In writing a Mechanic task, it may be necessary to do more than a single round of work. The task may need to act based on the content of a previous action, possibly rendering additional actions to follow.

To achieve enhanced flows of this sort, subscribe to the mechanic/actions/perform event topic. When a task includes this subscription, Mechanic will generate an event with that topic for every action that the task completes. This event will only ever be responded to by the task that generated it; it will never be sent to other tasks.

Inspecting the context

Events with the topic mechanic/actions/perform are, by nature, always child events. As such, their event variable contains a reference to the event's parent. This means a task may use event.parent.data to access all the data that informed the construction of the action in question. (Note that only the previous 5 generations of parents are available.)

A mechanic/actions/perform event also provides task runs with an action variable (containing the same information available in event.data). This variable contains the original action object (including its type, options, and meta information), and data from the run in which the action was performed.

Common sources of information for followup task runs:

• action.options — the original options defined for the action

• action.run.ok — true or false, indicating the action's success

• action.run.result — the value (if any) generated by the action run

• action.run.error — the error (if any) raised during the action run

• action.meta — the meta information given in the action's definition

Example

This example cycles between three different modes of operation, depending on the meta information recorded in the action definition. And, by checking on action.run.ok at the very beginning, the task is also able to "break" into failure-handling mode, in which an email is sent to an administrator.

mechanic/user/trigger
mechanic/actions/perform

{% if action.run.ok == false %}
  {% action "email" %}
    {
      "to": "[email protected]",
      "subject": "Action failure",
      "body": {{ "Alert!<br><br>" | append: action.run.error | json }}
    }
  {% endaction %}
{% elsif event.topic contains "trigger" %}
  {% action %}
    {
      "type": "cache",
      "options": ["set", "foo", "bar"],
      "meta": {
        "mode": "first"
      }
    }
  {% endaction %}
{% elsif action.meta.mode == "first" %}
  {% capture mutation %}
    mutation {
      tagsAdd(id: "gid://shopify/Customer/1234567890", tags: ["processed"]) {
        userErrors { field, message }
      }
    }
  {% endcapture %}
​
  {% action %}
    {
      "type": "shopify",
      "options": {{ mutation | json }},
      "meta": {
        "mode": "second"
      }
    }
  {% endaction %}
{% elsif action.meta.mode == "second" %}
  {% action "echo", "done" %}
{% endif %}