Lightning Emp API in Winter 19

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Emp API in Winter 19

Introduction

It’s August. After weeks of unusually sunny days, the schools in the UK have broken up and the weather has turned. As I sit looking out at cloudy skies, my thoughts turn to winter. Winter 19 to be specific - the release notes are in preview and some of the new functionality has hit my pre-release org. The first item that I’ve been playing with is the new Emp API component, which takes away a lot of the boilerplate code that I have to copy and paste every time I create a component that listens for platform events.

From the preview docs, this component

Exposes the EmpJs Streaming API library which subscribes to a streaming channel and listens to event messages using a shared CometD connection. This component is supported only in desktop browsers. This component requires API version 44.0 and later.

What I Used to Do

Previously, to connect to the streaming API and start listening for events, I’d need code to:

• Download the cometd library and put it into a static resource
• Add the static resource to my component
• Instantiate org.cometd.CometD
• Call the server to get a session id
• Configure cometd with the session id and the Salesforce endpoint
• Carry out the cometd handshake
• Subscribe to my platform event channel
• Wiat for messages

What I do Now

• Add the lightning:empApi component inside my custom component:
• Add an error handler in case anything goes wrong
• Subscribe to my platform event channel
• Wait for a message

In practice this means my controller code has dropped from 70 odd lines to around 20,

Example

The first thing I need for an example is a platform event - I’ve created one called Demo_Event__e, which contains a single field named ‘Message__c’. This holds the message that I’ll display to the user.

My example component (Demo Events) actually uses a couple of standard components - the Emp API and the notifications library - the latter is used to show a toast message when I receive an event:

<lightning:empApi aura:id="empApi" /> 
<lightning:notificationsLibrary aura:id="notifLib"/>

The controller handles all the setup via a method invoked when the standard init event is fired. Before I can do anything I need a reference for the Emp API component:

var empApi = component.find("empApi");

Once I have this I can subscribe to my demo event channel - I’ve chosen a replayId of -1 to say start with the next event published. Note that I also capture the subscription object returned by the promise so that I can unsubscribe later if I need to (although my sample component doesn’t actually do anything with it).

var channel='/event/Demo_Event__e';
var sub;
var replayId=-1;
empApi.subscribe(channel, replayId, callback).then(function(value) {
      console.log("Subscribed to channel " + channel);
      sub = value;
      component.set("v.sub", sub);
});

I also provide a callback function that gets invoked whenever I receive a message. This simply finds the notification library and executes the showToast aura method that it exposes.

var callback = function (message) {
	component.find('notifLib').showToast({
      	"title": "Message Received!",
        "message": message.data.payload.Message__c
	}); 
}.bind(this);

On the server side I have a class exposing a single static method that allows me to publish a platform event:

public class PlatformEventsDemo 
{
    public static void PublishDemoEvent(String message)
    {
	    Demo_Event__e event = new Demo_Event__e(Message__c=message);
                Database.SaveResult result = EventBus.publish(event);
                if (!result.isSuccess()) 
        {
            for (Database.Error error : result.getErrors()) 
            {
                System.debug('Error returned: ' +
                             error.getStatusCode() +' - '+
                             error.getMessage());
            }
        }
    }
}

Running the Example

I’ve added my component to a lightning page - s it doesn’t have any UI you’ll have to take my word for it! Using the execute anonymous feature of the dev console, I publish a message:

 

And on my lightning app page, shortly afterwards I see the toast message:

 

 

More Information

• The release notes for this component
• The Github repository for Winter 19 samples

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