WordPress: Flush rewrite rules on plugin activation

class-*-activator.php

<?php
/**
 * `includes/class-*-activator.php
 */

	/**
	 * Add setting to register the plugins activation.
	 *
	 * Create an option field in the database to check against later.
	 *
	 * @since    1.0.0
	 */
	public static function activate() {

    add_option( 'Activated_Plugin', 'plugin-name' );

  }

class-*-admin.php

<?php
/**
 * `admin/class-*-admin.php
 */
 
  /**
	 *  Flush rewrite rules when the option is present
	 * 
	 * @since    1.0.0
	 */
	public function load_plugin() {

		global $wp_rewrite;

    if ( is_admin() && get_option( 'Activated_Plugin' ) == 'plugin-name' ) {

      delete_option( 'Activated_Plugin' );

		  $wp_rewrite->init();
		  $wp_rewrite->flush_rules();

    }
	}

class-*.php

<?php
/**
 * `includes/class-*.php
 */

private function define_admin_hooks() {

/**  ...  */

  $this->loader->add_action( 'admin_init', $plugin_admin, 'load_plugin' );

  
}