Block API: Block Context: Support filtering default context

lib/compat.php

@@ -207,7 +207,8 @@ function gutenberg_render_block_with_assigned_block_context( $pre_render, $parse
 
 	/** This filter is documented in src/wp-includes/blocks.php */
 	$parsed_block = apply_filters( 'render_block_data', $parsed_block, $source_block );
-	$context      = array(
+
+	$context = array(
 		'postId'   => $post->ID,
 
 		/*
@@ -218,7 +219,16 @@ function gutenberg_render_block_with_assigned_block_context( $pre_render, $parse
 		 */
 		'postType' => $post->post_type,
 	);
-	$block        = new WP_Block( $parsed_block, $context );
+
+	/**
+	 * Filters the default context provided to a rendered block.
+	 *
+	 * @param array $context      Default context.
+	 * @param array $parsed_block Block being rendered, filtered by `render_block_data`.
+	 */
+	$context = apply_filters( 'render_block_context', $context, $parsed_block );
+
+	$block = new WP_Block( $parsed_block, $context );
 
 	return $block->render();
 }

phpunit/class-block-context-test.php

@@ -163,4 +163,38 @@ function test_provides_default_context() {
 		);
 	}
 
+	/**
+	 * Tests that default block context can be filtered.
+	 */
+	function test_default_context_is_filterable() {
+		$provided_context = array();
+
+		$this->register_block_type(
+			'gutenberg/test-context-consumer',
+			array(
+				'context'         => array( 'example' ),
+				'render_callback' => function( $attributes, $content, $block ) use ( &$provided_context ) {
+					$provided_context[] = $block->context;
+
+					return '';
+				},
+			)
+		);
+
+		$filter_block_context = function( $context ) {
+			$context['example'] = 'ok';
+			return $context;
+		};
+
+		$parsed_blocks = parse_blocks( '<!-- wp:gutenberg/test-context-consumer /-->' );
+
+		add_filter( 'render_block_context', $filter_block_context );
+
+		render_block( $parsed_blocks[0] );
+
+		remove_filter( 'render_block_context', $filter_block_context );
+
+		$this->assertEquals( array( 'example' => 'ok' ), $provided_context[0] );
+	}
+
 }