Start to port over alternative WordPress/gutenberg#67719

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -147,6 +147,19 @@ public function register_routes() {
 				'schema'      => array( $this, 'get_public_item_schema' ),
 			)
 		);
+
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base . '/count',
+			array(
+				array(
+					'methods'             => WP_REST_Server::READABLE,
+					'callback'            => array( $this, 'get_count' ),
+					'permission_callback' => array( $this, 'get_count_permissions_check' ),
+				),
+				'schema' => array( $this, 'get_count_schema' ),
+			)
+		);
 	}
 
 	/**
@@ -3296,4 +3309,86 @@ private function prepare_taxonomy_limit_schema( array $query_params ) {
 
 		return $query_params;
 	}
+
+	/**
+	 * Retrieves post counts for the post type.
+	 *
+	 * @since 6.8.0
+	 *
+	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
+	 */
+	public function get_count() {
+		$counts = wp_count_posts( $this->post_type );
+		$data   = array();
+
+		if ( ! empty( $counts ) ) {
+			/*
+			 * The fields comprise all non-internal post statuses,
+			 * including any custom statuses that may be registered.
+			 * 'trash' is an exception, so if it exists, it is added separately.
+			 */
+			$post_stati = get_post_stati( array( 'internal' => false ) );
+
+			if ( get_post_status_object( 'trash' ) ) {
+				$post_stati[] = 'trash';
+			}
+			// Include all public statuses in the response if there is a count.
+			foreach ( $post_stati as $status ) {
+				if ( isset( $counts->$status ) ) {
+					$data[ $status ] = (int) $counts->$status;
+				}
+			}
+		}
+		return rest_ensure_response( $data );
+	}
+
+	/**
+	 * Checks if a given request has access to read post counts.
+	 *
+	 * @since 6.8.0
+	 *
+	 * @return true|WP_Error True if the request has read access, WP_Error object otherwise.
+	 */
+	public function get_count_permissions_check() {
+		$post_type = get_post_type_object( $this->post_type );
+
+		if ( ! current_user_can( $post_type->cap->read ) ) {
+			return new WP_Error(
+				'rest_cannot_read',
+				__( 'Sorry, you are not allowed to read post counts for this post type.' ),
+				array( 'status' => rest_authorization_required_code() )
+			);
+		}
+
+		return true;
+	}
+
+	/**
+	 * Retrieves the post counts schema, conforming to JSON Schema.
+	 *
+	 * @since 6.8.0
+	 *
+	 * @return array Item schema data.
+	 */
+	public function get_count_schema() {
+		return array(
+			'$schema'              => 'http://json-schema.org/draft-04/schema#',
+			'title'                => 'post-counts',
+			'type'                 => 'object',
+			/*
+			 * Use a pattern matcher for post status keys.
+			 * This allows for custom post statuses to be included,
+			 * which can be registered after the schema is generated.
+			 */
+			'patternProperties'    => array(
+				'^\w+$' => array(
+					'description' => __( 'The number of posts for a given status.' ),
+					'type'        => 'integer',
+					'context'     => array( 'view', 'edit', 'embed' ),
+					'readonly'    => true,
+				),
+			),
+			'additionalProperties' => false,
+		);
+	}
 }

tests/phpunit/tests/rest-api/rest-posts-controller.php

@@ -161,6 +161,7 @@ public function test_register_routes() {
 		$this->assertCount( 2, $routes['/wp/v2/posts'] );
 		$this->assertArrayHasKey( '/wp/v2/posts/(?P<id>[\d]+)', $routes );
 		$this->assertCount( 3, $routes['/wp/v2/posts/(?P<id>[\d]+)'] );
+		$this->assertArrayHasKey( '/wp/v2/pages/count', $routes );
 	}
 
 	public function test_context_param() {