Source

File: wp-includes/class-wp-customize-setting.php

protected function update( $value ) {
		$id_base = $this->id_data['base'];
		if ( 'option' === $this->type || 'theme_mod' === $this->type ) {
			if ( ! $this->is_multidimensional_aggregated ) {
				return $this->set_root_value( $value );
			} else {
				$root = self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['root_value'];
				$root = $this->multidimensional_replace( $root, $this->id_data['keys'], $value );
				self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['root_value'] = $root;
				return $this->set_root_value( $root );
			}
		} else {
			/**
			 * Fires when the WP_Customize_Setting::update() method is called for settings
			 * not handled as theme_mods or options.
			 *
			 * The dynamic portion of the hook name, `$this->type`, refers to the type of setting.
			 *
			 * @since 3.4.0
			 *
			 * @param mixed                $value Value of the setting.
			 * @param WP_Customize_Setting $this  WP_Customize_Setting instance.
			 */
			do_action( "customize_update_{$this->type}", $value, $this );

			return has_action( "customize_update_{$this->type}" );
		}
	}