Eventbus

The core/eventbus component is a bus that extends the events module in NodeJs. If no observer is attached to the event that is being emitted, then a warning will be written to the console, through the core/console component.

const eventbus = core.locate('core/eventbus')

eventbus.on('foo', (event) => /* ... */)
eventbus.emit('foo', 'bar')

In above example, the event argument passed to the observer hold the string value bar.

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Mapping an observer to listen to an event

You can define observers in the configuration file of a component you add to the core context, under the namespace core.eventbus.observers. Below follows an example of a configuration that attaches an observer to the eventbus.

{
  core:
  {
    eventbus:
    {
      observers:
      {
        'foobar': { 'foobar/observer' : true }
      }
    },
    locator:
    {
      'foobar/observer' : '/absolute/path/to/observer'
    }
  }
}

As seen by the example above, we define an observer called foobar/observer that we map to observe the event foobar. We rely on the locator to locate the observer for us. Locating the observer, and attaching the observer to observe the event, will take place during the core bootstrap process.

The trueful flag in the mapper is indicating that the observer will observe the event. It is sometimes necessery to deactivate some observers when for instance running in a test environment. If you need to deactivate an observer, set the flag to false.

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The observer interface

An observer is a class that follows a simple interface. The class must have an observe method that accepts 2 arguements, the emitted data and the eventName of the triggered event.

class Observer
{
  observe(data, eventName)
  {
    /* ... */
  }
}

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Alternative observer interface

Alternative an observer can follow a naming convention on the interface, defining a method that is prefixed with the keyword on - followed by a camelcased version of the eventname. So for example, the event foo bar woukld be mapped to the method onFooBar, as the following example describes.

class Observer
{
  onFooBar(data, eventName)
  {
    /* ... */
  }
}