Add GoBackResult (#449)

src/core/Elsa.Core/Elsa.Core.csproj

@@ -17,6 +17,7 @@
         <RepositoryType>GitHub</RepositoryType>
         <PackageTags>elsa, workflows, orchard</PackageTags>
         <PackageIcon>icon.png</PackageIcon>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

src/core/Elsa.Core/Results/GoBackResult.cs

@@ -0,0 +1,29 @@
+using System.Linq;
+using Elsa.Services;
+using Elsa.Services.Extensions;
+using Elsa.Services.Models;
+
+namespace Elsa.Results
+{
+    public class GoBackResult : ActivityExecutionResult
+    {
+        private readonly int steps;
+
+        public GoBackResult(int steps = 1)
+        {
+            this.steps = steps;
+        }
+        
+        protected override void Execute(IWorkflowInvoker invoker, WorkflowExecutionContext workflowContext)
+        {
+            var previousEntry = workflowContext.Workflow.ExecutionLog.Skip(steps).FirstOrDefault();
+
+            if (previousEntry == null)
+                return;
+
+            var activityId = previousEntry.ActivityId;
+            var activity = workflowContext.Workflow.GetActivity(activityId);
+            workflowContext.ScheduleActivity(activity);
+        }
+    }
+}