Tabs: split animation logic into multiple separate composable utilities.

packages/components/src/tabs/tablist.tsx

@@ -10,8 +10,9 @@ import * as Ariakit from '@ariakit/react';
 import warning from '@wordpress/warning';
 import {
 	forwardRef,
+	useCallback,
 	useEffect,
-	useLayoutEffect,
+	useInsertionEffect,
 	useRef,
 	useState,
 } from '@wordpress/element';
@@ -25,57 +26,121 @@ import { TabListWrapper } from './styles';
 import type { WordPressComponentProps } from '../context';
 import clsx from 'clsx';
 
-function useTrackElementOffset(
-	targetElement?: HTMLElement | null,
-	onUpdate?: () => void
-) {
-	const [ indicatorPosition, setIndicatorPosition ] = useState( {
-		left: 0,
-		top: 0,
-		width: 0,
-		height: 0,
-	} );
+// TODO: move these into a separate utility file, for use in other components
+// such as ToggleGroupControl.
 
-	// TODO: replace with useEventCallback or similar when officially available.
-	const updateCallbackRef = useRef( onUpdate );
-	useLayoutEffect( () => {
-		updateCallbackRef.current = onUpdate;
+/**
+ * Any function.
+ */
+type AnyFunction = ( ...args: any ) => any;
+
+/**
+ * Creates a stable callback function that has access to the latest state and
+ * can be used within event handlers and effect callbacks. Throws when used in
+ * the render phase.
+ *
+ * @example
+ *
+ * ```tsx
+ * function Component(props) {
+ *   const onClick = useEvent(props.onClick);
+ *   React.useEffect(() => {}, [onClick]);
+ * }
+ * ```
+ */
+function useEvent< T extends AnyFunction >( callback?: T ) {
+	const ref = useRef< AnyFunction | undefined >( () => {
+		throw new Error( 'Cannot call an event handler while rendering.' );
 	} );
+	useInsertionEffect( () => {
+		ref.current = callback;
+	} );
+	return useCallback< AnyFunction >(
+		( ...args ) => ref.current?.( ...args ),
+		[]
+	) as T;
+}
+
+/**
+ * `useResizeObserver` options.
+ */
+type UseResizeObserverOptions = {
+	/**
+	 * Whether to trigger the callback when an element's ResizeObserver is
+	 * first set up.
+	 *
+	 * @default true
+	 */
+	fireOnObserve?: boolean;
+};
 
-	const observedElementRef = useRef< HTMLElement >();
+/**
+ * Fires `onResize` when the target element is resized.
+ *
+ * **The element must not be stored in a ref**, else it won't be observed
+ * or updated. Instead, it should be stored in a React state or equivalent.
+ *
+ * It sets up a `ResizeObserver` that tracks the element under the hood. The
+ * target element can be changed dynamically, and the observer will be
+ * updated accordingly.
+ *
+ * By default, `onResize` is called when the observer is set up, in addition
+ * to when the element is resized. This behavior can be disabled with the
+ * `fireOnObserve` option.
+ *
+ * @example
+ *
+ * ```tsx
+ * const [ targetElement, setTargetElement ] = useState< HTMLElement | null >();
+ *
+ * useResizeObserver( targetElement, ( element ) => {
+ *   console.log( 'Element resized:', element );
+ * } );
+ *
+ * <div ref={ setTargetElement } />;
+ * ```
+ */
+function useResizeObserver(
+	/**
+	 * The target element to observe. It can be changed dynamically.
+	 */
+	targetElement: HTMLElement | undefined | null,
+
+	/**
+	 * Callback to fire when the element is resized. It will also be
+	 * called when the observer is set up, unless `fireOnObserve` is
+	 * set to `false`.
+	 */
+	onResize: ( element: HTMLElement ) => void,
+	{ fireOnObserve = true }: UseResizeObserverOptions = {}
+) {
+	const onResizeEvent = useEvent( onResize );
+
+	const observedElementRef = useRef< HTMLElement | null >();
 	const resizeObserverRef = useRef< ResizeObserver >();
+
 	useEffect( () => {
 		if ( targetElement === observedElementRef.current ) {
 			return;
 		}
 
-		observedElementRef.current = targetElement ?? undefined;
-
-		function updateIndicator( element: HTMLElement ) {
-			setIndicatorPosition( {
-				// Workaround to prevent unwanted scrollbars, see:
-				// https://github.com/WordPress/gutenberg/pull/61979
-				left: Math.max( element.offsetLeft - 1, 0 ),
-				top: Math.max( element.offsetTop - 1, 0 ),
-				width: parseFloat( getComputedStyle( element ).width ),
-				height: parseFloat( getComputedStyle( element ).height ),
-			} );
-			updateCallbackRef.current?.();
-		}
+		observedElementRef.current = targetElement;
 
 		// Set up a ResizeObserver.
 		if ( ! resizeObserverRef.current ) {
 			resizeObserverRef.current = new ResizeObserver( () => {
 				if ( observedElementRef.current ) {
-					updateIndicator( observedElementRef.current );
+					onResizeEvent( observedElementRef.current );
 				}
 			} );
 		}
 		const { current: resizeObserver } = resizeObserverRef;
 
 		// Observe new element.
 		if ( targetElement ) {
-			updateIndicator( targetElement );
+			if ( fireOnObserve ) {
+				onResizeEvent( targetElement );
+			}
 			resizeObserver.observe( targetElement );
 		}
 
@@ -85,35 +150,115 @@ function useTrackElementOffset(
 				resizeObserver.unobserve( observedElementRef.current );
 			}
 		};
-	}, [ targetElement ] );
+	}, [ fireOnObserve, onResizeEvent, targetElement ] );
+}
+
+/**
+ * The position and dimensions of an element, relative to its offset parent.
+ */
+type ElementOffsetRect = {
+	/**
+	 * The distance from the left edge of the offset parent to the left edge of
+	 * the element.
+	 */
+	left: number;
+	/**
+	 * The distance from the top edge of the offset parent to the top edge of
+	 * the element.
+	 */
+	top: number;
+	/**
+	 * The width of the element.
+	 */
+	width: number;
+	/**
+	 * The height of the element.
+	 */
+	height: number;
+};
+
+/**
+ * An `ElementOffsetRect` object with all values set to zero.
+ */
+const NULL_ELEMENT_OFFSET_RECT = {
+	left: 0,
+	top: 0,
+	width: 0,
+	height: 0,
+} satisfies ElementOffsetRect;
+
+/**
+ * Returns the position and dimensions of an element, relative to its offset
+ * parent. This is useful in contexts where `getBoundingClientRect` is not
+ * suitable, such as when the element is transformed.
+ *
+ * **Note:** the `left` and `right` values are adjusted due to a limitation
+ * in the way the browser calculates the offset position of the element,
+ * which can cause unwanted scrollbars to appear. This adjustment makes the
+ * values potentially inaccurate within a range of 1 pixel.
+ */
+function getElementOffsetRect( element: HTMLElement ): ElementOffsetRect {
+	return {
+		// The adjustments mentioned in the documentation above are necessary
+		// because `offsetLeft` and `offsetTop` are rounded to the nearest pixel,
+		// which can result in a position mismatch that causes unwanted overflow.
+		// For context, see: https://github.com/WordPress/gutenberg/pull/61979
+		left: Math.max( element.offsetLeft - 1, 0 ),
+		top: Math.max( element.offsetTop - 1, 0 ),
+		// This is a workaround to obtain these values with a sub-pixel precision,
+		// since `offsetWidth` and `offsetHeight` are rounded to the nearest pixel.
+		width: parseFloat( getComputedStyle( element ).width ),
+		height: parseFloat( getComputedStyle( element ).height ),
+	};
+}
+
+/**
+ * Tracks the position and dimensions of an element, relative to its offset
+ * parent. The element can be changed dynamically.
+ */
+function useTrackElementOffsetRect(
+	targetElement: HTMLElement | undefined | null
+) {
+	const [ indicatorPosition, setIndicatorPosition ] =
+		useState< ElementOffsetRect >( NULL_ELEMENT_OFFSET_RECT );
+
+	useResizeObserver( targetElement, ( element ) =>
+		setIndicatorPosition( getElementOffsetRect( element ) )
+	);
 
 	return indicatorPosition;
 }
 
+/**
+ * Context object for the `onUpdate` callback of `useOnValueUpdate`.
+ */
 type ValueUpdateContext< T > = {
 	previousValue: T;
 };
 
+/**
+ * Calls the `onUpdate` callback when the `value` changes.
+ */
 function useOnValueUpdate< T >(
+	/**
+	 * The value to watch for changes.
+	 */
 	value: T,
+	/**
+	 * Callback to fire when the value changes.
+	 */
 	onUpdate: ( context: ValueUpdateContext< T > ) => void
 ) {
 	const previousValueRef = useRef( value );
-
-	// TODO: replace with useEventCallback or similar when officially available.
-	const updateCallbackRef = useRef( onUpdate );
-	useLayoutEffect( () => {
-		updateCallbackRef.current = onUpdate;
-	} );
-
+	const updateCallbackEvent = useEvent( onUpdate );
 	useEffect( () => {
 		if ( previousValueRef.current !== value ) {
-			updateCallbackRef.current( {
+			updateCallbackEvent( {
 				previousValue: previousValueRef.current,
 			} );
 			previousValueRef.current = value;
 		}
-	}, [ value ] );
+	}, [ updateCallbackEvent, value ] );
 }
 
 export const TabList = forwardRef<
@@ -123,7 +268,7 @@ export const TabList = forwardRef<
 	const context = useTabsContext();
 
 	const selectedId = context?.store.useState( 'selectedId' );
-	const indicatorPosition = useTrackElementOffset(
+	const indicatorPosition = useTrackElementOffsetRect(
 		context?.store.item( selectedId )?.element
 	);