Show custom fields option only if post type supports it

[Soean]

Description

Fixes #16333
The custom field option should only be visible, if the post type supports this feature. The old solution doesn't work anymore, because enableCustomFields has now a default value false (added in #14082).
So this new solution checks the supports array of the post type for the value custom-fields.

How has this been tested?

Register a post type without custom field support, so the option should be hidden. If you add the custom fields support, the option should be visible.

function register_custom_post_types() {
	register_post_type('test', [
		'labels' => [
			'name' => __('test'),
		],
		'public' => true,
		'supports' => [
			'title',
			'editor',
			// 'custom-fields',
		],
		'show_in_rest' => true,
	]);
}
add_action('init', 'register_custom_post_types');

Screenshot

[youknowriad]

What's the purpose of the previous enableCustomFields setting?
I assume it's meant to be set in WordPress Core. is there code we need to remove from there?

[Soean]

supportsCustomFields defines if the post type supports this feature. enableCustomFields is the setting if this meta-box is visible. Before enableCustomFields had the default value false we checked with !== undefined if the post type supports this feature. Now enableCustomFields can't be undefined anymore, so we need to check supportsCustomFields.

We still need enableCustomFields for checking if the meta-box should be visible. This logic is already merged into core.

[youknowriad]

Ok, makes sense. Changes look good to me. I'll wait for @talldan or @noisysocks to confirm.

[noisysocks]

Thanks for looking into this!

Unfortunately we can't rely on checking if the CPT supports custom fields because plugins will disable custom fields by modifying the $wp_meta_boxes global during the do_meta_boxes action.

You can see this problem by following these steps:

1. Check out this branch
2. Install the Advanced Custom Fields plugin
3. Create a new Field Group with a filed in it by going to Custom Fields > Add New
4. Create a new post by going to Posts > Add New
5. Select More tools & options > Options

Expected result: There should be no Custom Fields checkbox because ACF replaces the Core Custom Fields functionality.
Actual result: There's a Custom Fields checkbox which doesn't do anything.

This is why WordPress will check the $wp_meta_boxes global and unset the enableCustomFields editor setting (making it undefined) if custom fields were disabled by a plugin:

https://github.com/WordPress/wordpress-develop/blob/5fea17ba03c42d16a6517b164d3437bd53ac8ac9/src/wp-admin/edit-form-blocks.php#L366-L370

Are we able to stick with the undefined|true|false approach that existed before #14082? I think that the error is in having the default setting for enableCustomFields be false. The default should be undefined.

[kosso]

Still having this issue on Version 5.3.2.

My custom post type supports array( 'title', 'editor', 'thumbnail', 'custom-fields' )

And I can see the Options checkbox to activate 'Custom Fields'.
But after clicking the provided button to view them in the panel, no change occurs after the page reloads.

Am I missing something?

[paaljoachim]

Hello everyone.

What is needed to move this PR forward?
Or perhaps a new PR is needed?

It would be great with an update @Soean
Thank you!

@ntsekouras Nik perhaps this is a PR you are able to move forward?

[ntsekouras]

@ntsekouras Nik perhaps this is a PR you are able to move forward?

Hey Paal. I think some other person with previous work on custom fields could be more effective here. I'll keep this in my queue though.

[skorasaurus]

I believe this can be closed because of the work done at #33931; the custom fields preference does not display, when i create a custom post type ,following the directions in #16333 in a new gutenberg build that includes #33931