Source

File: wp-includes/customize/class-wp-customize-selective-refresh.php

public function add_dynamic_partials( $partial_ids ) {
		$new_partials = array();

		foreach ( $partial_ids as $partial_id ) {

			// Skip partials already created.
			$partial = $this->get_partial( $partial_id );
			if ( $partial ) {
				continue;
			}

			$partial_args  = false;
			$partial_class = 'WP_Customize_Partial';

			/**
			 * Filters a dynamic partial's constructor arguments.
			 *
			 * For a dynamic partial to be registered, this filter must be employed
			 * to override the default false value with an array of args to pass to
			 * the WP_Customize_Partial constructor.
			 *
			 * @since 4.5.0
			 *
			 * @param false|array $partial_args The arguments to the WP_Customize_Partial constructor.
			 * @param string      $partial_id   ID for dynamic partial.
			 */
			$partial_args = apply_filters( 'customize_dynamic_partial_args', $partial_args, $partial_id );
			if ( false === $partial_args ) {
				continue;
			}

			/**
			 * Filters the class used to construct partials.
			 *
			 * Allow non-statically created partials to be constructed with custom WP_Customize_Partial subclass.
			 *
			 * @since 4.5.0
			 *
			 * @param string $partial_class WP_Customize_Partial or a subclass.
			 * @param string $partial_id    ID for dynamic partial.
			 * @param array  $partial_args  The arguments to the WP_Customize_Partial constructor.
			 */
			$partial_class = apply_filters( 'customize_dynamic_partial_class', $partial_class, $partial_id, $partial_args );

			$partial = new $partial_class( $this, $partial_id, $partial_args );

			$this->add_partial( $partial );
			$new_partials[] = $partial;
		}
		return $new_partials;
	}