cpt-page-template-support.php

<?php
// Define custom post type string
define( 'CUSTOM_POST_TYPE', 'affiliates' );


/**
 * Register the meta box
 */
if ( !function_exists( 'page_templates_dropdown_metabox' ) ) {

	function page_templates_dropdown_metabox() {
		add_meta_box( CUSTOM_POST_TYPE . '-page-template', __( 'Template', 'TEXTDOMAIN' ), 'render_page_template_dropdown_metabox', CUSTOM_POST_TYPE, 'side', 'low' );
	}

	add_action( 'add_meta_boxes', 'page_templates_dropdown_metabox' );
}

/**
 * Render your metabox - This code is similar to what is rendered on the page post type
 * @return void
 */
if ( !function_exists( 'render_page_template_dropdown_metabox' ) ) {

	function render_page_template_dropdown_metabox() {
		global $post;
		$template = get_post_meta( $post->ID, '_wp_page_template', true );

		echo "<label class='screen-reader-text' for='page_template'>Page Template</label>
            <select name='_wp_page_template' id='page_template'>
            <option value='default'>Default Template</option>";
		page_template_dropdown( $template );
		echo "</select>";
	}

}

/**
 * Save the page template
 * @return void
 */
if ( !function_exists( 'save_page_template' ) ) {

	function save_page_template( $post_id ) {

		// Skip the auto saves
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return;
		} elseif ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {
			return;
		} elseif ( defined( 'DOING_CRON' ) && DOING_CRON ) {
			return;
		}

		// Only update the page template meta if we are on our specific post type
		elseif ( CUSTOM_POST_TYPE === $_POST[ 'post_type' ] ) {
			update_post_meta( $post_id, '_wp_page_template', esc_attr( $_POST[ '_wp_page_template' ] ) );
		}
	}

	add_action( 'save_post', 'save_page_template' );
}


/**
 * Set the page template
 * @param string $template The determined template from the WordPress brain
 * @return string $template Full path to predefined or custom page template
 */
if ( !function_exists( 'set_page_template' ) ) {

	function set_page_template( $template ) {
		global $post;

		if ( CUSTOM_POST_TYPE === $post->post_type ) {
			$custom_template = get_post_meta( $post->ID, '_wp_page_template', true );

			// since our dropdown only gives the basename, use the locate_template() function to easily find the full path
			if ( $custom_template && 'default' != $custom_template ) {
				return locate_template( $custom_template );
			}
		}

		return $template;
	}

	add_filter( 'single_template', 'set_page_template' );
}