block_core_navigation_get_fallback_blocks

block_core_navigation_get_fallback_blocks: 这个函数是用来获取导航块的后备块: 当没有导航块项目需要显示时，它就会被使用。

检索适当的回退，以便在没有为导航块分配菜单的情况下在网站前面使用。

这旨在反映wp_nav_menu的后退机制是如何工作的。见https://developer.wordpress.org/reference/functions/wp_nav_menu/#more-information。

function block_core_navigation_get_fallback_blocks() {
	$page_list_fallback = array(
		array(
			'blockName' => 'core/page-list',
		),
	);

	$registry = WP_Block_Type_Registry::get_instance();

	// If `core/page-list` is not registered then return empty blocks.
	$fallback_blocks = $registry->is_registered( 'core/page-list' ) ? $page_list_fallback : array();

	// Default to a list of Pages.

	$navigation_post = block_core_navigation_get_most_recently_published_navigation();

	// Prefer using the first non-empty Navigation as fallback if available.
	if ( $navigation_post ) {
		$maybe_fallback = block_core_navigation_filter_out_empty_blocks( parse_blocks( $navigation_post->post_content ) );

		// Normalizing blocks may result in an empty array of blocks if they were all `null` blocks.
		// In this case default to the (Page List) fallback.
		$fallback_blocks = ! empty( $maybe_fallback ) ? $maybe_fallback : $fallback_blocks;
	}

	/**
	 * Filters the fallback experience for the Navigation block.
	 *
	 * Returning a falsey value will opt out of the fallback and cause the block not to render.
	 * To customise the blocks provided return an array of blocks - these should be valid
	 * children of the `core/navigation` block.
	 *
	 * @since 5.9.0
	 *
	 * @param array[] default fallback blocks provided by the default block mechanic.
	 */
	return apply_filters( 'block_core_navigation_render_fallback', $fallback_blocks );
}