomainEvent class you could either inherit from the base Class DomainEvent or implement the IDomainEvent interface.

    /// <summary>
    /// Represents the play choosen by a player
    /// </summary>
    public class PlayMade : DomainEvent
    {
        public PlayMade(PlayerType player, PlayType play)
            : base( "PlayMade", "1.0")
        {
            Player = player;
            Play = play;
        }

        public PlayerType Player { get; private set; }
        public PlayType Play { get; private set; }

    }

2. Create an instance of the DomainEventDispatcher

From your business logic you can publish events or subscribe to events by means of a DomainEventDispatcher instance.

  // we create the domain event dispatcher and inject it into the objects of our domain model (normally done using a IoC container) 
  IDomainEventDispatcher dispatcher = new DomainEventDispatcher();
  Player j1 = new Player1(dispatcher);
  Player j2 = new Player2(dispatcher);
  Match match = new Match(dispatcher);
  Outcome outcome = new Outcome(dispatcher);

Note: a better option is to register the DomainEventDispatcher instance in your IoC container.

3. Trigger domain events from your business logic

To trigger an domain event immediately we should use the DomainEventDipatcher Publish<T> method:

  //publish an event notifying that the match ended and Player1 is the winner
  _dispatcher.Publish<MatchEnded>(new MatchEnded(PlayerType.Player1));

To add a delayed domain event we should use the Add<T> method :

  //delayed event to notify of the move choosen by the player
  _dispatcher.Add<PlayMade>(new PlayMade( _player, play ));

To trigger all the delayed domain events of a specific type we should use the Commit<T> method :

    //commit all registered delayed events
    _dispatcher.Commit<PlayMade>();

4. Respond to specific domain events from your business logic by subscribing to them

The easiest way to subscribe to a specific domain event is by inheriting from the HandleDomainEventsBase<T> class. This way the subscription is done automatically, you just have to override the HandleEvent method as shown:

    public class Outcome : HandleDomainEventsBase<MatchEnded>
    {
        PlayerType _lastWinner;
        public Outcome(IDomainEventDispatcher dispatcher):base(dispatcher)
        {}

        public override void HandleEvent(MatchEnded domainEvent)
        {
            _lastWinner = domainEvent.Winner;
        }
        
        public PlayerType LastWinner()
        {
            return _lastWinner;
        }

    }
    ```
If you want a class to subscribe to several domain events (this may be breaks the SR principle ) we should inherit from  IHandleDomainEvents<T> interface and subscribe explicitly. In this example we combine both approachs.

```cs
    public class Match : HandleDomainEventsBase<PlayMade>, 
        IHandleDomainEvents<InvalidPlay>
    {
     ...

        public Match( IDomainEventDispatcher dispatcher):base( dispatcher)
        {
            domainEventDispatcher = dispatcher;
            dispatcher.Subscribe<InvalidPlay>(this);
        }
        
        ...
        
        public override void HandleEvent(PlayMade domainEvent)
        {
            SavePlay(domainEvent);

            EvalOutcome();
        }

        public void HandleEvent(InvalidPlay domainEvent)
        {
            Console.WriteLine($"invalid play {domainEvent.Play.ToString()} made by {domainEvent.Player.ToString()}");
        }
    }
    ```

### See a example app that uses domain events 
    
    To see a full sample take a look to the StonePaperScissors app or specflow functional tests from the [github](https://github.com/pmilet/domainevents) repo