Source

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

public function get_available_widgets() {
		static $available_widgets = array();
		if ( ! empty( $available_widgets ) ) {
			return $available_widgets;
		}

		global $wp_registered_widgets, $wp_registered_widget_controls;
		require_once ABSPATH . 'wp-admin/includes/widgets.php'; // For next_widget_id_number().

		$sort = $wp_registered_widgets;
		usort( $sort, array( $this, '_sort_name_callback' ) );
		$done = array();

		foreach ( $sort as $widget ) {
			if ( in_array( $widget['callback'], $done, true ) ) { // We already showed this multi-widget.
				continue;
			}

			$sidebar = is_active_widget( $widget['callback'], $widget['id'], false, false );
			$done[]  = $widget['callback'];

			if ( ! isset( $widget['params'][0] ) ) {
				$widget['params'][0] = array();
			}

			$available_widget = $widget;
			unset( $available_widget['callback'] ); // Not serializable to JSON.

			$args = array(
				'widget_id'   => $widget['id'],
				'widget_name' => $widget['name'],
				'_display'    => 'template',
			);

			$is_disabled     = false;
			$is_multi_widget = ( isset( $wp_registered_widget_controls[ $widget['id'] ]['id_base'] ) && isset( $widget['params'][0]['number'] ) );
			if ( $is_multi_widget ) {
				$id_base            = $wp_registered_widget_controls[ $widget['id'] ]['id_base'];
				$args['_temp_id']   = "$id_base-__i__";
				$args['_multi_num'] = next_widget_id_number( $id_base );
				$args['_add']       = 'multi';
			} else {
				$args['_add'] = 'single';

				if ( $sidebar && 'wp_inactive_widgets' !== $sidebar ) {
					$is_disabled = true;
				}
				$id_base = $widget['id'];
			}

			$list_widget_controls_args = wp_list_widget_controls_dynamic_sidebar(
				array(
					0 => $args,
					1 => $widget['params'][0],
				)
			);
			$control_tpl               = $this->get_widget_control( $list_widget_controls_args );

			// The properties here are mapped to the Backbone Widget model.
			$available_widget = array_merge(
				$available_widget,
				array(
					'temp_id'      => isset( $args['_temp_id'] ) ? $args['_temp_id'] : null,
					'is_multi'     => $is_multi_widget,
					'control_tpl'  => $control_tpl,
					'multi_number' => ( 'multi' === $args['_add'] ) ? $args['_multi_num'] : false,
					'is_disabled'  => $is_disabled,
					'id_base'      => $id_base,
					'transport'    => $this->is_widget_selective_refreshable( $id_base ) ? 'postMessage' : 'refresh',
					'width'        => $wp_registered_widget_controls[ $widget['id'] ]['width'],
					'height'       => $wp_registered_widget_controls[ $widget['id'] ]['height'],
					'is_wide'      => $this->is_wide_widget( $widget['id'] ),
				)
			);

			$available_widgets[] = $available_widget;
		}

		return $available_widgets;
	}