Listen to a data store event

In this tutorial we will show how to listen to a components data store event.

Component data store

All your components are stored at runtime in a data store that you can access with System Runtime APIs. It sends:

• an update event when a component has been updated in the data store,
• an insert event when a component has been created in the data store and
• a remove event when a component has been deleted from the data store.

Define your system

First create a schema that:

• has for name Person
• has a name property

{
  "_id": "eb74b2c8-a1b7-4a08-aa6e-eaf029e2e3f2",
  "_name": "Person",
  "_inherit": [
    "_Component"
  ],
  "name": "property"
}

• then create a component,
• click on it to edit it,
• change the value of _id to luke,

{
  "_id": "luke",
  "name": ""
}

• then edit the start method of your system and
• add this code:

function start() { 
  // get luke component
  const luke = this.require('luke');
  
  // set the name property of luke component
  luke.name('luke');
}

Listen to an update event of the data store

• click on the import button on the left toolbar,
• a dialog is shown,
• click on from the library radio button,
• select Listen to components collection events from the proposed systems,
• click on the Compose button,

This system has now been composed in your system.

• on Behaviors tab, click on _Database model (on the right panel under Models),

• click on the update behavior to edit it,
• add this code:

function update(event) { 
  // get logger component
  const logger = this.require('logger');
  
  // react only if the update occurs on Person
  if (event.collection === 'Person') {
    logger.info('component ' + event.id + ' has for new property ' + event.field + ' : ' + event.value);
  }
}

• save it and
• run your system.

You will see a message telling that the property of the component has changed.

A system makes a lot of update on the data store when running, so for performance reason always filter on a specific collection.