InnerBlocks: introduce prop to specify render callback for each block.

packages/block-editor/src/components/block-list/block.js

@@ -93,6 +93,7 @@ function BlockListBlock( {
 	toggleSelection,
 	index,
 	enableAnimation,
+	__experimentalRenderCallback: renderCallback,
 } ) {
 	// In addition to withSelect, we should favor using useSelect in this
 	// component going forward to avoid leaking new props to the public API
@@ -241,6 +242,10 @@ function BlockListBlock( {
 		block = <Block.div { ...wrapperProps }>{ blockEdit }</Block.div>;
 	}
 
+	if ( renderCallback ) {
+		block = renderCallback( block );
+	}
+
 	return (
 		<BlockListBlockContext.Provider value={ memoizedValue }>
 			<BlockCrashBoundary onError={ onBlockError }>

packages/block-editor/src/components/block-list/index.js

@@ -28,6 +28,7 @@ function BlockList(
 		className,
 		rootClientId,
 		renderAppender,
+		__experimentalItemCallback,
 		__experimentalTagName = 'div',
 		__experimentalAppenderTagName,
 		__experimentalPassedProps = {},
@@ -110,6 +111,9 @@ function BlockList(
 									isDropTarget &&
 									orientation === 'horizontal',
 							} ) }
+							__experimentalRenderCallback={
+								__experimentalItemCallback
+							}
 						/>
 					</AsyncModeProvider>
 				);

packages/block-editor/src/components/block-list/index.native.js

@@ -260,6 +260,7 @@ export class BlockList extends Component {
 			parentWidth,
 			marginVertical = styles.defaultBlock.marginTop,
 			marginHorizontal = styles.defaultBlock.marginLeft,
+			__experimentalItemCallback,
 		} = this.props;
 		return (
 			<BlockListItem
@@ -279,6 +280,7 @@ export class BlockList extends Component {
 				onCaretVerticalPositionChange={
 					this.onCaretVerticalPositionChange
 				}
+				__experimentalRenderCallback={ __experimentalItemCallback }
 			/>
 		);
 	}

packages/block-editor/src/components/inner-blocks/index.js

@@ -40,6 +40,7 @@ function UncontrolledInnerBlocks( props ) {
 	const {
 		clientId,
 		allowedBlocks,
+		__experimentalItemCallback: itemCallback,
 		template,
 		templateLock,
 		forwardedRef,
@@ -93,6 +94,7 @@ function UncontrolledInnerBlocks( props ) {
 			{ ...props }
 			ref={ forwardedRef }
 			rootClientId={ clientId }
+			__experimentalItemCallback={ itemCallback }
 			className={ classes }
 		/>
 	);
@@ -157,7 +159,11 @@ ForwardedInnerBlocks.DefaultBlockAppender = DefaultBlockAppender;
 ForwardedInnerBlocks.ButtonBlockAppender = ButtonBlockAppender;
 
 ForwardedInnerBlocks.Content = withBlockContentContext(
-	( { BlockContent } ) => <BlockContent />
+	( { BlockContent, __experimentalItemCallback } ) => (
+		<BlockContent
+			__experimentalItemCallback={ __experimentalItemCallback }
+		/>
+	)
 );
 
 /**

packages/block-editor/src/components/inner-blocks/index.native.js

@@ -49,6 +49,7 @@ function UncontrolledInnerBlocks( props ) {
 		marginHorizontal,
 		horizontalAlignment,
 		filterInnerBlocks,
+		__experimentalItemCallback,
 	} = props;
 
 	const block = useSelect(
@@ -82,6 +83,7 @@ function UncontrolledInnerBlocks( props ) {
 			onAddBlock={ onAddBlock }
 			onDeleteBlock={ onDeleteBlock }
 			filterInnerBlocks={ filterInnerBlocks }
+			__experimentalItemCallback={ __experimentalItemCallback }
 		/>
 	);
 

packages/block-editor/src/components/use-block-drop-zone/index.js

@@ -1,3 +1,8 @@
+/**
+ * External dependencies
+ */
+import { difference } from 'lodash';
+
 /**
  * WordPress dependencies
  */
@@ -50,11 +55,6 @@ export function getNearestBlockIndex( elements, position, orientation ) {
 	let candidateDistance;
 
 	elements.forEach( ( element, index ) => {
-		// Ensure the element is a block. It should have the `wp-block` class.
-		if ( ! element.classList.contains( 'wp-block' ) ) {
-			return;
-		}
-
 		const rect = element.getBoundingClientRect();
 		const cursorLateralPosition = isHorizontal ? y : x;
 		const cursorForwardPosition = isHorizontal ? x : y;
@@ -326,7 +326,16 @@ export default function useBlockDropZone( {
 
 	useEffect( () => {
 		if ( position ) {
-			const blockElements = Array.from( element.current.children );
+			// Get the root elements of blocks inside the element, ignoring
+			// InnerBlocks item wrappers and the children of the blocks.
+			const blockElements = difference(
+				Array.from( element.current.querySelectorAll( '.wp-block' ) ),
+				Array.from(
+					element.current.querySelectorAll(
+						':scope .wp-block .wp-block'
+					)
+				)
+			);
 
 			const targetIndex = getNearestBlockIndex(
 				blockElements,

packages/block-editor/src/components/use-block-drop-zone/test/index.js

@@ -83,22 +83,6 @@ describe( 'getNearestBlockIndex', () => {
 		expect( result ).toBeUndefined();
 	} );
 
-	it( 'returns `undefined` if the elements do not have the `wp-block` class', () => {
-		const nonBlockElements = [
-			{ classList: createMockClassList( 'some-other-class' ) },
-		];
-		const position = { x: 0, y: 0 };
-		const orientation = 'horizontal';
-
-		const result = getNearestBlockIndex(
-			nonBlockElements,
-			position,
-			orientation
-		);
-
-		expect( result ).toBeUndefined();
-	} );
-
 	describe( 'Vertical block lists', () => {
 		const orientation = 'vertical';
 

packages/blocks/src/api/serializer.js

@@ -6,7 +6,12 @@ import { isEmpty, reduce, isObject, castArray, startsWith } from 'lodash';
 /**
  * WordPress dependencies
  */
-import { Component, cloneElement, renderToString } from '@wordpress/element';
+import {
+	Component,
+	RawHTML,
+	cloneElement,
+	renderToString,
+} from '@wordpress/element';
 import { hasFilter, applyFilters } from '@wordpress/hooks';
 import isShallowEqual from '@wordpress/is-shallow-equal';
 
@@ -21,10 +26,16 @@ import {
 import { normalizeBlockType } from './utils';
 import BlockContentProvider from '../block-content-provider';
 
+/** @typedef {import('@wordpress/element').WPElement} WPElement */
+
 /**
  * @typedef {Object} WPBlockSerializationOptions Serialization Options.
  *
- * @property {boolean} isInnerBlocks Whether we are serializing inner blocks.
+ * @property {boolean}   isInnerBlocks
+ * Whether we are serializing inner blocks.
+ * @property {WPElement} [__experimentalRenderCallback]
+ * Callback to define HTML surrounding block, outside of the comment
+ * delimiters. Used by InnerBlocks API.
  */
 
 /**
@@ -307,20 +318,41 @@ export function getCommentDelimitedContent(
  *
  * @return {string} Serialized block.
  */
-export function serializeBlock( block, { isInnerBlocks = false } = {} ) {
+export function serializeBlock(
+	block,
+	{ isInnerBlocks = false, __experimentalRenderCallback: renderCallback } = {}
+) {
 	const blockName = block.name;
 	const saveContent = getBlockContent( block );
 
+	// Serialized block content before wrapping it with an InnerBlocks item
+	// wrapper.
+	let unwrappedContent;
+
 	if (
 		blockName === getUnregisteredTypeHandlerName() ||
 		( ! isInnerBlocks && blockName === getFreeformContentHandlerName() )
 	) {
-		return saveContent;
+		unwrappedContent = saveContent;
+	} else {
+		const blockType = getBlockType( blockName );
+		const saveAttributes = getCommentAttributes(
+			blockType,
+			block.attributes
+		);
+		unwrappedContent = getCommentDelimitedContent(
+			blockName,
+			saveAttributes,
+			saveContent
+		);
 	}
 
-	const blockType = getBlockType( blockName );
-	const saveAttributes = getCommentAttributes( blockType, block.attributes );
-	return getCommentDelimitedContent( blockName, saveAttributes, saveContent );
+	if ( renderCallback ) {
+		return renderToString(
+			renderCallback( <RawHTML>{ unwrappedContent }</RawHTML> )
+		);
+	}
+	return unwrappedContent;
 }
 
 /**

packages/blocks/src/block-content-provider/index.js

@@ -33,9 +33,12 @@ const { Consumer, Provider } = createContext( () => {} );
  * @return {WPComponent} Element with BlockContent injected via context.
  */
 const BlockContentProvider = ( { children, innerBlocks } ) => {
-	const BlockContent = () => {
+	const BlockContent = ( { __experimentalItemCallback } ) => {
 		// Value is an array of blocks, so defer to block serializer
-		const html = serialize( innerBlocks, { isInnerBlocks: true } );
+		const html = serialize( innerBlocks, {
+			isInnerBlocks: true,
+			__experimentalRenderCallback: __experimentalItemCallback,
+		} );
 
 		// Use special-cased raw HTML tag to avoid default escaping
 		return <RawHTML>{ html }</RawHTML>;