Proof of concept for adapting existing AMP Legacy Reader mode post templates as part of a Reader theme in AMP plugin v2.0

readme.md

⚠️ WARNING! The code here is not guaranteed to work in the future. It is intended as a short-term solution to utilize legacy AMP post templates with AMP plugin v2.0.

functions.php

<?php
/**
 * ⚠️ WARNING! The code here is not guaranteed to work in the future. It is intended as a short-term solution to
 * utilize legacy AMP post templates with AMP plugin v2.0.
 */

// Prevent the normal AMP Customizer initialization.
remove_action( 'after_setup_theme', 'amp_init_customizer', 12 );

/**
 * Customize AMP initialization to force legacy settings to be added.
 *
 * @see amp_init_customizer()
 */
function custom_legacy_amp_init_customizer() {
	if (
		! class_exists( 'AMP_Template_Customizer' )
		||
		! method_exists( 'AMP_Template_Customizer', 'register_legacy_settings' )
		||
		! method_exists( 'AMP_Template_Customizer', 'register_legacy_ui' )
		||
		! class_exists( 'AMP_Customizer_Design_Settings' )
		||
		! method_exists( 'AMP_Customizer_Design_Settings', 'init' )
	) {
		return;
	}

	// Fire up the AMP Customizer.
	add_action( 'customize_register', 'custom_legacy_init_amp_template_customizer', 500 );

	// Add some basic design settings + controls to the Customizer.
	add_action( 'amp_init', [ 'AMP_Customizer_Design_Settings', 'init' ] );

	// Add a link to the AMP Customizer in Reader mode.
	add_action( 'admin_menu', 'amp_add_customizer_link' );
}
add_action( 'after_setup_theme', 'custom_legacy_amp_init_customizer', 12 );

/**
 * Wrapped version of `AMP_Template_Customizer::init()` which forces legacy AMP initialization and undoes other initialization.
 *
 * @see AMP_Template_Customizer::init()
 * @param WP_Customize_Manager $wp_customize Customize manager.
 */
function custom_legacy_init_amp_template_customizer( WP_Customize_Manager $wp_customize ) {
	$self = AMP_Template_Customizer::init( $wp_customize );

	/** This action is documented in amp/includes/admin/class-amp-template-customizer.php */
	do_action( 'amp_customizer_init', $self );

	$self->register_legacy_settings();
	$self->register_legacy_ui();

	add_action( 'customize_controls_print_footer_scripts', [ $self, 'print_legacy_controls_templates' ] );
	add_action( 'customize_preview_init', [ $self, 'init_legacy_preview' ] );

	add_action( 'customize_controls_enqueue_scripts', [ $self, 'add_legacy_customizer_scripts' ] );

	// Undo what AMP_Template_Customizer::init() did for Reader themes.
	remove_action( 'customize_controls_enqueue_scripts', [ $self, 'add_customizer_scripts' ] );
	remove_action( 'customize_controls_print_footer_scripts', [ $self, 'render_setting_import_section_template' ] );
}

index.php

<?php
if (
	! function_exists( 'amp_is_available' )
	||
	! function_exists( 'amp_get_slug' )
	||
	! class_exists( 'AMP_Post_Template' )
	||
	! defined( 'AMP__DIR__' )
	||
	! file_exists( AMP__DIR__ . '/includes/templates/reader-template-loader.php' )
) {
	wp_die( 'AMP plugin not active.', 'Temporarily unavailable', [ 'response' => 503 ] );
}

if ( ! amp_is_available() || ! is_singular() ) {
	if ( wp_safe_redirect( remove_query_arg( amp_get_slug() ) ) ) {
		die( 'AMP unavailable. Redirecting to non-AMP version.' );
	}
	return;
}

/*
// Uncomment to override the AMP legacy template directory to be different from using the templates in the AMP plugin.
add_filter(
	'amp_post_template_dir',
	static function () {
		return __DIR__ . '/templates';
	}
);
*/

/*
// Uncomment to override an individual AMP legacy template file to be different from using the templates in the AMP plugin.
add_filter(
	'amp_post_template_file',
	static function () {
		// ...
	}
);
*/

require_once AMP__DIR__ . '/includes/templates/reader-template-loader.php';

style.css

/*
Theme Name: Custom AMP Legacy
Version: 0.1
Description: Legacy AMP templates ported over to be used as a Reader theme.
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
AMP Compatibility: theme-support
*/