Domain event

In Domain-Driven Design, a domain event is a pattern used to represent a significant change or occurrence within a domain. It is a way of capturing and communicating an event that has happened in the system and might be of interest to other parts of the system.

Packages

Package	Link	Application in section
LeanCode.DomainModels		IAggregateRoot, IDomainEvent
LeanCode.DomainModels.Generators		Ids

Example

Let's imagine that we want to perform some action after a employee has been assigned to a assignment. We will modify AssignEmployeeToAssignment method from the Project class:

public class Project : IAggregateRoot<ProjectId>
{
    // . . .

    public void AssignEmployeeToAssignment(
        AssignmentId assignmentId,
        EmployeeId employeeId)
    {
        assignments.Single(t => t.Id == assignmentId)
            .AssignEmployee(employeeId);

        DomainEvents.Raise(new EmployeeAssignedToAssignment(
            AssignmentId assignmentId,
            EmployeeId employeeId));
    }
}

public class EmployeeAssignedToAssignment : IDomainEvent
{
    public Guid Id { get; private init; }
    public DateTime DateOccurred { get; private init; }

    public AssignmentId AssignmentId { get; private init; }
    public EmployeeId EmployeeId { get; private init; }

    public EmployeeAssignedToAssignment(
        AssignmentId assignmentId,
        EmployeeId employeeId)
    {
        Id = Guid.NewGuid();
        DateOccurred = Time.UtcNow;

        AssignmentId = assignmentId;
        EmployeeId = employeeId;
    }

    [JsonConstructor]
    public EmployeeAssignedToAssignment(
        Guid id,
        DateTime dateOccurred,
        AssignmentId assignmentId,
        EmployeeId employeeId
    )
    {
        Id = id;
        DateOccurred = dateOccurred;
        AssignmentId = assignmentId;
        EmployeeId = employeeId;
    }
}

Ensure that EmployeeAssignedToAssignment implements the IDomainEvent interface, and it has a constructor with the [JsonConstructor] attribute. [JsonConstructor] attribute is used during JSON deserialization to specify which constructor should be used to create an object from JSON data. This attribute ensures that when a message is deserialized from JSON, the correct constructor is invoked to create the object.

After being raised, the event can be handled by the matching IConsumer to perform wanted action.

Tip

To read how to handle domain events, visit here.