In C#, a class can define own events and other classes can register it to be notified when something happen. This is useful for a desktop application or standalone windows service. But, for a web application it's a bit problematic since objects are created in a web request and they are short-lived. It's hard to register some class's events. Also, directly registering to another class's event makes classes tightly coupled.

Domain events can be used to decouple business logic and react to important domain changes in an application.

EventBus

EventBus is a singleton object that is shared by all other classes to trigger and handle events. To use the event bus, you should get a reference to it. You can do it in two way.

Injecting IEventBus

You can use dependency injection to get a reference to IEventBus. Here, we used property injection pattern:

public class TaskAppService : ApplicationService
{
    public IEventBus EventBus { get; set; }

    public TaskAppService()
    {
        EventBus = NullEventBus.Instance;
    }
}

Property injection is more proper than constructor injection to inject the event bus. Thus, your class can work without event bus. NullEventBus implements null object pattern. When you call it's methods, it does nothing at all.

Getting The Default Instance

If you can not inject it, you can directly use EventBus.Default. It's the global event bus and can be used as shown below:

EventBus.Default.Trigger(...); //trigger an event

It's not suggested to directly use EventBus.Default wherever possible since it makes unit testing harder.

Defining Events

Before trigger an event, you should define the event first. An event is represented by a class that is derived from EventData. Assume that we want to trigger an event when a task is completed:

public class TaskCompletedEventData : EventData
{
    public int TaskId { get; set; }
}

This class contains properties those are needed by class that handles the event. EventData class defines EventSource (which object triggered the event) and EventTime (when it's triggered) properties.

Predefined Events

Handled Exceptions

ASP.NET Boilerplate defines AbpHandledExceptionData and triggers this event when it automatically handles any exception. This is especially useful if you want to get more information about exceptions (even ASP.NET Boilerplate automatically logs all exceptions). You can register to this event to be informed when an exception occurs.

Entity Changes

There are also generic event data classes for entity changes: EntityCreatingEventData<TEntity>, EntityCreatedEventData<TEntity>, EntityUpdatingEventData<TEntity>, EntityUpdatedEventData<TEntity>, EntityDeletingEventData<TEntity> and EntityDeletedEventData<TEntity>. Also, there are EntityChangingEventData<TEntity> and EntityChangedEventData<TEntity>. A change can be insert, update or delete.

'ing' events (ex: EntityUpdating) are triggered before saving changes. So, you can rollback the unit of work to prevent the operation by throwing exceptions in these events. 'ed' events (ex: EntityUpdated) are triggered after saving changes and no chance to rollback the unit of work.

Entity change events are defined in Abp.Events.Bus.Entities namespace and are automatically triggered by ASP.NET Boilerplate when an entity is inserted, updated or deleted. If you have a Person entity, can register to EntityCreatedEventData<Person> to be informed when a new Person created and inserted to database. These events also supports inheritance. If Student class derived from Person class and you registered to EntityCreatedEventData<Person>, you will be informed when a Person or Student is inserted.

Triggering Events

Triggering an event is simple:

public class TaskAppService : ApplicationService
{
    public IEventBus EventBus { get; set; }

    public TaskAppService()
    {
        EventBus = NullEventBus.Instance;
    }

    public void CompleteTask(CompleteTaskInput input)
    {
        //TODO: complete the task on database...
        EventBus.Trigger(new TaskCompletedEventData {TaskId = 42});
    }
}

There are some overloads of the trigger method:

EventBus.Trigger<TaskCompletedEventData>(new TaskCompletedEventData { TaskId = 42 }); //Explicitly declare generic argument
EventBus.Trigger(this, new TaskCompletedEventData { TaskId = 42 }); //Set 'event source' as 'this'
EventBus.Trigger(typeof(TaskCompletedEventData), this, new TaskCompletedEventData { TaskId = 42 }); //Call non-generic version (first argument is the type of the event class)

Another way of triggering events can be using DomainEvents collection of AggregateRoot class (see related section in the Entity documentation).

Handling Events

To handle an event, you should implement IEventHandler<T> interface as shown below:

public class ActivityWriter : IEventHandler<TaskCompletedEventData>, ITransientDependency
{
    public void HandleEvent(TaskCompletedEventData eventData)
    {
        WriteActivity("A task is completed by id = " + eventData.TaskId);
    }
}

IEventHandler defines HandleEvent method and we implemented it as shown above.

EventBus is integrated to dependency injection system. As we implemented ITransientDependency above, when a TaskCompleted event occured, it creates a new instance of ActivityWriter class and calls it's HandleEvent method, then disposes it. See dependency injection for more.

Handling Base Events

Eventbus supports inheritance of events. For example, you can create a TaskEventData and two derived classes: TaskCompletedEventData and TaskCreatedEventData:

public class TaskEventData : EventData
{
    public Task Task { get; set; }
}

public class TaskCreatedEventData : TaskEventData
{
    public User CreatorUser { get; set; }
}

public class TaskCompletedEventData : TaskEventData
{
    public User CompletorUser { get; set; }
}

Then you can implement IEventHandler<TaskEventData> to handle both of the events:

public class ActivityWriter : IEventHandler<TaskEventData>, ITransientDependency
{
    public void HandleEvent(TaskEventData eventData)
    {
        if (eventData is TaskCreatedEventData)
        {
            //...
        }
        else if (eventData is TaskCompletedEventData)
        {
            //...
        }
    }
}

That also means you can implement IEventHandler<EventData> to handle all events in the application. You probably don't want that, but it's possible.

Handler Exceptions

EventBus triggers all handlers even one/some of them throws exception. If only one of them throws exception, then it's directly thrown by the Trigger method. If more than one handler throws exception, EventBus throws a single AggregateException for all of them.

Handling Multiple Events

You can handle multiple events in a single handler. In this time, you should implement IEventHandler<T> for each event. Example:

public class ActivityWriter : 
    IEventHandler<TaskCompletedEventData>, 
    IEventHandler<TaskCreatedEventData>, 
    ITransientDependency
{
    public void HandleEvent(TaskCompletedEventData eventData)
    {
        //TODO: handle the event...
    }

    public void HandleEvent(TaskCreatedEventData eventData)
    {
        //TODO: handle the event...
    }
}

Registration Of Handlers

We must register the handler to the event bus in order to handle events.

Automatically

ASP.NET Boilerplate finds all classes those implement IEventHandler and registered to dependency injection (for example, by implementing ITransientDependency as the samples above). Then it registers them to the eventbus automatically. When an event occures, it uses dependency injection to get a reference to the handler and releases the handler after handling the event. This is the suggested way of using event bus in ASP.NET Boilerplate.

Manually

It is also possible to manually register to events but use it with caution. In a web application, event registration should be done at application start. It's not a good approach to register to an event in a web request since registered classes remain registered after request completion and re-registering for each request. This may cause problems for your application since registered class can be called multiple times. Also keep in mind that manual registration does not use dependency injection system.

There are some overloads of register method of the event bus. The simplest one waits a delegate (or a lambda):

EventBus.Register<TaskCompletedEventData>(eventData =>
    {
        WriteActivity("A task is completed by id = " + eventData.TaskId);
    });
			

Thus, then a 'task completed' event occurs, this lambda method is called. Second one waits an object that implements IEventHandler<T>:

EventBus.Register<TaskCompletedEventData>(new ActivityWriter());

Same instance of ActivityWriter is called for events. This method has also a non-generic overload. Another overload accepts two generic arguments:

EventBus.Register<TaskCompletedEventData, ActivityWriter>();

In this time, event bus creates a new ActivityWriter for each event. It calls ActivityWriter.Dispose method if it's disposable.

Lastly, you can register an event handler factory to handle creation of handlers. A handler factory has two methods: GetHandler and ReleaseHandler. Example:

public class ActivityWriterFactory : IEventHandlerFactory
{
    public IEventHandler GetHandler()
    {
        return new ActivityWriter();
    }

    public void ReleaseHandler(IEventHandler handler)
    {
        //TODO: release/dispose the activity writer instance (handler)
    }
}

There is also a special factory class, the IocHandlerFactory, that can be used to use dependency injection system to create/release handlers. ASP.NET Boilerplate also uses this class in automatic registration. So, if you want to use dependency injection system, directly use automatic registration defined before.

Unregistration

When you manually register to event bus, you may want to unregister to the event later. Simplest way of unregistering an event is disposing the return value of the Register method. Example:

//Register to an event...
var registration = EventBus.Register<TaskCompletedEventData>(eventData => WriteActivity("A task is completed by id = " + eventData.TaskId) );

//Unregister from event
registration.Dispose();
			

Surely, unregistration will be somewhere and sometime else. Keep registration object and dispose it when you want to unregister. All overloads of the Register method returns a disposable object to unregister to the event.

EventBus also provides Unregister method. Example usage:

//Create a handler
var handler = new ActivityWriter();

//Register to the event
EventBus.Register<TaskCompletedEventData>(handler);

//Unregister from event
EventBus.Unregister<TaskCompletedEventData>(handler);
			

It also provides overloads to unregister delegates and factories. Unregistering handler object must be the same object which is registered before.

Lastly, EventBus provides a UnregisterAll<T>() method to unregister all handlers of an event and UnregisterAll() method to unregister all handlers of all events.