Adding notification publisher strategies

[jbogard]

I did not pull all the possible options from @remcoros 's samples. Just the basic couple options.

[jbogard]

This PR introduces the idea of notification publishers as a separate class to be injected into the Mediator class. Instead of only being able to override the PublishCore method in a derived class, this lets you inject your own implementation.:

public class Mediator : IMediator
{
    public Mediator(IServiceProvider serviceProvider) 
-        => _serviceProvider = serviceProvider;
+        : this(serviceProvider, new ForeachAwaitPublisher()) { }

+    public Mediator(IServiceProvider serviceProvider, INotificationPublisher publisher)
+    {
+        _serviceProvider = serviceProvider;
+        _publisher = publisher;
+    }

Two new publishing strategies added:

• ForeachAwaitPublisher (the default, existing strategy)
• TaskWhenAllPublisher - uses Task.WhenAll to publish

The publishing strategies now can accept the handler instance, for cases when ordering is needed:

+public interface INotificationPublisher
+{
+    Task Publish(IEnumerable<NotificationHandlerExecutor> handlerExecutors, INotification notification,
+        CancellationToken cancellationToken);
+ }
+ public record NotificationHandlerExecutor(object HandlerInstance, Func<INotification, CancellationToken, Task> HandlerCallback);

This required a breaking change in the PublishCore method in Mediator:

public class Mediator : IMediator {
-    protected virtual async Task PublishCore(IEnumerable<Func<INotification, CancellationToken, Task>> allHandlers, INotification notification, CancellationToken cancellationToken)
-    {
-        foreach (var handler in allHandlers)
-        {
-            await handler(notification, cancellationToken).ConfigureAwait(false);
-        }
-    }
+    protected virtual Task PublishCore(IEnumerable<NotificationHandlerExecutor> handlerExecutors, INotification notification, CancellationToken cancellationToken) 
+       => _publisher.Publish(handlerExecutors, notification, cancellationToken);
}

The notification publisher can be configured via MediatrServiceConfiguation:

+    public INotificationPublisher NotificationPublisher { get; set; } = new ForeachAwaitPublisher();
+    public Type? NotificationPublisherType { get; set; }

The NotificationPublisherType value overrides the instance property.

[remcoros]

awesome!

[neozhu]

Will this feature be included in the next version? Very much looking forward to