Bagaimana cara menambahkan template ke tema menggunakan plugin?

Tidak ada cara langsung untuk ini, sepengetahuan saya. Saya telah mencoba mencari juga, dan tidak menemukan solusi yang lebih baik daripada solusi di bawah ini.

Kode di bawah ini akan memberi Anda apa yang Anda cari:

class PageTemplater {

    /**
     * A Unique Identifier
     */
     protected $plugin_slug;

    /**
     * A reference to an instance of this class.
     */
    private static $instance;

    /**
     * The array of templates that this plugin tracks.
     */
    protected $templates;


    /**
     * Returns an instance of this class. 
     */
    public static function get_instance() {

            if( null == self::$instance ) {
                    self::$instance = new PageTemplater();
            } 

            return self::$instance;

    } 

    /**
     * Initializes the plugin by setting filters and administration functions.
     */
    private function __construct() {

            $this->templates = array();


            // Add a filter to the attributes metabox to inject template into the cache.
            add_filter(
                'page_attributes_dropdown_pages_args',
                 array( $this, 'register_project_templates' ) 
            );


            // Add a filter to the save post to inject out template into the page cache
            add_filter(
                'wp_insert_post_data', 
                array( $this, 'register_project_templates' ) 
            );


            // Add a filter to the template include to determine if the page has our 
            // template assigned and return it's path
            add_filter(
                'template_include', 
                array( $this, 'view_project_template') 
            );


            // Add your templates to this array.
            $this->templates = array(
                    'goodtobebad-template.php'     => 'It\'s Good to Be Bad',
            );

    } 


    /**
     * Adds our template to the pages cache in order to trick WordPress
     * into thinking the template file exists where it doens't really exist.
     *
     */

    public function register_project_templates( $atts ) {

            // Create the key used for the themes cache
            $cache_key = 'page_templates-' . md5( get_theme_root() . "https://wordpress.stackexchange.com/" . get_stylesheet() );

            // Retrieve the cache list. 
            // If it doesn't exist, or it's empty prepare an array
            $templates = wp_get_theme()->get_page_templates();
            if ( empty( $templates ) ) {
                    $templates = array();
            } 

            // New cache, therefore remove the old one
            wp_cache_delete( $cache_key , 'themes');

            // Now add our template to the list of templates by merging our templates
            // with the existing templates array from the cache.
            $templates = array_merge( $templates, $this->templates );

            // Add the modified cache to allow WordPress to pick it up for listing
            // available templates
            wp_cache_add( $cache_key, $templates, 'themes', 1800 );

            return $atts;

    } 

    /**
     * Checks if the template is assigned to the page
     */
    public function view_project_template( $template ) {

            global $post;

            if (!isset($this->templates[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 
            );

            // Just to be safe, we check if the file exist first
            if( file_exists( $file ) ) {
                    return $file;
            } 
            else { echo $file; }

            return $template;

    } 


}
add_action( 'plugins_loaded', array( 'PageTemplater', 'get_instance' ) );

Berikut adalah tutorial lengkap dengan detail tentang apa yang terjadi dan alasannya.