WP_Customize_Setting::preview() WordPress Method

	public function preview() {
		if ( ! isset( $this->_previewed_blog_id ) ) {
			$this->_previewed_blog_id = get_current_blog_id();
		}

		// Prevent re-previewing an already-previewed setting.
		if ( $this->is_previewed ) {
			return true;
		}

		$id_base                 = $this->id_data['base'];
		$is_multidimensional     = ! empty( $this->id_data['keys'] );
		$multidimensional_filter = array( $this, '_multidimensional_preview_filter' );

		/*
		 * Check if the setting has a pre-existing value (an isset check),
		 * and if doesn't have any incoming post value. If both checks are true,
		 * then the preview short-circuits because there is nothing that needs
		 * to be previewed.
		 */
		$undefined     = new stdClass();
		$needs_preview = ( $undefined !== $this->post_value( $undefined ) );
		$value         = null;

		// Since no post value was defined, check if we have an initial value set.
		if ( ! $needs_preview ) {
			if ( $this->is_multidimensional_aggregated ) {
				$root  = self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['root_value'];
				$value = $this->multidimensional_get( $root, $this->id_data['keys'], $undefined );
			} else {
				$default       = $this->default;
				$this->default = $undefined; // Temporarily set default to undefined so we can detect if existing value is set.
				$value         = $this->value();
				$this->default = $default;
			}
			$needs_preview = ( $undefined === $value ); // Because the default needs to be supplied.
		}

		// If the setting does not need previewing now, defer to when it has a value to preview.
		if ( ! $needs_preview ) {
			if ( ! has_action( "customize_post_value_set_{$this->id}", array( $this, 'preview' ) ) ) {
				add_action( "customize_post_value_set_{$this->id}", array( $this, 'preview' ) );
			}
			return false;
		}

		switch ( $this->type ) {
			case 'theme_mod':
				if ( ! $is_multidimensional ) {
					add_filter( "theme_mod_{$id_base}", array( $this, '_preview_filter' ) );
				} else {
					if ( empty( self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['previewed_instances'] ) ) {
						// Only add this filter once for this ID base.
						add_filter( "theme_mod_{$id_base}", $multidimensional_filter );
					}
					self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['previewed_instances'][ $this->id ] = $this;
				}
				break;
			case 'option':
				if ( ! $is_multidimensional ) {
					add_filter( "pre_option_{$id_base}", array( $this, '_preview_filter' ) );
				} else {
					if ( empty( self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['previewed_instances'] ) ) {
						// Only add these filters once for this ID base.
						add_filter( "option_{$id_base}", $multidimensional_filter );
						add_filter( "default_option_{$id_base}", $multidimensional_filter );
					}
					self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['previewed_instances'][ $this->id ] = $this;
				}
				break;
			default:
				/**
				 * Fires when the WP_Customize_Setting::preview() method is called for settings
				 * not handled as theme_mods or options.
				 *
				 * The dynamic portion of the hook name, `$this->id`, refers to the setting ID.
				 *
				 * @since 3.4.0
				 *
				 * @param WP_Customize_Setting $setting WP_Customize_Setting instance.
				 */
				do_action( "customize_preview_{$this->id}", $this );

				/**
				 * Fires when the WP_Customize_Setting::preview() method is called for settings
				 * not handled as theme_mods or options.
				 *
				 * The dynamic portion of the hook name, `$this->type`, refers to the setting type.
				 *
				 * @since 4.1.0
				 *
				 * @param WP_Customize_Setting $setting WP_Customize_Setting instance.
				 */
				do_action( "customize_preview_{$this->type}", $this );
		}

		$this->is_previewed = true;

		return true;
	}