Over the past few weeks i have been working with Platform Events in a number of my Lightning projects, such as the Custom Metadata Services and Event Logging. This component allows you to receive Platform Events in your own components.
This component can be deployed from this repository here. I have been working on this for a while and I have yet to highlight this component on my blog. I wanted to rectify that now and I also wanted to take some time to explain lessons learned along the way and if anyone else has any thoughts on improving it even further!
Using the Component
Firstly using the component is very easy, you just need two attributes as you can see above. The channel defines the event and the onMessage attribute the handler for the event. I learnt along the way that if you define a registerEvent in your component it automatically exposes an attribute that takes a callback handler, much nicer for users of your component! The payload event parameter contains the platform event fields.
Wrapping CometD in a Component World
The component uses the CometD JavaScript library internally. This library does require a certain amount of configuration and authentication tweaks to initialise (such as disabling WebSockets, since LEX does not currently support it). Also, as i found out eventually, code to unsubscribe from events is also needed.
In the Visualforce page examples this was less of a concern in a page centric world. In the world of components this becomes very important! Without it, i was finding that when i navigated around Lightning Experience and back to components using this component i would accumulate handlers, that would effectively appear to be repeating events, not good.
In order to find the correct moment to unsubscribe and disconnect, the component listens to the aura:valueDestroy event. This does not fire immediately however, so some additional defensive work in the internal handler is needed. I am certainly open to further comments on how to make this component more robust.
This Trialhead module also includes example code on how to use the CometD library in Lightning, though the result is not a generic component like the one in this blog. I am also less sure about its design in terms of unsubscribing only on page unload.
Summary
Access to creating, sending and receiving Platform Events are available in Apex, REST API’s, Process Builder and Flow. Hopefully one day we will see a platform provided Lightning Base Component to replace the one i have created here. Meanwhile have fun!