October 16, 2021

How to subscribe to Platform Event using Lightning Web Component in Salesforce?

HTML:
<template>
    <lightning-card title="EmpApi Example" icon-name="custom:custom14">
        <div class="slds-m-around_medium">
            <lightning-button variant="success" label="Subscribe" title="Subscribe"
                onclick={handleSubscribe} disabled={isSubscribeDisabled}
                class="slds-m-left_x-small"></lightning-button>
            <lightning-button variant="destructive" label="Unsubscribe" title="Unsubscribe"
                onclick={handleUnsubscribe} disabled={isUnsubscribeDisabled}
                class="slds-m-left_x-small"></lightning-button>
        </div>
        <lightning-datatable 
            key-field="ReplayId"
            data={listEvents}
            columns={columns}
            hide-checkbox-column="true">
        </lightning-datatable>  
    </lightning-card>
</template>

JavaScript:
import { LightningElement } from 'lwc';
import { subscribe, unsubscribe, onError } from 'lightning/empApi';

export default class SamplePlatformEventsListener extends LightningElement {

    channelName = '/event/Test__e';
    isSubscribeDisabled = false;
    isUnsubscribeDisabled = !this.isSubscribeDisabled;
    subscription = {};
    listEvents;
    columns = [   
        { label: 'Serial No', fieldName: 'SerialNo' }, 
        { label: 'Replay Id', fieldName: 'ReplayId' }, 
        { label: 'Schema', fieldName: 'Schema' }
    ];

    connectedCallback() {   

        this.registerErrorListener();      

    }

    handleSubscribe() {
        
        const messageCallback = ( response ) => {
            console.log( 'New message received: ', JSON.stringify( response ) );
            let val =  { 
                'SerialNo' : response.data.payload.Serial_No__c, 
                'Schema' : response.data.schema, 
                'ReplayId' : response.data.event.replayId
            };
            console.log( 'Val is ' + JSON.stringify( val ) );
            console.log( 'Events before assignment are ' + JSON.stringify( this.listEvents ) );
            
            if ( this.listEvents ) {
                this.listEvents = [ ...this.listEvents,  val ];
            } else {
                this.listEvents = [ val ];
            }

            console.log( 'Events after assignment are ' + JSON.stringify( this.listEvents ) );
        };

        subscribe( this.channelName, -1, messageCallback ).then( response => {
            console.log( 'Subscription request sent to: ', JSON.stringify( response.channel ) );
            this.subscription = response;
            this.toggleSubscribeButton( true );
        });

    }

    handleUnsubscribe() {

        this.toggleSubscribeButton( false );

        unsubscribe( this.subscription, response => {
            console.log( 'unsubscribe() response: ', JSON.stringify( response ) );
        });

    }

    toggleSubscribeButton( enableSubscribe ) {

        this.isSubscribeDisabled = enableSubscribe;
        this.isUnsubscribeDisabled = !enableSubscribe;

    }

    registerErrorListener() {

        onError( error => {
            console.log( 'Received error from server: ', JSON.stringify( error ) );
        });

    }

}

meta.xml:
<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>52.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>

Output:


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