For arlanix/notification 1.0.0

Declare events

An event is something your module tells, such as a feed failed or a stock level fell, that the store's rules can answer: each rule chooses an event, conditions on its fields, a topic, and a message worded with the fields. Publishing yourself, as in Publish from your module, sends one message to one topic the store chose; declaring an event leaves what is sent, when and where to the store.

arlanix/module-notification-event (orders, customers, reviews) and arlanix/module-notification-monitoring (cron, errors, admin sign-ins, imports and exports) are both made this way, and are the examples to read.

To add a field to an event another module declares, or a variable to every event, see Customise messages.

Declare it

etc/arlanix_notification_events.xml, validated by urn:magento:module:Arlanix_Notification:etc/arlanix_notification_events.xsd:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Arlanix_Notification:etc/arlanix_notification_events.xsd">
    <event code="vendor_feed_failed" label="Feed failed" group="Feeds" severity="error">
        <field path="feed.code" label="Feed" attribute="true" sample="google-us"/>
        <field path="feed.products" label="Products written" type="number" sample="0"/>
        <field path="feed.channel" label="Channel" type="select"
               source="Vendor\Module\Model\Source\Channels" sample="google"/>
        <subject>Feed {feed.code} failed</subject>
        <body>{feed.code} wrote {feed.products} products.</body>
    </event>
</config>

Emit it

With arlanix/module-notification required, through Arlanix\Notification\Api\EmitterInterface:

public function __construct(private readonly \Arlanix\Notification\Api\EmitterInterface $emitter)
{
}

public function afterFailure(Feed $feed): void
{
    if (!$this->emitter->wants('vendor_feed_failed')) {
        return;
    }

    $this->emitter->emit('vendor_feed_failed', [
        'feed' => [
            'code' => $feed->getCode(),
            'products' => $feed->getProductCount(),
            'channel' => $feed->getChannel(),
        ],
    ]);
}

• wants() says whether any active rule answers the event: ask first, and gather the data only when one does.
• emit() never throws. The data is nested as the paths name it. Each active rule of the event whose conditions it meets publishes its message to its topic.
• A rule left on As the event says takes the event's severity, unless the data says another under _severity (Arlanix\Notification\Model\Rule\MessageBuilder::SEVERITY): info, warning or error. An event that tells a failure and its recovery, as monitoring's Cron stopped does, emits '_severity' => 'error' for one and 'info' for the other. A rule with a severity of its own keeps it, and a value that is none of the three is ignored.
• Inside a database transaction, the message waits for the commit and is dropped on a rollback: an order a checkout gives up on is never told. Emit from *_save_commit_after, or anywhere: the wait is done for you.

With the module only suggested, dispatch an event instead, which needs none of its classes:

$this->eventManager->dispatch('arlanix_notification_emit', [
    'code' => 'vendor_feed_failed',
    'data' => ['feed' => ['code' => $feed->getCode(), 'products' => 0]],
]);

The key is code, not event: Magento's own observers already take event.

Check it

Arlanix > Notifications > Rules > Add rule lists the event under its group; Variables lists its fields, and Preview renders its subject and body with the samples. bin/magento cache:clean config after a change to the XML.