本地事件总线和事务

通过重写Ef Core的SaveChanges/SaveChangesAsync来实现事务。当然，如果您愿意实现仓储层，可以在仓储层实现展开对应实体包含的事件，并且调整事件的处理顺序。

Github仓库地址：soda-event-bus

实现`AggregateRoot`类

AggregateRoot类主要通过一个集合来记录本次事务的所有事件，到保存前再展开读取，在Abp中采用的ICollection记录的本地事件，通过实现一个排序器来保证顺序问题，我这里直接采用了ConcurrentQueue，保证原子操作的同时保证了顺序性，实现更简单一些。

public abstract class AggregateRoot
{
    public ConcurrentQueue<object> LocalEvents { get; } = new();

    public void AddLocalEvent<TEvent>(TEvent eventData) where TEvent : IEvent
    {
        LocalEvents.Enqueue(eventData);
    }

    public bool GetLocalEvent(out object? @event)
    {
        LocalEvents.TryDequeue(out var eventData);

        @event = eventData;
        return @event is not null;
    }

    public void ClearLocalEvents()
    {
        LocalEvents.Clear();
    }
}

重写`DbContext`

主要是从ServiceProvider中获取对应实体类包含的事件，并且找到对应的Handler进行处理，然后再当作一个事务提交。

public class EventBusDbContext<TDbContext> : DbContext
    where TDbContext : DbContext
{
    private readonly IServiceProvider _serviceProvider;

    public EventBusDbContext(DbContextOptions<TDbContext> options, IServiceProvider serviceProvider) : base(options)
    {
        _serviceProvider = serviceProvider;
    }

    public override int SaveChanges()
    {
        return base.SaveChanges();
    }

    public override int SaveChanges(bool acceptAllChangesOnSuccess)
    {
        return base.SaveChanges(acceptAllChangesOnSuccess);
    }

    public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
    {
        await HandleEventsAsync();

        return await base.SaveChangesAsync(cancellationToken);
    }

    public override async Task<int> SaveChangesAsync(bool acceptAllChangesOnSuccess, CancellationToken cancellationToken = default)
    {
        await HandleEventsAsync();

        return await base.SaveChangesAsync(acceptAllChangesOnSuccess, cancellationToken);
    }

    private async Task HandleEventsAsync()
    {
        foreach (var entityEntry in ChangeTracker.Entries<AggregateRoot>())
        {
            while (entityEntry.Entity.GetLocalEvent(out var @event))
            {
                if (@event is null) break;

                await HandleEventAsync(@event);
            }

            entityEntry.Entity.ClearLocalEvents();
        }
    }

    private async Task HandleEventAsync(object @event)
    {
        var eventHandlerType = typeof(IAsyncEventHandler<>).MakeGenericType(@event.GetType());
        var eventHandler = _serviceProvider.GetRequiredService(eventHandlerType);

        var method = eventHandler.GetType().GetMethod(nameof(IAsyncEventHandler<IEvent>.HandleAsync));
        var exceptionHandleMethod = eventHandlerType.GetMethod(nameof(IAsyncEventHandler<IEvent>.HandleException));

        try
        {
            await (Task)method!.Invoke(eventHandler, new[] { @event })!;
        }
        catch (Exception ex)
        {
            exceptionHandleMethod!.Invoke(eventHandler, new[] { @event, ex });
        }
    }
}