Add "connections" block attribute and get the meta field

docs/reference-guides/core-blocks.md

@@ -480,7 +480,7 @@ Start with the basic building block of all narrative. ([Source](https://github.c
 -	**Name:** core/paragraph
 -	**Category:** text
 -	**Supports:** __unstablePasteTextInline, anchor, color (background, gradients, link, text), spacing (margin, padding), typography (fontSize, lineHeight), ~~className~~
--	**Attributes:** align, content, direction, dropCap, placeholder
+-	**Attributes:** align, connections, content, direction, dropCap, placeholder
 
 ## Pattern placeholder
 

lib/experimental/blocks.php

@@ -121,3 +121,98 @@ function gutenberg_register_metadata_attribute( $args ) {
 	return $args;
 }
 add_filter( 'register_block_type_args', 'gutenberg_register_metadata_attribute' );
+
+
+
+
+// TODO: Wrap this with a check for the experimental Custom Sources API.
+
+/**
+ * Renders the block meta attributes.
+ *
+ * @param string   $block_content Block Content.
+ * @param array    $block Block attributes.
+ * @param WP_Block $block_instance The block instance.
+ */
+function gutenberg_render_custom_sources( $block_content, $block, $block_instance ) {
+	$meta_custom_source = require __DIR__ . '/custom-sources/meta.php';
+	$block_type         = $block_instance->block_type;
+
+	// Not sure if we need it, it was in Riad's PR.
+	if ( null === $block_type ) {
+		return $block_content;
+	}
+
+	// Whitelist of the block types that support custom sources
+	// Currently, we only allow the Paragraph and Image blocks to use custom sources.
+	if ( ! in_array( $block_type->name, array( 'core/paragraph', 'core/image' ), true ) ) {
+		return $block_content;
+	}
+
+	// Get all the attributes that have a connection.
+	$connected_attributes = _wp_array_get( $block_type->attributes, array( 'connections', 'attributes' ), false );
+	if ( ! $connected_attributes ) {
+		return $block_content;
+	}
+
+	foreach ( $connected_attributes as $attribute_name => $attribute_value ) {
+		// If the source value is not meta, skip it because we only support meta
+		// sources for now.
+		if ( 'meta' !== $attribute_value['source'] ) {
+			continue;
+		}
+
+		// If the attribute does not have a source, skip it.
+		if ( ! isset( $block_type->attributes[ $attribute_name ]['source'] ) ) {
+			continue;
+		}
+
+		// If the attribute does not specify the name of the custom field, skip it.
+		if ( ! isset( $attribute_value['value'] ) ) {
+			continue;
+		}
+
+		$block_content = $meta_custom_source['apply_source'](
+			$block_content,
+			$block_instance,
+			$attribute_value['value'],
+			$block_type->attributes[ $attribute_name ]
+		);
+	}
+
+	return $block_content;
+}
+
+add_filter( 'render_block', 'gutenberg_render_custom_sources', 10, 3 );
+
+
+
+
+// ----- This is just for testing, remove later -----
+
+/**
+ * Registers a custom meta
+ */
+function init_test_summary_meta_field() {
+	register_meta(
+		'post',
+		'test_custom_field',
+		array(
+			'show_in_rest' => true,
+			'single'       => true,
+			'type'         => 'string',
+			'default'      => 'hello this is a custom field test',
+		)
+	);
+	register_meta(
+		'post',
+		'second_test_custom_field',
+		array(
+			'show_in_rest' => true,
+			'single'       => true,
+			'type'         => 'string',
+			'default'      => 'second custom field test',
+		)
+	);
+}
+add_action( 'init', 'init_test_summary_meta_field' );

lib/experimental/custom-sources/meta.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * Meta Custom Source
+ *
+ * @package gutenberg
+ */
+
+return array(
+	'name'         => 'meta',
+	'apply_source' => function ( $block_content, $block_instance, $meta_field, $attribute_config ) {
+		$meta_value = get_post_meta( $block_instance->context['postId'], $meta_field, true );
+		$p          = new WP_HTML_Tag_Processor( $block_content );
+		$found      = $p->next_tag(
+			array(
+				// TODO: build the query from CSS selector.
+				'tag_name' => $attribute_config['selector'],
+			)
+		);
+		if ( ! $found ) {
+			return $block_content;
+		}
+		$tag_name = $p->get_tag();
+		$markup   = "<$tag_name>$meta_value</$tag_name>";
+		$p2       = new WP_HTML_Tag_Processor( $markup );
+		$p2->next_tag();
+		$names = $p->get_attribute_names_with_prefix( '' );
+		foreach ( $names as $name ) {
+			$p2->set_attribute( $name, $p->get_attribute( $name ) );
+		}
+
+		return $p2 . '';
+	},
+);

packages/block-library/src/paragraph/block.json

@@ -7,6 +7,7 @@
 	"description": "Start with the basic building block of all narrative.",
 	"keywords": [ "text" ],
 	"textdomain": "default",
+	"usesContext": [ "postId" ],
 	"attributes": {
 		"align": {
 			"type": "string"
@@ -28,6 +29,14 @@
 		"direction": {
 			"type": "string",
 			"enum": [ "ltr", "rtl" ]
+		},
+		"connections": {
+			"type": "object",
+			"attributes": {
+				"content": {
+					"source": "meta"
+				}
+			}
 		}
 	},
 	"supports": {