From 3fd1c24b4bd3077493c8b5e0e92cecaec745b39a Mon Sep 17 00:00:00 2001
From: James Mockett <1166188+jamesmockett@users.noreply.github.com>
Date: Thu, 28 Nov 2024 12:36:20 +0000
Subject: [PATCH] Carousels: Optimise updating button state (#12923)

* Toggle button state when card half in or out of view

* Throttle scroll events with requestAnimationFrame

* Throttle scroll events with timeout as no animation

* Add comments to event throttling functions
---
 .../src/components/ScrollableCarousel.tsx     | 32 +++++++++++++----
 .../SlideshowCarousel.importable.tsx          | 34 ++++++++++++-------
 2 files changed, 47 insertions(+), 19 deletions(-)

diff --git a/dotcom-rendering/src/components/ScrollableCarousel.tsx b/dotcom-rendering/src/components/ScrollableCarousel.tsx
index 7656f66e15..ea72821dc4 100644
--- a/dotcom-rendering/src/components/ScrollableCarousel.tsx
+++ b/dotcom-rendering/src/components/ScrollableCarousel.tsx
@@ -288,9 +288,9 @@ export const ScrollableCarousel = ({
 	 * Updates state of navigation buttons based on carousel's scroll position.
 	 *
 	 * This function checks the current scroll position of the carousel and sets
-	 * the styles of the previous and next buttons accordingly. The previous
-	 * button is disabled if the carousel is at the start, and the next button
-	 * is disabled if the carousel is at the end.
+	 * the styles of the previous and next buttons accordingly. The button state
+	 * is toggled when the midpoint of the first or last card has been scrolled
+	 * in or out of view.
 	 */
 	const updateButtonVisibilityOnScroll = () => {
 		const carouselElement = carouselRef.current;
@@ -299,9 +299,27 @@ export const ScrollableCarousel = ({
 		const scrollLeft = carouselElement.scrollLeft;
 		const maxScrollLeft =
 			carouselElement.scrollWidth - carouselElement.clientWidth;
+		const cardWidth = carouselElement.querySelector('li')?.offsetWidth ?? 0;
 
-		setPreviousButtonEnabled(scrollLeft > 0);
-		setNextButtonEnabled(scrollLeft < maxScrollLeft);
+		setPreviousButtonEnabled(scrollLeft > cardWidth / 2);
+		setNextButtonEnabled(scrollLeft < maxScrollLeft - cardWidth / 2);
+	};
+
+	/**
+	 * Throttle scroll events to optimise performance. As we're only using this
+	 * to toggle button state as the carousel is scrolled we don't need to
+	 * handle every event. This function ensures the callback is only called
+	 * once every 200ms, no matter how many scroll events are fired.
+	 */
+	const throttleEvent = (callback: () => void) => {
+		let isThrottled: boolean = false;
+		return function () {
+			if (!isThrottled) {
+				callback();
+				isThrottled = true;
+				setTimeout(() => (isThrottled = false), 200);
+			}
+		};
 	};
 
 	useEffect(() => {
@@ -310,13 +328,13 @@ export const ScrollableCarousel = ({
 
 		carouselElement.addEventListener(
 			'scroll',
-			updateButtonVisibilityOnScroll,
+			throttleEvent(updateButtonVisibilityOnScroll),
 		);
 
 		return () => {
 			carouselElement.removeEventListener(
 				'scroll',
-				updateButtonVisibilityOnScroll,
+				throttleEvent(updateButtonVisibilityOnScroll),
 			);
 		};
 	}, []);
diff --git a/dotcom-rendering/src/components/SlideshowCarousel.importable.tsx b/dotcom-rendering/src/components/SlideshowCarousel.importable.tsx
index 2fe1be68b2..544d408add 100644
--- a/dotcom-rendering/src/components/SlideshowCarousel.importable.tsx
+++ b/dotcom-rendering/src/components/SlideshowCarousel.importable.tsx
@@ -124,26 +124,36 @@ export const SlideshowCarousel = ({
 	 * Updates state of navigation buttons based on carousel's scroll position.
 	 *
 	 * This function checks the current scroll position of the carousel and sets
-	 * the styles of the previous and next buttons accordingly. The previous
-	 * button is disabled if the carousel is at the start, and the next button
-	 * is disabled if the carousel is at the end.
+	 * the styles of the previous and next buttons accordingly. The button state
+	 * is toggled when the midpoint of the first or last card has been scrolled
+	 * in or out of view.
 	 */
 	const updatePaginationStateOnScroll = () => {
 		const carouselElement = carouselRef.current;
 		if (!carouselElement) return;
 
 		const scrollLeft = carouselElement.scrollLeft;
-
 		const maxScrollLeft =
 			carouselElement.scrollWidth - carouselElement.clientWidth;
-
-		setPreviousButtonEnabled(scrollLeft > 0);
-		setNextButtonEnabled(scrollLeft < maxScrollLeft);
-
 		const cardWidth = carouselElement.querySelector('li')?.offsetWidth ?? 0;
-		const page = Math.round(scrollLeft / cardWidth);
 
-		setCurrentPage(page);
+		setPreviousButtonEnabled(scrollLeft > cardWidth / 2);
+		setNextButtonEnabled(scrollLeft < maxScrollLeft - cardWidth / 2);
+		setCurrentPage(Math.round(scrollLeft / cardWidth));
+	};
+
+	/**
+	 * Throttle scroll events to optimise performance. As the scroll events are
+	 * used to trigger the pagination dot animation we're using
+	 * `requestAnimationFrame` rather than `setTimeout` to ensure this animates
+	 * smoothly in sync with the carousel being scrolled.
+	 */
+	const throttleEvent = (callback: () => void) => {
+		let requestId: number;
+		return function () {
+			cancelAnimationFrame(requestId);
+			requestId = requestAnimationFrame(callback);
+		};
 	};
 
 	useEffect(() => {
@@ -152,13 +162,13 @@ export const SlideshowCarousel = ({
 
 		carouselElement.addEventListener(
 			'scroll',
-			updatePaginationStateOnScroll,
+			throttleEvent(updatePaginationStateOnScroll),
 		);
 
 		return () => {
 			carouselElement.removeEventListener(
 				'scroll',
-				updatePaginationStateOnScroll,
+				throttleEvent(updatePaginationStateOnScroll),
 			);
 		};
 	}, []);