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.

There are also generic event data classes for entity changes: EntityCreatedEventData<TEntity>, EntityUpdatedEventData<TEntity> and EntityDeletedEventData<TEntity>. They are defined in Abp.Events.Bus.Entities namespace. These events 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 inserted.

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

EventBus is integrated with 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.

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; }
}

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...
    }
}

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 keep 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 ıf 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 created a new ActivityWriter for each event. It calls ActivityWriter.Dispose method if it's disposable.

Lastly, you can register a 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 mode. So, if you want to use dependency injection system, directly use automatic registration.

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 theRegister 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.