Defining preview events

During task preview, Mechanic scans the task's subscriptions. For each event topic found, Mechanic constructs a synthetic preview event, resembling one that the task might encounter during live use.

By default, each preview event's data is sampled from previous events that the Mechanic account has seen, for the same topic.

However, developers may define their own preview events, containing whatever data the developer wishes to use for preview. This may be useful for several reasons:

  • Most tasks conditionally respond to events based on their data. Controlling the event data present during preview allows the developer to deterministically verify the results of the action.

  • Further, by deterministically/predictably generating actions, the developer can consistently demonstrate the permissions they need to Mechanic. (To learn more about this, see Previews.)

  • Defining preview event data is usually simpler than defining stub data.

    • Stubbing the event variable (or any of the subject variables) removes any intelligence from the objects Mechanic generates from event data, a drawback avoided by defining a preview event and its data. Using the Order object as an example, a task may typically access its custom attributes via order.note_attributes.color, or via order.note_attributes[0].value. This dynamic behavior is lost if the event variable is stubbed out, which can result in behaviors that are difficult to diagnose.

  • Multiple preview events may be defined per event topic. This allows developers to verify that their task renders the appropriate results under a variety of circumstances.

    • Defined preview events can be labeled with a description, which is visible in the task preview pane. This makes it easy to identify the scenario that a preview event is meant to represent.

Preview event definitions cannot provide for return values from Shopify query operations (i.e. output from the shopify filter, or the result of traversing Shopify Liquid objects, as in customer.orders.first). For those purposes, use the stub data technique.


Preview events may be defined using the "Edit preview events" button, in the task preview pane.

The configuration area for preview events contains a quickstart link for each event topic the task subscribes to, allowing developers to get started using sample data if the event topic is known to Mechanic. Or, the developer may start with a blank preview event definition, filling in whatever topic and data are useful.

A developer may define any number of preview events per topic. If no preview events are defined for a given topic, Mechanic will construct its own ad-hoc event during preview.



Displayed beneath the event topic in the preview pane, allowing the developer to distinguish one scenario from another.


Identifies the event definition to Mechanic, when Mechanic goes to construct preview events by topic.


Used to construct, and may be set to whatever values are useful in representing a specific scenario. The data structures used here should resemble what Mechanic will receive for a live event of the same topic.

Notably, the data here can be limited to just the properties that are useful. For example, while Mechanic might normally generate a complete payload for shopify/orders/create, the developer might only care about the "email" property of the order – and so their defined preview event data might be limited to just that property. (Note that the inverse may not be true: defining preview event data for traversals into other objects, e.g. using preview event data to define a value for order.line_items[0].product.title, will not work.)


For a trivial task, subscribing to shopify/customers/create, and having the following task code...

{% if contains "" %}
  {% log message: "got a gmail user!", email: %}
{% else %}
  {% log message: "got someone else!", email: %}
{% endif %}

... we define two preview events, one which represents a Gmail user, and one which does not. This allows us to easily assert that the task behaves properly in both scenarios.


Preview event definitions are stored along with the task itself, and thus are present in the tasks version history (and, naturally, in task exports).

Because definitions are a part of the task itself, they're appropriate for use as a testing tool, allowing the developer to verify that a task behaves as intended at every stage of the task's development.

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