How to Access Payload Parameter in IEventPublisher.PublishAsync

[bashinamwila]

Hello,

I am working with Elsa Workflows and trying to access the payload parameter when publishing an event using IEventPublisher.PublishAsync. My goal is to retrieve the payload in the workflow when the event is triggered.

Here’s my workflow definition:

using Elsa;
using Elsa.Workflows;
using Elsa.Workflows.Runtime;
using Elsa.Workflows.Runtime.Activities;
using Elsa.Workflows.Activities;
using Elsa.Workflows.Memory;
using Elsa.Workflows.Models;
using Elsa.Extensions;
using Elsa.Expressions.Models;

public class MyWorkflow : WorkflowBase
{
    protected override void Build(IWorkflowBuilder builder)
    {
        // Create a variable to store the payload
        var payloadVar = new Variable<object>();

        builder.Root = new Sequence
        {
            Variables = { payloadVar },
            Activities =
            {
                new WriteLine("Starting workflow..."),
                new Event("ButtonClicked")
                {
                    CanStartWorkflow = true,
                },
                new WriteLine("Retrieving the payload..."),
                new WriteLine("Workflow completed...")
            }
        };
    }
}

I am triggering the event with the following code:

var myPayload = new Dictionary<string, object?>
{
    ["MyKey"] = "MyValue"
};

await eventPublisher.PublishAsync(
    eventName: "ButtonClicked",
    workflowInstanceId: result.WorkflowState.Id,
    payload: myPayload
);

Issue:
I want to retrieve myPayload inside my workflow when the "ButtonClicked" event is received. How can I access the payload parameter from IEventPublisher.PublishAsync inside my workflow?

Any guidance or code examples would be greatly appreciated.

Thanks in advance!

[sfmskywalker]

payload is the result of the Event activity. Bind the Event's result to a workflow variable (or use that result directly) and then inspect it in the following activity.

For example, publish a typed/anonymous payload to the waiting workflow instance:

await eventPublisher.PublishAsync(
    "ButtonClicked",
    workflowInstanceId: result.WorkflowState.Id,
    payload: new { MyKey = "MyValue" },
    cancellationToken: cancellationToken);

The Event activity completes by setting its result to the received payload. In the designer, bind the Event result to a variable and reference that variable in the next activity; in code, use the Event activity's result as the input to the next activity. The runtime stores the payload as workflow input under the event-payload key before resuming the event, so it is available even when the workflow resumes asynchronously.

The same pattern, including an example for a following decision activity, is covered in discussion #6898.

Relevant source: Elsa.Workflows.Runtime/Services/EventPublisher.cs, Activities/Event.cs, and Extensions/ActivityExecutionContextEventExtensions.cs.

Closing this Q&A as answered.