Block Bindings: Support Image block's caption attribute, remove <figcaption> if empty

[ockham]

Implement "conditional" Block Bindings, using the example of the Image block's caption attribute. The following cases need to be supported when that attribute is bound to a Block Bindings source:

• If the block was saved with a caption, and the Block Bindings source value is not empty, the caption is replaced with the latter.
• If the block was saved with a caption, and the Block Bindings source value is empty, the <figcaption> element is removed.
• If the block was saved without a caption, and the Block Bindings source value is not empty, a <figcaption> element is added (with the caption set to the Block Bindings source value).
  • We can implement this by changing save.js to include a <figcaption>. This is implemented by Image block: Save <figcaption> if caption attr is bound gutenberg#71483. If the caption attribute is empty after binding, the empty <figcaption> element will be removed by the logic that handles the case described by the second item in this list.

The logic implemented in this PR isn't specific to the Image block; it can be extended to any block that conditionally includes an HTML element that corresponds to a rich-text block attribute (e.g. the Pullquote block's citation attribute, which corresponds to a <cite> element). See here for some discussion related to this solution.

See the screenshots in the "Testing Instructions" section for an illustrative example.

Testing Instructions

The first two cases from the above list are covered by unit tests.

To test manually, follow these instructions:

Create a new post, and paste the following markup:

<!-- wp:group {"layout":{"type":"flex","flexWrap":"nowrap"}} -->
<div class="wp-block-group"><!-- wp:image {"width":"126px","height":"auto","sizeSlug":"large","metadata":{"bindings":{"__default":{"source":"core/pattern-overrides"}},"name":"Bulbasur Override"}} -->
<figure class="wp-block-image size-large is-resized"><img src="https://img.pokemondb.net/artwork/large/bulbasaur.jpg" alt="" style="width:126px;height:auto"/><figcaption class="wp-element-caption">Replace</figcaption></figure>
<!-- /wp:image -->

<!-- wp:image {"width":"107px","height":"auto","sizeSlug":"large","metadata":{"bindings":{"__default":{"source":"core/pattern-overrides"}},"name":"Charmander Override"}} -->
<figure class="wp-block-image size-large is-resized"><img src="https://img.pokemondb.net/artwork/large/charmander.jpg" alt="" style="width:107px;height:auto"/><figcaption class="wp-element-caption"></figcaption></figure>
<!-- /wp:image -->

<!-- wp:image {"width":"118px","height":"auto","sizeSlug":"large","metadata":{"bindings":{"__default":{"source":"core/pattern-overrides"}},"name":"Squirtle Override"}} -->
<figure class="wp-block-image size-large is-resized"><img src="https://img.pokemondb.net/artwork/large/squirtle.jpg" alt="" style="width:118px;height:auto"/><figcaption class="wp-element-caption">Remove</figcaption></figure>
<!-- /wp:image --></div>
<!-- /wp:group -->

The resulting content should look like this:

In the Visual Editor, select all three image blocks. Then, from the block toolbar, select "Create Pattern". Assign a name of your choice to the pattern (e.g. "Pokemons").

Below the three image blocks that now form a pattern, insert another instance of the "Pokemons" pattern. Then, edit the newly inserted instance as follows:

1. Change the caption of the first image to "Bulbasaur".
2. Add a caption to the second image ("Charmander").
3. Remove the caption of third image.

Publish the post, and verify that the result looks like this:

(Testing instructions largely copied from WordPress/gutenberg#70642.)

Supersedes ockham#5.

Trac ticket: https://core.trac.wordpress.org/ticket/63919

---

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[mukeshpanchal27]

Missing function doc.

[mukeshpanchal27]

Do we need these condition?

[ockham]

Yeah, this is needed. It's a loop that advances the internal cursor until a matching tag closer is found for the opener that we started from.

[gziolo]

Based on my testing experience described in WordPress/gutenberg#71483 (comment), we might need some coverage when replacing HTML attributes, too. In that particular case, src in the <img /> didn't get replaced for unknown reasons with the Pattern Overrides mechanism.

[gziolo]

I see it's covered in test_update_block_with_value_from_source_image_placeholder test:

wordpress-develop/tests/phpunit/tests/block-bindings/render.php

Lines 414 to 423 in e2560f1

	$this->assertSame(
	'https://example.com/image.jpg',
	$block->attributes['url'],
	"The 'url' attribute should be updated with the value returned by the source."
	);
	$this->assertSame(
	'<figure class="wp-block-image"><img src="https://example.com/image.jpg" alt=""/></figure>',
	trim( $result ),
	'The block content should be updated with the value returned by the source.'
	);

That might be an issue in the Gutenberg implementation. However, adding more diverse cases here could still be beneficial.

[ockham]

Based on my testing experience described in WordPress/gutenberg#71483 (comment), we might need some coverage when replacing HTML attributes, too. In that particular case, src in the <img /> didn't get replaced for unknown reasons with the Pattern Overrides mechanism.

Test coverage for this scenario was added on the GB side as part of the fix to the bug you mentioned (src not being replaced).

The bug was specific to GB's compat layer (and can't manifest in Core in the same way), but we can carry over the test coverage to Core just for parity's sake.

[ockham]

Done over at the new PR: 339fbe8

[ockham]

Closing this due to reasons laid out in this comment.

I'll follow-up with a trimmed-down PR that only adds support for the Image block's caption attribute (but doesn't remove <figcaption> if it's empty).