Sibling Communication with Lightning Message Service

@api and CustomEvent both rely on a direct parent/child relationship — a parent passing data down, or a child's event bubbling up to a listening ancestor. Two components that are not nested — for instance, two independent components both placed directly on the same Lightning Record Page — have no such relationship to lean on.

Lightning Message Service solves this with a publish/subscribe model, built around a Lightning Message Channel — a piece of metadata that defines a named "channel" any component can publish to or subscribe from, regardless of where it sits in the page.

LMS's code below uses something called @wire for the first time in this course, so before that code makes sense, here's just enough to read it correctly:

@wire is a decorator (Module 4 introduced @api as a decorator; @wire is a different one) that connects a component property to a reactive Salesforce-provided data source or service — it automatically supplies a value to that property, and can automatically update it again later if the underlying source changes. Module 6 is the full, dedicated lesson on @wire for reading Salesforce record data specifically — this primer covers only what's needed here: @wire can also connect a component to non-record services the platform provides, and MessageContext is exactly one such service — the piece of infrastructure LMS's publish/subscribe functions need in order to work. Writing @wire(MessageContext) messageContext; is simply how a component obtains that infrastructure access.

// Publishing (in one component)
import { LightningElement, wire } from 'lwc';
import { publish, MessageContext } from 'lightning/messageService';
import RECORD_SELECTED_CHANNEL from '@salesforce/messageChannel/RecordSelected__c';

export default class FilterPanel extends LightningElement {
    @wire(MessageContext) messageContext;

    handleFilterClick(recordId) {
        publish(this.messageContext, RECORD_SELECTED_CHANNEL, { recordId });
    }
}

// Subscribing (in a completely unrelated component)
import { subscribe, MessageContext } from 'lightning/messageService';
import RECORD_SELECTED_CHANNEL from '@salesforce/messageChannel/RecordSelected__c';

connectedCallback() {
    subscribe(this.messageContext, RECORD_SELECTED_CHANNEL, (message) => {
        this.selectedRecordId = message.recordId;
    });
}

@wire(MessageContext), per the primer above, gives access to the messaging infrastructure this all runs on top of — Module 6 covers @wire in full for reading actual Salesforce record data, a related but distinct use of the same decorator.

Real Lightning pages commonly have several independent components — a filter panel, a results list, a detail panel — that all need to coordinate without being nested inside each other. LMS is the standard, supported way to make that happen.