render_block_context filter and block context inheritance

Question

I am attempting to add some context to the Query Loop block and inherit that on the inner blocks like post title (so nested inside the Post Template block). I can filter it in on render_block_context and the context is available on the Query Loop block, but I was hoping to figure out how to automatically inherit that in the descendant blocks.

Here's my minimal example:

add_filter( 'render_block_context', function( $context, $parsed_block ) {
    if ( 'core/query' === $parsed_block['blockName'] ) {
        $context['helga/tacos'] = 'we love tacos';
    }
    return $context;
}, 10, 2 );
 
add_filter( 'block_type_metadata', function( $metadata ) {
    if ( 'core/query' === $metadata['name'] ) {
        $metadata['attributes'] ??= [];
        $metadata['attributes']['tacos'] = [
            'type' => 'string',
            'default' => 'default tacos',
        ];
 
        $metadata['providesContext'] ??= [];
        $metadata['providesContext']['helga/tacos'] = 'tacos';
    }
    if ( 'core/post-title' === $metadata['name'] ) {
        $metadata['usesContext'] ??= [];
        $metadata['usesContext'][] = 'helga/tacos';
    }
    return $metadata;
} );
 
add_filter( 'render_block_core/post-title', function( $block_content, $block, $instance) {
    return $block_content . json_encode( $instance->context );
}, 10, 3 );

Then insert a Query Loop block anywhere and look at the resulting output. I have the context json encoded after each post title... and it never includes the helga/tacos context key that I defined both in render_block_context but also set up providesContext and usesContext for in the loop and title blocks respectively.

I am trying to get the helga/tacos key available in the context in the post title block AND for it's value to be we love tacos.

Wongjn · Accepted Answer · 2025-01-10 17:45:48Z

Presumably, within your Query Loop block, you also have a Post Template block. When rendering the page server side, there is "barrier" in the context inheritance caused by the render_block_core_post_template():

$block_content = ( new WP_Block( $block_instance ) )->render( array( 'dynamic' => false ) );

It renders the content without any of its context (it is the second parameter to the WP_Block constructor).

Hence, we need to "save" the context value and pass them through this barrier. We can do this by hooking into render_block_context for child blocks. We do this by adding this hook just before the Post Template block is rendered and then remove the hook after rendering it:

// render_block_data is the last hook just before a block is rendered.
add_filter(
  'render_block_data',
  function( $block ) {
    if ( $block['blockName'] === 'core/post-template' ) {
      // This will add the context for any child block within this
      // `core/post-template` block we are about to render.
      $add_context = function ( $context ) {
        $context['helga/tacos'] = 'we love tacos';
        return $context;
      };
      add_filter( 'render_block_context', $add_context );

      // Unhook the context filter above, after rendering this
      // `core/post-template` block.
      $unhook = function ( $content ) use ( &$unhook, $add_context ) {
        // Unhook.
        remove_filter( 'render_block_context', $add_context );
        // Unhook this function too since it is no longer needed.
        remove_filter( 'render_block_core/post-template', $unhook );

        return $content;
      };
      add_filter( 'render_block_core/post-template', $unhook );
    }

    return $block;
  }
);

I have been stuck on this for a while and I also just finally noticed that WP_Block is not getting that 2nd param inside the post template render callback. I created an issue for that. I think you are on to something as a workaround, but I added it after my above snippets and there's no change, so something isn't quite right somewhere. — helgatheviking
– helgatheviking, Commented Jan 10 at 18:04
Seems to be working in this WordPress Playground which would suggest there could be some code (or otherwise) in your WordPress instance that is interfering. — Wongjn
– Wongjn, Commented Jan 10 at 18:32
You're right, my apologies, it was probably some copy/paste shenanigangs on my part. I hope the issue is fixed upstream, but this works for right now. Thank you! — helgatheviking
– helgatheviking, Commented Jan 10 at 19:04

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