Using Stimulus To Preload Links On Hover In Hotwire And Lucee CFML

<div
	data-controller="hover-preload"
	data-hover-preload-delay-value="500">

	<a href="my-link.htm" data-hover-preload-target="link">
		My link....
	</a>
</li>

<cfmodule template="./tags/page.cfm" section="home">
	<cfoutput>

		<h2>
			Welcome to Our Site!
		</h2>

		<p>
			Copy, copy, copy....
		</p>

		<h3>
			Check Out Our Products
		</h3>

		<ul class="products">
			<cfloop index="productID" from="101" to="110">
				<li
					data-controller="hover-preload"
					data-hover-preload-delay-value="500"
					class="products__item">

					<a
						href="product.htm?id=#productID#"
						data-hover-preload-target="link"
						class="products__link">
						Product #ucase( productID )#
					</a>
				</li>
			</cfloop>
		</ul>

	</cfoutput>
</cfmodule>

<cfscript>

	param name="url.id" type="string";

	// Adding a sleep to exaggerate the benefits of preloading the content.
	sleep( 500 );

</cfscript>
<cfmodule template="./tags/page.cfm">
	<cfoutput>

		<h2>
			Product #encodeForHtml( url.id )#
		</h2>

		<p>
			Copy, copy, copy....
		</p>

		<script type="text/javascript">

			// Logging the render time so that we can see CACHED vs LIVE rendering.
			console.log(
				"Product #encodeForJavaScript( url.id )# rendered at",
				new Date().toLocaleTimeString()
			);

		</script>

	</cfoutput>
</cfmodule>

// Import core modules.
import { Application } from "@hotwired/stimulus";
import { Controller } from "@hotwired/stimulus";
import * as Turbo from "@hotwired/turbo";

// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //

class HoverPreloadController extends Controller {

	// Every link target contained within the controller's scope will be preloaded when
	// the hover timer has completed.
	static targets = [ "link" ];
	// The hover threshold (ie, duration the user has to hover before the link preloading
	// kicks-in) can be configured.
	static values = {
		delay: {
			type: Number,
			default: 300
		}
	};

	// ---
	// LIFE-CYCLE METHODS.
	// ---

	/**
	* I run once when the controller has been instantiated, but before it has been
	* connected to the host DOM element. A single instantiated controller may be connected
	* to the host element multiples times during the life-time of a page.
	*/
	initialize() {

		this.hoverTimer = 0;

	}


	/**
	* I run once after the component instance has been bound to the host DOM element. At
	* this point, all of the classes, targets, and values have already been bound.
	*/
	connect() {

		if ( this.hasLinkTarget ) {

			this.setupEvents();

		}

	}


	/**
	* I get called once after the component instance has been unbound from the host DOM
	* element. At this point, all of the targets have already been disconnected as well.
	*/
	disconnect() {

		this.teardownEvents();

	}

	// ---
	// PRIVATE METHODS.
	// ---

	/**
	* I handle the mouseenter event on the host element.
	*/
	handleMouseenter = ( event ) => {

		this.hoverTimer = window.setTimeout( this.handleTimeout, this.delayValue );

	};


	/**
	* I handle the mouseleave event on the host element.
	*/
	handleMouseleave = ( event ) => {

		window.clearTimeout( this.hoverTimer );
		this.hoverTimer = 0;

	};


	/**
	* I handle the timeout event triggered from the hover interactions.
	*/
	handleTimeout = () => {

		this.teardownEvents();
		this.preloadLinks();

	};


	/**
	* I ask the Turbo session to preload all of the link targets embedded within this
	* controller.
	*/
	preloadLinks() {

		for ( var target of this.linkTargets ) {

			console.log( "Preloading links for:", target.href.split( "/" ).at( -1 ) );
			// CAUTION: The "preloader" is an UNDOCUMENTED PROPERTY of the session (at
			// least as far as I can tell) - I only found it by looking at the source code
			// for the Turbo project. As such, use this technique at your own risk!
			Turbo.session.preloader.preloadURL( target );

		}

	}


	/**
	* I setup the hover events on the host element.
	*/
	setupEvents() {

		this.element.addEventListener( "mouseenter", this.handleMouseenter );
		this.element.addEventListener( "mouseleave", this.handleMouseleave );

	}


	/**
	* I teardown the hover events on the host element. Any pending timer will be cleared.
	*/
	teardownEvents() {

		window.clearTimeout( this.hoverTimer );
		this.hoverTimer = 0;

		this.element.removeEventListener( "mouseenter", this.handleMouseenter );
		this.element.removeEventListener( "mouseleave", this.handleMouseleave );

	}

}

// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //

window.Stimulus = Application.start();
// When not using the Ruby On Rails asset pipeline / build system, Stimulus doesn't know
// how to map controller classes to data-controller attributes. As such, we have to
// explicitly register the Controllers on Stimulus startup.
Stimulus.register( "hover-preload", HoverPreloadController );