Add a WordPress Page Template from a Plugin from a functional programming point of view

add-page-template.php

<?php
/**
 * Inspiration: https://www.wpexplorer.com/wordpress-page-templates-plugin/
 */

add_filter( 'theme_page_templates', 'add_new_template' );
add_filter( 'wp_insert_post_data', 'register_templates' );
add_filter( 'template_include', 'view_template' );

function register_templates( $atts ) {
	$cache_key = 'page-templates-' . md5( get_theme_root() . '/' . get_stylesheet() );

	$templates = wp_get_theme()->get_page_templates();
	if ( empty( $templates ) ) {
		$templates = array();
	}

	wp_cache_delete( $cache_key, 'themes' );
	$templates = array_merge(
		$templates,
		array( plugin_dir_url( __DIR__ ) . '/plugin-name/templates/template-name.php' => 'Template Name' )
	);

	wp_cache_add( $cache_key, $templates, 'themes', 1800 );

	return $atts;
}

function view_template( $template ) {
	$page_template = array( plugin_dir_url( __DIR__ ) . '/plugin-name/templates/template-name.php' => 'Template Name' );

	global $post;

	if ( ! $post ) {
		return $template;
	}

	if ( ! isset(
		$page_template[ get_post_meta(
			$post->ID,
			'_wp_page_template',
			true
		) ]
	) ) {
		return $template;
	}

	$file = plugin_dir_path( __FILE__ ) . get_post_meta(
		$post->ID,
		'_wp_page_template',
		true
	);

	if ( file_exists( $file ) ) {
		return $file;
	} else {
		echo $file;
	}

	return $template;

}

function add_new_template( $posts_templates ) {
	$posts_templates = array_merge(
		$posts_templates,
		array( plugin_dir_url( __DIR__ ) . '/plugin-name/templates/template-name.php' => 'Template Name' )
	);
	return $posts_templates;
}