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When you interact with the Public API, and with the content Repository in particular, Signals may be sent out, allowing you to react on actions triggered by the Repository. Those signals can be received by dedicated services called Slots.
This recipe will describe how to register a Slot for a dedicated Signal.
Registering a Slot for a given Signal
As described in the SignalSlot documentation, a Slot is roughly like an event listener and must extend eZ\Publish\Core\SignalSlot\Slot.
A typical implementation is the following:
OnPublishSlot now needs to be registered as a service in the ServiceContainer and identified as a valid Slot:
services.yml (in your bundle)
Service tag ezpublish.api.slot identifies your service as a valid Slot. The signal part (mandatory) says that this slot is listening to ContentService\PublishVersionSignal (shortcut for \eZ\Publish\Core\SignalSlot\Signal\ContentService\PublishVersionSignal).
Internal signals emitted by Repository services are always relative to eZ\Publish\Core\SignalSlot\Signal namespace.
Hence ContentService\PublishVersionSignal means eZ\Publish\Core\SignalSlot\Signal\ContentService\PublishVersionSignal.
You can register a slot for any kind of signal by setting signal to * in the service tag.
Using a basic Symfony event listener
eZ Platform comes with a generic slot that converts signals (including ones defined by user code) to regular event objects and exposes them via the EventDispatcher. This makes it possible to implement a simple event listener/subscriber if you're more comfortable with this approach.
All you need to do is to implement an event listener or subscriber and register it.
This very simple example will just log the received signal.