/// <summary>
/// Creates the current state of an entity from the event stream.
/// </summary>
/// <typeparam name="T">Type of the resulting entity</typeparam>
/// <param name="stream">Event stream</param>
/// <param name="resourceType">Resource type</param>
/// <param name="id"></param>
/// <returns>The resulting entity</returns>
public static async Task <T> GetCurrentEntityAsync <T>(this IEventStream stream, ResourceType resourceType, int id) where T : class, new()
{
if (ResourceType.ResourceTypeDictionary.ContainsKey(resourceType.Name))
{
var targetType = typeof(T);
if (!targetType.IsAssignableFrom(resourceType.Type))
{
throw new ArgumentException("The type parameter doesn't match up with the associated type of the ResourceType");
}
var enumerator = stream.GetEnumerator();
var obj = default(T);
while (await enumerator.MoveNextAsync())
{
switch (enumerator.Current)
{
case CreatedEvent createdEv:
if (createdEv.ResourceTypeName.Equals(resourceType.Name) && createdEv.Id == id)
{
obj = Activator.CreateInstance <T>();
}
break;
case PropertyChangedEvent propertyEv:
if (!Equals(obj, default(T)) && Equals(propertyEv.ResourceTypeName, resourceType.Name) && propertyEv.Id == id)
{
var propertyInfo = targetType.GetProperty(propertyEv.PropertyName);
propertyInfo.SetValue(obj, propertyEv.Value);
}
break;
case DeletedEvent deletedEv:
if (Equals(deletedEv.ResourceTypeName, resourceType.Name) && deletedEv.Id == id)
{
obj = default(T); // entity might be recreated later
}
break;
}
}
return(obj);
}
else
{
throw new ArgumentException("A resource type with the given name does not exist");
}
}
/// <summary>
/// Walks through the specified event stream to obtain a summary of when the specified entity
/// was created, modified and deleted. This is a potentially expensive operation.
/// </summary>
/// <remarks>
/// This method only looks for standard CRUD events (<see cref="ICreateEvent"/>, <see cref="IUpdateEvent"/>,
/// <see cref="IDeleteEvent"/>). If for your specific entity type there are other event types the semantically
/// represent a change to the entity, these won't be part of the generated summary.
///
/// In case the entity lived different lives (i.e. has been recreated after deletion), only the most
/// recent "life" is considered for the history. Consider the following example event stream (with timestamps):
/// create (12:00), update (12:04), delete (13:20), create (13:55), delete (14:00), create (16:10), update (16:25).
/// In this case only the events from 16:10 and 16:25 will be considered and the property
/// <see cref="HistorySummary.Deleted"/> will not be set.
///
/// The event stream is assumed to be consistent. If the stream is inconsistent (e.g. has a create event
/// immediately followed by another create event), the behavior and resulting summary is undefined.
/// </remarks>
public static async Task <HistorySummary> GetSummaryAsync(IEventStream eventStream, EntityId entityId)
{
var enumerator = eventStream.GetEnumerator();
var summary = new HistorySummary();
while (await enumerator.MoveNextAsync())
{
if (enumerator.Current is ICrudEvent crudEvent &&
crudEvent.GetEntityType() == entityId.Type && crudEvent.Id == entityId.Id)
{
var timestamp = crudEvent.Timestamp;
var user = (crudEvent as IUserActivityEvent)?.UserId;
switch (crudEvent)
{
case ICreateEvent _:
if (summary.Created.HasValue)
{
// assumption: entity was deleted before and is now recreated (we don't check if there
// was a delete event before; it's not our job to validate the stream's consistency)
summary = new HistorySummary();
}
summary.Owner = user;
summary.Created = timestamp;
summary.LastModified = timestamp;
summary.Changes.Add(new HistorySummary.Change(timestamp, "Created", user));
break;
case IUpdateEvent _:
summary.LastModified = timestamp;
summary.Changes.Add(new HistorySummary.Change(timestamp, "Updated", user));
break;
case IDeleteEvent _:
summary.LastModified = timestamp;
summary.Deleted = timestamp;
summary.Changes.Add(new HistorySummary.Change(timestamp, "Deleted", user));
break;
}
}
}
return(summary);
}
public IAsyncEnumerator <IEvent> GetExistingEvents() => _stream.GetEnumerator();
