Activity Preconditions (#4507)

* Add CanExecuteAsync precondition method

* Rename GetInput to GetWorkflowInput to disambiguate between input sent to workflow and input sent to indiviudal activities.

src/modules/Elsa.AzureServiceBus/Activities/MessageReceived.cs

@@ -88,7 +88,7 @@ public class MessageReceived : Trigger
     protected override async ValueTask ExecuteAsync(ActivityExecutionContext context)
     {
         // If we did not receive external input, it means we are just now encountering this activity.
-        if (!context.TryGetInput<ReceivedServiceBusMessageModel>(InputKey, out var receivedMessage))
+        if (!context.TryGetWorkflowInput<ReceivedServiceBusMessageModel>(InputKey, out var receivedMessage))
         {
             // Create bookmarks for when we receive the expected HTTP request.
             context.CreateBookmark(GetBookmarkPayload(context.ExpressionExecutionContext), Resume);
@@ -102,7 +102,7 @@ public class MessageReceived : Trigger
 
     private async ValueTask Resume(ActivityExecutionContext context)
     {
-        var receivedMessage = context.GetInput<ReceivedServiceBusMessageModel>(InputKey);
+        var receivedMessage = context.GetWorkflowInput<ReceivedServiceBusMessageModel>(InputKey);
         await SetResultAsync(receivedMessage, context);
         await context.CompleteActivityAsync();
     }

src/modules/Elsa.Http/Activities/HttpEndpoint.cs

@@ -198,7 +198,7 @@ public class HttpEndpoint : Trigger<HttpRequest>
         context.Set(Result, request);
 
         // Read route data, if any.
-        var path = context.GetInput<PathString>(RequestPathInputKey);
+        var path = context.GetWorkflowInput<PathString>(RequestPathInputKey);
         var routeData = GetRouteData(httpContext, path);
 
         var routeDictionary = routeData.Values.ToDictionary(route => route.Key, route => route.Value!);

src/modules/Elsa.MassTransit/Activities/MessageReceived.cs

@@ -32,7 +32,7 @@ public class MessageReceived : Trigger<object>
     protected override async ValueTask ExecuteAsync(ActivityExecutionContext context)
     {
         // If we did not receive external input, it means we are just now encountering this activity and we need to block execution by creating a bookmark.
-        if (!context.TryGetInput<object>(InputKey, out var message))
+        if (!context.TryGetWorkflowInput<object>(InputKey, out var message))
         {
             // Create bookmarks for when we receive the expected HTTP request.
             context.CreateBookmark(GetBookmarkPayload(context.ExpressionExecutionContext));

src/modules/Elsa.Scheduling/Activities/StartAt.cs

@@ -76,7 +76,7 @@ public class StartAt : Trigger
     protected override void Execute(ActivityExecutionContext context)
     {
         // If external input was received, it means this activity got triggered and does not need to create a bookmark.
-        if (context.TryGetInput<DateTimeOffset>(InputKey, out _)) 
+        if (context.TryGetWorkflowInput<DateTimeOffset>(InputKey, out _)) 
             return;
         
         // No external input received, so create a bookmark.

src/modules/Elsa.Telnyx/Activities/AnswerCallBase.cs

@@ -67,7 +67,7 @@ public abstract class AnswerCallBase : Activity<CallAnsweredPayload>
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallAnsweredPayload>();
+        var payload = context.GetWorkflowInput<CallAnsweredPayload>();
         context.Set(Result, payload);
         await HandleConnectedAsync(context);
     }

src/modules/Elsa.Telnyx/Activities/BridgeCallsBase.cs

@@ -79,7 +79,7 @@ public abstract class BridgeCallsBase : Activity<BridgedCallsOutput>
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallBridgedPayload>()!;
+        var payload = context.GetWorkflowInput<CallBridgedPayload>()!;
         var callControlIdA = CallControlIdA.Get(context);
         var callControlIdB = CallControlIdB.Get(context);;
 

src/modules/Elsa.Telnyx/Activities/CallAnswered.cs

@@ -40,7 +40,7 @@ public class CallAnswered : Activity<CallAnsweredPayload>
 
     private async ValueTask Resume(ActivityExecutionContext context)
     {
-        var input = context.GetInput<CallAnsweredPayload>(WebhookSerializerOptions.Create());
+        var input = context.GetWorkflowInput<CallAnsweredPayload>(WebhookSerializerOptions.Create());
         context.Set(Result, input);
         await context.CompleteActivityAsync();
     }

src/modules/Elsa.Telnyx/Activities/CallHangup.cs

@@ -40,7 +40,7 @@ public class CallHangup : Activity<CallHangupPayload>
 
     private async ValueTask Resume(ActivityExecutionContext context)
     {
-        var input = context.GetInput<CallHangupPayload>(WebhookSerializerOptions.Create());
+        var input = context.GetWorkflowInput<CallHangupPayload>(WebhookSerializerOptions.Create());
         context.Set(Result, input);
         await context.CompleteActivityAsync();
     }

src/modules/Elsa.Telnyx/Activities/DialAndWait.cs

@@ -89,14 +89,14 @@ public class DialAndWait : Activity<CallPayload>
 
     private async ValueTask OnCallAnswered(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallAnsweredPayload>();
+        var payload = context.GetWorkflowInput<CallAnsweredPayload>();
         Result.Set(context, payload);
         await context.CompleteActivityWithOutcomesAsync("Answered");
     }
     
     private async ValueTask OnCallHangup(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallHangupPayload>();
+        var payload = context.GetWorkflowInput<CallHangupPayload>();
         Result.Set(context, payload);
         await context.CompleteActivityWithOutcomesAsync("Hangup");
     }

src/modules/Elsa.Telnyx/Activities/GatherUsingAudio.cs

@@ -150,7 +150,7 @@ public class GatherUsingAudio : Activity<CallGatherEndedPayload>, IBookmarksPers
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallGatherEndedPayload>();
+        var payload = context.GetWorkflowInput<CallGatherEndedPayload>();
         var outcome = payload.Status == "valid" ? "Valid input" : "Invalid input";
         context.Set(Result, payload);
         await context.CompleteActivityWithOutcomesAsync(outcome);

src/modules/Elsa.Telnyx/Activities/GatherUsingSpeak.cs

@@ -181,7 +181,7 @@ public class GatherUsingSpeak : Activity<CallGatherEndedPayload>
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallGatherEndedPayload>();
+        var payload = context.GetWorkflowInput<CallGatherEndedPayload>();
         var outcome = payload.Status == "valid" ? "Valid input" : "Invalid input";
         context.Set(Result, payload);
         await context.CompleteActivityWithOutcomesAsync(outcome);

src/modules/Elsa.Telnyx/Activities/IncomingCall.cs

@@ -63,7 +63,7 @@ public class IncomingCall : Trigger<CallInitiatedPayload>
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var webhookModel = context.GetInput<TelnyxWebhook>(WebhookSerializerOptions.Create());
+        var webhookModel = context.GetWorkflowInput<TelnyxWebhook>(WebhookSerializerOptions.Create());
         var callInitiatedPayload = (CallInitiatedPayload)webhookModel.Data.Payload;
 
         // Store webhook payload as output.

src/modules/Elsa.Telnyx/Activities/StartRecordingBase.cs

@@ -101,7 +101,7 @@ public abstract class StartRecordingBase : Activity<CallRecordingSavedPayload>
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallRecordingSavedPayload>();
+        var payload = context.GetWorkflowInput<CallRecordingSavedPayload>();
         context.Set(Result, payload);
         await HandleCallRecordingSavedAsync(context);
     }

src/modules/Elsa.Telnyx/Activities/TransferCall.cs

@@ -105,7 +105,7 @@ public class TransferCall : Activity<CallPayload>
 
     private ValueTask InitiatedAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallInitiatedPayload>();
+        var payload = context.GetWorkflowInput<CallInitiatedPayload>();
         var callControlId = payload.CallControlId;
         var answeredBookmark = new WebhookEventBookmarkPayload(WebhookEventTypes.CallAnswered, callControlId);
         var hangupBookmark = new WebhookEventBookmarkPayload(WebhookEventTypes.CallHangup, callControlId);
@@ -116,14 +116,14 @@ public class TransferCall : Activity<CallPayload>
     
     private async ValueTask AnsweredAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallAnsweredPayload>();
+        var payload = context.GetWorkflowInput<CallAnsweredPayload>();
         Result.Set(context, payload);
         await context.CompleteActivityWithOutcomesAsync("Transferred");
     }
 
     private async ValueTask HangupAsync(ActivityExecutionContext context)
     {
-        var payload = context.GetInput<CallHangupPayload>();
+        var payload = context.GetWorkflowInput<CallHangupPayload>();
         Result.Set(context, payload);
         await context.CompleteActivityWithOutcomesAsync("Hangup");
     }

src/modules/Elsa.Telnyx/Activities/WebhookEvent.cs

@@ -54,7 +54,7 @@ public class WebhookEvent : Activity<Payload>
 
     private async ValueTask Resume(ActivityExecutionContext context)
     {
-        var input = context.GetInput<TelnyxWebhook>(WebhookSerializerOptions.Create());
+        var input = context.GetWorkflowInput<TelnyxWebhook>(WebhookSerializerOptions.Create());
         context.Set(Result, input.Data.Payload);
         await CompleteAsync(context);
     }

src/modules/Elsa.Workflows.Core/Abstractions/Activity.cs

@@ -88,6 +88,23 @@ public abstract class Activity : IActivity, ISignalHandler
     [JsonIgnore]
     public ICollection<IBehavior> Behaviors { get; } = new List<IBehavior>();
 
+    /// <summary>
+    /// Override this method to return a value indicating whether the activity can execute.
+    /// </summary>
+    protected virtual ValueTask<bool> CanExecuteAsync(ActivityExecutionContext context)
+    {
+        var result = CanExecute(context);
+        return new(result);
+    }
+    
+    /// <summary>
+    /// Override this method to return a value indicating whether the activity can execute.
+    /// </summary>
+    protected virtual bool CanExecute(ActivityExecutionContext context)
+    {
+        return true;
+    }
+
     /// <summary>
     /// Override this method to implement activity-specific logic.
     /// </summary>
@@ -187,6 +204,11 @@ public abstract class Activity : IActivity, ISignalHandler
     {
         await context.CompleteActivityAsync();
     }
+    
+    async ValueTask<bool> IActivity.CanExecuteAsync(ActivityExecutionContext context)
+    {
+        return await CanExecuteAsync(context);
+    }
 
     async ValueTask IActivity.ExecuteAsync(ActivityExecutionContext context)
     {

src/modules/Elsa.Workflows.Core/Contexts/ActivityExecutionContext.cs

@@ -42,6 +42,7 @@ public class ActivityExecutionContext : IExecutionContext
         Activity = activity;
         ActivityDescriptor = activityDescriptor;
         StartedAt = startedAt;
+        Status = ActivityStatus.Pending;
         Tag = tag;
         CancellationToken = cancellationToken;
         Id = Guid.NewGuid().ToString();
@@ -140,7 +141,7 @@ public class ActivityExecutionContext : IExecutionContext
     /// <summary>
     /// Returns the <see cref="ActivityNode"/> metadata about the current activity.
     /// </summary>
-    public ActivityNode? ActivityNode => WorkflowExecutionContext.FindNodeByActivity(Activity);
+    public ActivityNode ActivityNode => WorkflowExecutionContext.FindNodeByActivity(Activity)!;
 
     /// <summary>
     /// Returns the global node ID for the current activity within the graph.

src/modules/Elsa.Workflows.Core/Contexts/WorkflowExecutionContext.cs

@@ -157,7 +157,7 @@ public class WorkflowExecutionContext : IExecutionContext
 
         workflowExecutionContext.MemoryRegister = workflow.CreateRegister();
         workflowExecutionContext.ExpressionExecutionContext = new ExpressionExecutionContext(serviceProvider, workflowExecutionContext.MemoryRegister, cancellationToken: cancellationTokens.ApplicationCancellationToken);
-        
+
         await workflowExecutionContext.SetWorkflowAsync(workflow);
         return workflowExecutionContext;
     }
@@ -339,7 +339,9 @@ public class WorkflowExecutionContext : IExecutionContext
     /// <summary>
     /// A list of <see cref="ActivityExecutionContext"/>s that are currently active.
     /// </summary>
-    public IReadOnlyCollection<ActivityExecutionContext> ActiveActivityExecutionContexts => ActivityExecutionContexts.Where(x => !x.IsCompleted || x.ParentActivityExecutionContext == null).ToList();
+    public IReadOnlyCollection<ActivityExecutionContext> ActiveActivityExecutionContexts => ActivityExecutionContexts
+        .Where(x => !x.IsCompleted || x.ParentActivityExecutionContext == null && x.Status != ActivityStatus.Pending)
+        .ToList();
 
     /// <summary>
     /// A list of <see cref="ActivityExecutionContext"/>s that are currently active.
@@ -363,7 +365,7 @@ public class WorkflowExecutionContext : IExecutionContext
     /// <summary>
     /// The expression execution context for the current workflow execution.
     /// </summary>
-    public ExpressionExecutionContext? ExpressionExecutionContext { get; private set; } = default!;
+    public ExpressionExecutionContext? ExpressionExecutionContext { get; private set; }
 
     /// <inheritdoc />
     public IEnumerable<Variable> Variables => Workflow.Variables;
@@ -450,7 +452,7 @@ public class WorkflowExecutionContext : IExecutionContext
     /// Returns the <see cref="ActivityNode"/> containing the specified activity from the workflow graph.
     /// </summary>
     public ActivityNode? FindNodeByActivity(IActivity activity) => NodeActivityLookup[activity];
-    
+
     /// <summary>
     /// Returns the <see cref="ActivityNode"/> associated with the specified activity ID.
     /// </summary>

src/modules/Elsa.Workflows.Core/Contracts/IActivity.cs

@@ -43,6 +43,11 @@ public interface IActivity
     /// </summary>
     IDictionary<string, object> Metadata { get; set; }
     
+    /// <summary>
+    /// Returns a value indicating whether the activity can execute.
+    /// </summary>
+    ValueTask<bool> CanExecuteAsync(ActivityExecutionContext context);
+    
     /// <summary>
     /// Invoked when the activity executes.
     /// </summary>

src/modules/Elsa.Workflows.Core/Enums/ActivityStatus.cs

@@ -5,6 +5,11 @@ namespace Elsa.Workflows.Core;
 /// </summary>
 public enum ActivityStatus
 {
+    /// <summary>
+    /// The activity is in the Pending state.
+    /// </summary>
+    Pending,
+    
     /// <summary>
     /// The activity is in the Running state. Note that event if an activity is running, it may not be executing.
     /// </summary>

src/modules/Elsa.Workflows.Core/Extensions/ActivityExecutionContextExtensions.cs

@@ -29,7 +29,7 @@ public static class ActivityExecutionContextExtensions
     /// <summary>
     /// Attempts to get a value from the input provided via <see cref="WorkflowExecutionContext"/>. If a value was found, an attempt is made to convert it into the specified type <code>T</code>.
     /// </summary>
-    public static bool TryGetInput<T>(this ActivityExecutionContext context, string key, out T value, JsonSerializerOptions? serializerOptions = default)
+    public static bool TryGetWorkflowInput<T>(this ActivityExecutionContext context, string key, out T value, JsonSerializerOptions? serializerOptions = default)
     {
         var wellKnownTypeRegistry = context.GetRequiredService<IWellKnownTypeRegistry>();
 
@@ -46,12 +46,12 @@ public static class ActivityExecutionContextExtensions
     /// <summary>
     /// Gets a value from the input provided via <see cref="WorkflowExecutionContext"/>. If a value was found, an attempt is made to convert it into the specified type <code>T</code>.
     /// </summary>
-    public static T GetInput<T>(this ActivityExecutionContext context, JsonSerializerOptions? serializerOptions = default) => context.GetInput<T>(typeof(T).Name, serializerOptions);
+    public static T GetWorkflowInput<T>(this ActivityExecutionContext context, JsonSerializerOptions? serializerOptions = default) => context.GetWorkflowInput<T>(typeof(T).Name, serializerOptions);
 
     /// <summary>
     /// Gets a value from the input provided via <see cref="WorkflowExecutionContext"/>. If a value was found, an attempt is made to convert it into the specified type <code>T</code>.
     /// </summary>
-    public static T GetInput<T>(this ActivityExecutionContext context, string key, JsonSerializerOptions? serializerOptions = default)
+    public static T GetWorkflowInput<T>(this ActivityExecutionContext context, string key, JsonSerializerOptions? serializerOptions = default)
     {
         var wellKnownTypeRegistry = context.GetRequiredService<IWellKnownTypeRegistry>();
         return context.WorkflowInput[key].ConvertTo<T>(new ObjectConverterOptions(serializerOptions, wellKnownTypeRegistry))!;

src/modules/Elsa.Workflows.Core/Extensions/ExpressionExecutionContextExtensions.cs

@@ -72,6 +72,16 @@ public static class ExpressionExecutionContextExtensions
     /// </summary>
     public static IDictionary<string, object> GetInput(this ExpressionExecutionContext context) => (IDictionary<string, object>)context.TransientProperties[InputKey];
     
+    /// <summary>
+    /// Returns input sent to the workflow.
+    /// </summary>
+    public static T GetWorkflowInput<T>(this ExpressionExecutionContext context, string key) => context.GetActivityExecutionContext().GetWorkflowInput<T>(key);
+    
+    /// <summary>
+    /// Returns input sent to the workflow.
+    /// </summary>
+    public static T GetWorkflowInput<T>(this ExpressionExecutionContext context) => context.GetActivityExecutionContext().GetWorkflowInput<T>();
+    
     /// <summary>
     /// Returns the value of the specified input.
     /// </summary>

src/modules/Elsa.Workflows.Core/Middleware/Activities/DefaultActivityInvokerMiddleware.cs

@@ -42,6 +42,16 @@ public class DefaultActivityInvokerMiddleware : IActivityExecutionMiddleware
         // Evaluate input properties.
         await EvaluateInputPropertiesAsync(context);
 
+        // Check if the activity can be executed.
+        if (!await context.Activity.CanExecuteAsync(context))
+        {
+            context.Status = ActivityStatus.Pending;
+            context.AddExecutionLogEntry("Precondition Failed", "Cannot execute at this time");
+            return;
+        }
+
+        context.Status = ActivityStatus.Running;
+
         // Execute activity.
         await ExecuteActivityAsync(context);
 

src/modules/Elsa.Workflows.Runtime/Activities/RunTask.cs

@@ -94,7 +94,7 @@ public class RunTask : Activity<object>, IBookmarksPersistedHandler
 
     private async ValueTask ResumeAsync(ActivityExecutionContext context)
     {
-        var input = context.GetInput<object>(InputKey);
+        var input = context.GetWorkflowInput<object>(InputKey);
         context.Set(Result, input);
         await context.CompleteActivityAsync();
     }

test/integration/Elsa.IntegrationTests/Scenarios/CanExecute/Activities/CustomActivity.cs

@@ -0,0 +1,37 @@
+using System;
+using Elsa.Expressions.Models;
+using Elsa.Extensions;
+using Elsa.Workflows.Core;
+using Elsa.Workflows.Core.Contracts;
+using Elsa.Workflows.Core.Models;
+using Elsa.Workflows.Core.Services;
+
+namespace Elsa.IntegrationTests.Scenarios.CanExecute.Activities;
+
+public class CustomActivity : CodeActivity
+{
+    public CustomActivity(int magicNumber)
+    {
+        MagicNumber = new (magicNumber);
+    }
+    
+    public CustomActivity(Func<ExpressionExecutionContext, int> magicNumber)
+    {
+        MagicNumber = new (magicNumber);
+    }
+    
+    public Input<int> MagicNumber { get; set; }
+    
+    protected override bool CanExecute(ActivityExecutionContext context)
+    {
+        var magicNumber = MagicNumber.Get(context);
+        return magicNumber == 42;
+    }
+
+    protected override void Execute(ActivityExecutionContext context)
+    {
+        var provider = context.GetService<IStandardOutStreamProvider>() ?? new StandardOutStreamProvider(Console.Out);
+        var textWriter = provider.GetTextWriter();
+        textWriter.WriteLine("Welcome to the world of Might and Magic!");
+    }
+}

test/integration/Elsa.IntegrationTests/Scenarios/CanExecute/Tests.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Elsa.IntegrationTests.Scenarios.CanExecute.Activities;
+using Elsa.IntegrationTests.Scenarios.CanExecute.Workflows;
+using Elsa.Testing.Shared;
+using Elsa.Workflows.Core.Contracts;
+using Elsa.Workflows.Core.Options;
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Elsa.IntegrationTests.Scenarios.CanExecute;
+
+public class CanExecuteTests
+{
+    private readonly CapturingTextWriter _capturingTextWriter = new();
+    private readonly IServiceProvider _services;
+    private readonly IWorkflowRunner _workflowRunner;
+
+    public CanExecuteTests(ITestOutputHelper testOutputHelper)
+    {
+        _services = new TestApplicationBuilder(testOutputHelper)
+            .WithCapturingTextWriter(_capturingTextWriter)
+            .AddActivitiesFrom<CustomActivity>()
+            .Build();
+
+        _workflowRunner = _services.GetRequiredService<IWorkflowRunner>();
+    }
+
+    [Theory(DisplayName = "Activities are executed only when they report that they can execute.")]
+    [InlineData(12, "Magic number is 12")]
+    [InlineData(42, "Magic number is 42\nWelcome to the world of Might and Magic!\nDone")]
+    public async Task Test1(int magicNumber, string expectedLines)
+    {
+        await _services.PopulateRegistriesAsync();
+        var runOptions = new RunWorkflowOptions
+        {
+            Input = new Dictionary<string, object>
+            {
+                ["MagicNumber"] = magicNumber
+            }
+        };
+        await _workflowRunner.RunAsync<MagicWorkflow>(runOptions);
+        var lines = _capturingTextWriter.Lines.ToList();
+        Assert.Equal(expectedLines.Split('\n', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries), lines);
+    }
+}

test/integration/Elsa.IntegrationTests/Scenarios/CanExecute/Workflows/MagicWorkflow.cs

@@ -0,0 +1,26 @@
+using Elsa.Extensions;
+using Elsa.IntegrationTests.Scenarios.CanExecute.Activities;
+using Elsa.Workflows.Core;
+using Elsa.Workflows.Core.Activities;
+using Elsa.Workflows.Core.Contracts;
+
+namespace Elsa.IntegrationTests.Scenarios.CanExecute.Workflows;
+
+public class MagicWorkflow : WorkflowBase
+{
+    protected override void Build(IWorkflowBuilder builder)
+    {
+        var magicNumberVariable = builder.WithVariable<int>();
+
+        builder.Root = new Sequence
+        {
+            Activities =
+            {
+                new SetVariable<int>(magicNumberVariable, context => context.GetWorkflowInput<int>("MagicNumber")),
+                new WriteLine(context => $"Magic number is {magicNumberVariable.Get(context)}"),
+                new CustomActivity(context => magicNumberVariable.Get(context)),
+                new WriteLine("Done")
+            }
+        };
+    }
+}

test/integration/Elsa.IntegrationTests/Scenarios/FlowchartNextActivity/Activities/CustomActivity.cs

@@ -9,5 +9,5 @@ public class CustomActivity : CodeActivity
     protected override async ValueTask ExecuteAsync(ActivityExecutionContext context)
     {
         await context.CompleteActivityAsync();
-    }    
+    }
 }