Howto create a Trigger Activity

[robverhoef-united]

Hello,

What is needed to create a trigger activity like the HttpEndpoint or Schedule Activity. As i understand you have to derive it from the ITrigger interface. But other than that... So what are the specifics? Like i want to start a workflow with to be developed FileWatcher activity? How do i build this activity and add it to the system..

Thanks

[glime-ai[bot]]

To create a custom trigger activity like HttpEndpoint or Schedule that can start a workflow, you need to implement the ITrigger interface in your activity class. Here are the steps you would typically follow to create and integrate a custom FileWatcher trigger activity into Elsa Workflows:

1. Implement the ITrigger Interface:
   Your FileWatcher activity class should inherit from Activity and implement the ITrigger interface. This interface requires you to implement the GetTriggerPayloadsAsync method, which provides payloads that can be used to start workflows.

2. Create the Activity Class:
   Define the FileWatcher class with the necessary properties to configure the file watching behavior (e.g., directory path, filter, etc.). Implement the Execute method to create a bookmark that will be used to resume the workflow when a file event occurs.

3. Handle File Events:
   Within your FileWatcher activity, you will need to set up the actual file watching logic using FileSystemWatcher or a similar mechanism. When a file event that you want to trigger the workflow occurs, use the WorkflowExecutionContext to resume the workflow using the bookmark created earlier.

4. Register the Activity:
   To make your custom activity available to Elsa, you need to register it with the Elsa services. This can be done in the Startup.cs or Program.cs file of your application, where you configure the Elsa services.

5. Use the Activity in Workflows:
   Once registered, you can use your FileWatcher activity in workflows. You can do this programmatically by adding it to a workflow definition or by using Elsa Designer if you expose your activity to the designer.

Here's a simplified example of what the FileWatcher activity class might look like:

public class FileWatcher : Activity, ITrigger
{
    // Define properties for directory path, filter, etc.

    protected override void Execute(ActivityExecutionContext context)
    {
        // Create a bookmark to suspend the workflow
        context.CreateBookmark();

        // Set up file watching logic and handle events
        // ...
    }

    public ValueTask<IEnumerable<object>> GetTriggerPayloadsAsync(TriggerIndexingContext context)
    {
        // Return payloads if necessary for the trigger
        return new ValueTask<IEnumerable<object>>(Enumerable.Empty<object>());
    }
}

And to register the activity:

services.AddE

[robverhoef-united]

Hello,
Thanks for the answer. I already found a simular example on the elsav3 page, but still having trouble to get this going. I've made a simple test example with a Task.Delay(1000) which should start this workflow every 1 second. My activity looks like this

public class TestTrigger : Activity, ITrigger
{
    private ILogger<TestTrigger> _logger;
    private IWorkflowRuntime _workflowRuntime;

    protected override async ValueTask ExecuteAsync(ActivityExecutionContext context)
    {
        _logger = context.GetRequiredService<ILogger<TestTrigger>>();
        _workflowRuntime = context.GetRequiredService<IWorkflowRuntime>();
        _logger.LogInformation("Trigger Test Activity Started");

        context.CreateBookmark();

        trigger(context);
    }

    public ValueTask<IEnumerable<object>> GetTriggerPayloadsAsync(TriggerIndexingContext context)
    {
        //_workflowRuntime = context.ExpressionExecutionContext.ServiceProvider.GetRequiredService<IWorkflowRuntime>();
        //_workflowRuntime.StartWorkflowAsync(context.WorkflowIndexingContext.Workflow.Identity.DefinitionId, new StartWorkflowRuntimeOptions());
        return new ValueTask<IEnumerable<object>>(Enumerable.Empty<object>());
    }

    private void trigger(ActivityExecutionContext context)
    {
        var b = context.Bookmarks.Where(b => b.ActivityInstanceId == context.Id).FirstOrDefault();
        Task.Delay(TimeSpan.FromSeconds(1));
        var options = new ResumeWorkflowRuntimeOptions()
        {
            BookmarkId = b.Id,
        };

        _logger.LogInformation("Resume Trigger Test Activity");

        _workflowRuntime.ResumeWorkflowAsync(context.WorkflowExecutionContext.Id, options);
    }
}

I've noticed during testing that the "GetTriggerPayloadsAsync" is called two times during the start of the application so i though i may have to add code to start the particular workflow from the "TriggerIndexingContext" but that does not seems to do the trick.

This is the test workflow:

    public class TestTriggerWorkflow: WorkflowBase
    {
        protected override void Build(IWorkflowBuilder builder)
        {
            builder.DefinitionId = "TestTriggerWorkflow";
            builder.Name = "Test Trigger Workflow";
            builder.Version = 1;
            builder.Root = new Sequence
            {

                Activities = {
                    new TestTrigger
                    {
                        CanStartWorkflow = true,
                    },
                    new WriteLine($"Timer is triggered {DateTime.Now}")
                }
            };
        }
    }

And added this to program.cs

builder.Services.AddElsa(elsa =>
{
\\\\\\\\\\
\\\\\\\\\\\

    elsa.AddActivity<TestTrigger>();

    elsa.AddWorkflow<TestTriggerWorkflow>();
});

I also tested with a httpendpoint in a workflow using elsa.AddHttp() and that works perfectly. Every time you call the endpoint a new workflow instance in created and executed.

So what am i missing?

Any help is very much appreciated.

Rob

[GersonDias]

I'm trying something similar, did you find out how to implement custom triggers properly?

[robverhoef-united]

Tobe honest, no i have not. You might want to take a look at the way triggers are changed and expanded in the current V3.5 though. I have not been able to test these yet.
Regards