Admin PWA proof of concept for https://github.com/GoogleChromeLabs/pwa-wp/issues/295

admin-pwa.php

<?php
/**
 * Plugin Name:      Admin PWA
 * Plugin URI:       https://github.com/GoogleChromeLabs/pwa-wp/issues/295
 * Description:      Proof of concept to turn the WordPress admin into a PWA.
 * Version:          0.1.0
 * Author:           Weston Ruter
 * Author URI:       https://weston.ruter.net/
 * Text Domain:      pwa
 * License:          GPL-2.0+
 * License URI:      http://www.gnu.org/licenses/gpl-2.0.txt
 * Requires Plugins: pwa
 * Requires PHP:     5.6
 */

namespace AdminPWA;

/**
 * Gets manifest overrides which are merged on top of the non-admin manifest.
 *
 * @return array<string, mixed> Manifest overrides.
 */
function get_admin_web_app_manifest_overrides(): array {
	$manifest = array();

	$manifest['start_url'] = admin_url( '/' );
	$manifest['scope']     = admin_url( '/' );
	$manifest['display']   = 'standalone';

	// Add shortcuts for adding new items of each public post type.
	foreach ( get_post_types( array( 'public' => true ), 'objects' ) as $post_type ) {
		if ( 'attachment' === $post_type->name || ! current_user_can( $post_type->cap->create_posts ) ) {
			continue;
		}
		$manifest['shortcuts'][] = array(
			'name' => $post_type->labels->new_item,
			'url'  => add_query_arg( 'post_type', $post_type->name, admin_url( 'post-new.php' ) ),
			// TODO: Add 'icons'.
		);
	}

	return $manifest;
}

add_action(
	'admin_head',
	static function () {
		global $wp_web_app_manifest;

		if ( empty( $wp_web_app_manifest ) || ! method_exists( $wp_web_app_manifest, 'get_manifest' ) ) {
			// Bail if PWA plugin is not active or the class name changed in core merge.
			return;
		}

		$manifest = array_merge( $wp_web_app_manifest->get_manifest(), get_admin_web_app_manifest_overrides() );
		// Props ellatrix for the idea to use a data: URL for the web app manifest.
		?>
		<link rel="manifest" href="<?php echo esc_attr( 'data:application/manifest+json;base64,' . base64_encode( wp_json_encode( $manifest ) ) ); ?>">
		<?php
	}
);

To avoid the use of the data: URL (which can apparently not always be ideal), there is this alternative. Replace the admin_head action callback above with:

use WP_Web_App_Manifest;
const SCOPE_QUERY_VAR = 'scope';
const ADMIN_SCOPE = 'admin';

add_action(
	'admin_head',
	static function () {
		if ( ! method_exists( 'WP_Web_App_Manifest', 'get_url' ) ) {
			// Bail if PWA plugin is not active or the class name changed in core merge.
			return;
		}

		$url = add_query_arg(
			array(
				// The scope query arg here is added to customize the output of the web_app_manifest filter below.
				SCOPE_QUERY_VAR => ADMIN_SCOPE,
				// The nonce allows for the web-app-manifest to have access to the user's authentication state.
				// This must be paired with the crossorigin=use-credentials attribute on the link[rel=manifest] tag.
				'_wpnonce' => wp_create_nonce( 'wp_rest' ),
			),
			WP_Web_App_Manifest::get_url()
		);

		?>
		<link rel="manifest" href="<?php echo esc_url( $url ); ?>" crossorigin="use-credentials">
		<?php
	}
);


add_filter(
	'web_app_manifest',
	static function ( array $manifest ): array {
		if ( isset( $_GET[ SCOPE_QUERY_VAR ] ) && ADMIN_SCOPE === $_GET[ SCOPE_QUERY_VAR ] ) {
			$manifest = array_merge( $manifest, get_admin_web_app_manifest_overrides() );
		}
		return $manifest;
	}
);