Make overriding and back porting classes from gutenberg repo easier

[spacedmonkey]

The gutenberg repo overrides a number of core classes, including but not limited to

• WP_Theme_JSON -> ​WP_Theme_JSON_Gutenberg
• WP_Theme_JSON_Data -> ​WP_Theme_JSON_Data_Gutenberg
• WP_Theme_JSON_Resolver -> ​WP_Theme_JSON_Resolver
• WP_Theme_JSON_Schema -> ​WP_Theme_JSON_Schema_Gutenberg
• WP_Duotone -> ​WP_Duotone_Gutenberg

Currently this has a number of issues, including

• Overrides are not perfect, so it is unperdictable what version of this class will be used, core or gutenberg.
• It makes backporting changes to these classes a manaul process.
• It makes it hard to write unit test the code.

Recommendation

Add filters similar to parse_blocks function.

function parse_blocks( $content ) {
        /**
         * Filter to allow plugins to replace the server-side block parser.
         *
         * @since 5.0.0
         *
         * @param string $parser_class Name of block parser class.
         */
        $parser_class = apply_filters( 'block_parser_class', 'WP_Block_Parser' );

        $parser = new $parser_class();
        return $parser->parse( $content );
}

Possible issues:

A number of these classes, have static methods and consts, that make referencing class name by string a little harder.

[joemcgill]

This same concern was previously raised in ​https://github.com/WordPress/gutenberg/issues/62594.

In that discussion, another option that was discussed was to make these classes in the WordPress repo read-only and sync them the way we do PHP code in block packages.

While that would make maintainability a bit easier, it still doesn't allow the same code paths to be tested by the Gutenberg plugin that will be run once the code is synced, since many times the static methods are referenced directly in code, as you rightly point out.

I agree that being able to filter the entire class being used would help here and pointed out _wp_image_editor_choose and rest_get_server a couple other examples of functions where Core provides filters for overriding entire classes.