refactor the IAsyncEnumerable out of the saga

BREAKING CHANGE: The ISaga.Evaluate no longer accepts an IAsyncEnumerable<IReason> as return value. Add reasons through the context.Result property instead.

Author: corstian

samples/ConsoleSample/Domain/TodoItemAddedSaga.cs

@@ -7,10 +7,8 @@ namespace ConsoleSample.Domain;
 
 public class TodoItemAddedSaga : ISaga<TodoItemCreated>
 {
-    public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, TodoItemCreated @event)
+    public async Task Evaluate(ISagaContext context, TodoItemCreated @event)
     {
         await context.InvokeService(new NestedService());
-
-        yield break;
     }
 }

samples/Skyhop.Domain/AircraftContext/Sagas/OnAircraftChanged.cs

@@ -11,14 +11,14 @@ namespace Skyhop.Domain.AircraftContext.Sagas;
 
 internal class OnAircraftChanged : ISaga<AircraftSet>
 {
-    public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, AircraftSet @event)
+    public async Task Evaluate(ISagaContext context, AircraftSet @event)
     {   
         var flight = await context.Factory
             .Instantiate<Flight>(context.AggregateId!)
             .Snapshot<FlightSnapshot>();
 
         // There is no need to make a change as the flight is up to date with the latest state
-        if (flight.AircraftId != @event.AircraftId) yield break;
+        if (flight.AircraftId != @event.AircraftId) return;
 
         var aircraft = await context.Factory
             .Instantiate<Aircraft>(@event.AircraftId)

samples/Skyhop.Domain/AircraftContext/Sagas/OnAircraftRemoved.cs

@@ -11,14 +11,14 @@ namespace Skyhop.Domain.AircraftContext.Sagas;
 
 public class OnAircraftRemoved : ISaga<AircraftRemoved>
 {
-    public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, AircraftRemoved @event)
+    public async Task Evaluate(ISagaContext context, AircraftRemoved @event)
     {
         var flight = await context.Factory
             .Instantiate<Flight>(context.AggregateId!)
             .Snapshot<FlightSnapshot>();
 
         // No need to make a change; nothing to remove here.
-        if (flight.AircraftId == @event.AircraftId) yield break;
+        if (flight.AircraftId == @event.AircraftId) return;
 
         var aircraft = await context.Factory
             .Instantiate<Aircraft>(@event.AircraftId)

samples/Skyhop.Domain/AircraftContext/Sagas/OnArrival.cs

@@ -9,7 +9,7 @@ namespace Skyhop.Domain.AircraftContext.Sagas;
 
 internal class OnArrival : ISaga<ArrivalTimeSet>
 {
-    public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, ArrivalTimeSet @event)
+    public async Task Evaluate(ISagaContext context, ArrivalTimeSet @event)
     {
         var snapshot = await context.Factory
             .Instantiate<Flight>(context.AggregateId!)
@@ -23,7 +23,5 @@ public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, ArrivalTim
                     context.AggregateId!,
                     snapshot.DepartureTime,
                     @event.ArrivalTime));
-
-        yield break;
     }
 }

samples/Skyhop.Domain/AircraftContext/Sagas/OnDeparture.cs

@@ -9,7 +9,7 @@ namespace Skyhop.Domain.AircraftContext.Sagas;
 
 public class OnDeparture : ISaga<DepartureTimeSet>
 {
-    public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, DepartureTimeSet @event)
+    public async Task Evaluate(ISagaContext context, DepartureTimeSet @event)
     {
         var snapshot = await context.Factory
             .Instantiate<Flight>(context.AggregateId!)
@@ -23,7 +23,5 @@ public async IAsyncEnumerable<IReason> Evaluate(ISagaContext context, DepartureT
                     context.AggregateId!,
                     @event.DepartureTime,
                     snapshot.ArrivalTime));
-
-        yield break;
     }
 }

src/Whaally.Domain.Abstractions/Saga/ISaga.cs

@@ -4,14 +4,14 @@ namespace Whaally.Domain.Abstractions;
 
 public interface ISaga
 {
-    public IAsyncEnumerable<IReason> Evaluate(ISagaContext context, IEvent @event);
+    public Task Evaluate(ISagaContext context, IEvent @event);
 }
 
 public interface ISaga<TEvent> : ISaga
     where TEvent : class, IEvent
 {
-    IAsyncEnumerable<IReason> ISaga.Evaluate(ISagaContext context, IEvent @event)
+    Task ISaga.Evaluate(ISagaContext context, IEvent @event)
         => Evaluate(context, (TEvent)@event);
 
-    public IAsyncEnumerable<IReason> Evaluate(ISagaContext context, TEvent @event);
+    public Task Evaluate(ISagaContext context, TEvent @event);
 }

src/Whaally.Domain.Abstractions/Saga/ISagaContext.cs

@@ -9,20 +9,25 @@ public interface ISagaContext : IContext
     /// </summary>
     public IReadOnlyList<CommandEnvelope> Commands { get; }
 
+    /// <summary>
+    ///     Contains the failure or success reasons for this evaluation.
+    /// </summary>
+    public Result Result { get; }
+    
     /// <summary>
     ///     Stages a command as the optimistic result of this saga.
     /// </summary>
     /// <param name="command">The command staged as a result of saga evaluation</param>
     public void StageCommands(
         string aggregateId, 
         params ICommand[] command);
-    
+
     /// <summary>
     ///     Evaluates a service and stages the resulting commands as the optimistic result of this saga.
     /// </summary>
     /// <param name="service">The service to evaluate as part of the saga evaluation</param>
     /// <returns>A Result object indicating success or error states.</returns>
-    public Task<IResultBase> InvokeService(IService service);
+    public Task InvokeService(IService service);
 
     /// <summary>
     ///     The AggregateHandlerFactory provides access to AggregateHandler instances.

src/Whaally.Domain/DefaultEvaluationAgent.cs

@@ -154,12 +154,12 @@ public async Task<IResultBase> Invoke(ISaga saga, EventEnvelope eventEnvelope)
 
         var result = new Result();
 
-        await foreach (var reason in saga.Evaluate(
-                           _contextFactory.CreateSagaContext(eventEnvelope.Metadata),
-                           eventEnvelope.Messages.Single()))
-        {
-            result.WithReason(reason);
-        }
+        var context = _contextFactory.CreateSagaContext(eventEnvelope.Metadata);
+        await saga.Evaluate(
+            context,
+            eventEnvelope.Messages.Single());
+
+        result.WithReasons(context.Result.Reasons);
 
         return result;
     }

src/Whaally.Domain/Saga/SagaContext.cs

@@ -27,6 +27,7 @@ public SagaContext(
     }
 
     public string? AggregateId { get; init; }
+    public Result Result { get; init; } = new();
     public string? TransactionId { get; init; }
     public ActivityContext? ParentContext { get; init; }
     public IReadOnlyDictionary<string, object> Attributes { get; init; } 
@@ -66,7 +67,7 @@ public virtual void StageCommands(string aggregateId, params ICommand[] commands
         _envelopes.Add(aggregateId, envelope);
     }
 
-    public virtual async Task<IResultBase> InvokeService(IService service)
+    public virtual async Task InvokeService(IService service)
     {
         var result = await _evaluationAgent.Evaluate(
             new ServiceEnvelope(
@@ -77,13 +78,13 @@ public virtual async Task<IResultBase> InvokeService(IService service)
                     TransactionId = TransactionId
                 }, service));
 
-        if (!result.IsSuccess) return result.ToResult();
+        Result.WithReasons(result.Reasons);
+        
+        if (!result.IsSuccess) return;
 
         foreach (var envelope in result.Value)
             StageCommands(
                 envelope.Metadata.AggregateId, 
                 envelope.Messages.ToArray());
-
-        return result.ToResult();
     }
 }

tests/Skyhop.Domain.Tests/AircraftContext/Sagas/OnDepartureTests.cs

@@ -58,10 +58,10 @@ await AggregateFactory
         var @event = new DepartureTimeSet(DateTime.Now);
 
         // Evaluate
-        var result = saga.Evaluate(context, @event).ToBlockingEnumerable();
+        await saga.Evaluate(context, @event);
 
         // Assert
-        result.Should().BeEmpty();
+        context.Result.Reasons.Should().BeEmpty();
         Assert.Single(context.Commands);
         Assert.IsType<SetFlightInfo>(context.Commands.Single().Messages.Single());
     }