Introduce AmbientActivityExecutionContext to support direct access to composite activity input properties from child activities (#648)

src/core/Elsa.Abstractions/Services/Activity.cs

@@ -19,7 +19,7 @@ namespace Elsa.Services
         public bool PersistWorkflow { get; set; }
         public bool LoadWorkflowContext { get; set; }
         public bool SaveWorkflowContext { get; set; }
-        public JObject Data { get; set; } = new();
+        public virtual JObject Data { get; set; } = new();
         public ValueTask<bool> CanExecuteAsync(ActivityExecutionContext context) => OnCanExecuteAsync(context);
         public ValueTask<IActivityExecutionResult> ExecuteAsync(ActivityExecutionContext context) => OnExecuteAsync(context);
         public ValueTask<IActivityExecutionResult> ResumeAsync(ActivityExecutionContext context) => OnResumeAsync(context);

src/core/Elsa.Abstractions/Services/CompositeActivity.cs

@@ -4,6 +4,7 @@ using Elsa.ActivityResults;
 using Elsa.Builders;
 using Elsa.Models;
 using Elsa.Services.Models;
+using Newtonsoft.Json.Linq;
 
 namespace Elsa.Services
 {
@@ -21,6 +22,29 @@ namespace Elsa.Services
             set => SetState(value);
         }
 
+        public override JObject Data
+        {
+            get
+            {
+                // When executing a composite activity's workflow, child activities might try and reference the composite activity's "input" properties.
+                // Out of the box, that doesn't work, since "this" points to a new instance with an empty Data object.
+                // Instead, we need to capture the Data object of the composite activity (the parent of the currently executing child activity) using the ambient activity execution context.
+                if (AmbientActivityExecutionContext.Current == null) 
+                    return base.Data;
+                
+                var context = AmbientActivityExecutionContext.Current;
+                    
+                // Check if the currently executing activity is something other than this composite activity.
+                if (context.ActivityBlueprint.Type == Type) 
+                    return base.Data;
+                
+                // A child activity is attempting to retrieve property data from its parent composite activity.
+                var parentId = context.ActivityBlueprint.Parent!.Id;
+                return context.WorkflowInstance.ActivityData.GetItem(parentId)!;
+            }
+            set => base.Data = value;
+        }
+
         protected override async ValueTask<IActivityExecutionResult> OnExecuteAsync(ActivityExecutionContext context)
         {
             if (!IsScheduled)

src/core/Elsa.Abstractions/Services/Models/AmbientActivityExecutionContext.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Threading;
+
+namespace Elsa.Services.Models
+{
+    public static class AmbientActivityExecutionContext
+    {
+        private static readonly AsyncLocal<ActivityExecutionContext?> ActivityExecutionContext = new();
+
+        public static ActivityExecutionContext? Current
+        {
+            get => ActivityExecutionContext.Value;
+            private set => ActivityExecutionContext.Value = value;
+        }
+
+        public static ActivityExecutionContextScope EnterScope(ActivityExecutionContext activityExecutionContext)
+        {
+            Current = activityExecutionContext;
+            return new ActivityExecutionContextScope(() => Current = null);
+        }
+    }
+
+    public record ActivityExecutionContextScope(Action Reset) : IDisposable
+    {
+        public void Dispose()
+        {
+            Reset();
+        }
+    }
+}

src/core/Elsa.Core/Services/WorkflowRunner.cs

@@ -287,6 +287,7 @@ namespace Elsa.Services
                 resuming,
                 cancellationToken);
 
+            using var executionScope = AmbientActivityExecutionContext.EnterScope(activityExecutionContext);
             var activity = await activityExecutionContext.ActivateActivityAsync(cancellationToken);
             var canExecute = await activity.ActivityType.CanExecuteAsync(activityExecutionContext);
 
@@ -328,6 +329,8 @@ namespace Elsa.Services
                 var activityExecutionContext = new ActivityExecutionContext(scope, workflowExecutionContext, activityBlueprint, scheduledActivity.Input, resuming, cancellationToken);
                 var activity = await activityExecutionContext.ActivateActivityAsync(cancellationToken);
 
+                using var executionScope = AmbientActivityExecutionContext.EnterScope(activityExecutionContext);
+                
                 if (resuming)
                     await _mediator.Publish(new ActivityResuming(activityExecutionContext), cancellationToken);
 

src/samples/console/Elsa.Samples.ProgrammaticCompositeActivitiesConsole/Activities/CountdownActivity.cs

@@ -12,7 +12,7 @@ namespace Elsa.Samples.ProgrammaticCompositeActivitiesConsole.Activities
     public class CountdownActivity : CompositeActivity
     {
         // Exposed properties on composite activities must store their values in the State property bag so that it gets persisted & available to child activities.
-        // Activities are scheduled one by one, which means that when the child activities are executed, the container activity (such as this one) will not be in memory - the default value will be captured when the property is used directly.
+        // Think of composite activities as workflows and their properties as inputs into the workflow.
         [ActivityProperty(Hint = "The start number.")]
         public int Start
         {
@@ -23,9 +23,7 @@ namespace Elsa.Samples.ProgrammaticCompositeActivitiesConsole.Activities
         public override void Build(ICompositeActivityBuilder activity)
         {
             activity
-                // IMPORTANT: Notice that we need to get the "parent" state to get the `Start` value, since here we are in a different context than the CountDownActivity itself.
-                // Accessing `Start` directly would return the captured value at the time this `Build` method executed when creating workflow blueprints.
-                .For(context => context.GetContainerState<int>(nameof(Start)), _ => 0, _ => -1, iterate =>
+                .For(() => Start, () => 0, () => -1, iterate =>
                 {
                     iterate.WriteLine(context => $"{context.ForScope().CurrentValue}...");
                 }, Operator.GreaterThan)