# $$ Syntax: Two-Way Synchronization of Built-in Components


The $$ operator provides a TS variable by-reference to a built-in component so that the variable value and the internal state of that component are kept in sync.


What the internal state is depends on the component. For example, for the [bindPopup](../reference/arkui-ts/ts-universal-attributes-popup.md) attribute method, it is the **show** parameter.


## Rules of Use

- $$ supports variables of simple types and variables decorated by **\@State**, **\@Link**, or **\@Prop**.

- $$ supports the components listed below.

  | Component                                                        | Supported Parameter/Attribute| Initial API Version|
  | ------------------------------------------------------------ | --------------- | ----------- |
  | [bindPopup](../reference/arkui-ts/ts-universal-attributes-popup.md) | show            | 8           |
  | [Checkbox](../reference/arkui-ts/ts-basic-components-checkbox.md) | select          | 10          |
  | [CheckboxGroup](../reference/arkui-ts/ts-basic-components-checkboxgroup.md) | selectAll       | 10          |
  | [DatePicker](../reference/arkui-ts/ts-basic-components-datepicker.md) | selected        | 10          |
  | [TimePicker](../reference/arkui-ts/ts-basic-components-timepicker.md) | selected        | 10          |
  | [MenuItem](../reference/arkui-ts/ts-basic-components-menuitem.md) | selected        | 10          |
  | [Panel](../reference/arkui-ts/ts-container-panel.md)         | mode            | 10          |
  | [Radio](../reference/arkui-ts/ts-basic-components-radio.md)  | checked         | 10          |
  | [Rating](../reference/arkui-ts/ts-basic-components-rating.md) | rating          | 10          |
  | [Refresh](../reference/arkui-ts/ts-container-refresh.md)     | refreshing      | 8           |
  | [Search](../reference/arkui-ts/ts-basic-components-search.md) | value           | 10          |
  | [SideBarContainer](../reference/arkui-ts/ts-container-sidebarcontainer.md) | showSideBar     | 10          |
  | [Slider](../reference/arkui-ts/ts-basic-components-slider.md) | value           | 10          |
  | [Stepper](../reference/arkui-ts/ts-basic-components-stepper.md) | index           | 10          |
  | [Swiper](../reference/arkui-ts/ts-container-swiper.md)       | isOn            | 10          |
  | [Tabs](../reference/arkui-ts/ts-container-tabs.md)           | index           | 10          |
  | [TextArea](../reference/arkui-ts/ts-basic-components-textarea.md) | text            | 10          |
  | [TextInput](../reference/arkui-ts/ts-basic-components-textinput.md) | text            | 10          |
  | [TextPicker](../reference/arkui-ts/ts-basic-components-textpicker.md) | selected, value| 10          |
  | [Toggle](../reference/arkui-ts/ts-basic-components-toggle.md) | isOn            | 10          |
  | [AlphabetIndexer](../reference/arkui-ts/ts-container-alphabet-indexer.md) | selected        | 10          |
  | [Select](../reference/arkui-ts/ts-basic-components-select.md) | selected, value| 10          |
  | [BindSheet](../reference/arkui-ts/ts-universal-attributes-sheet-transition.md) | isShow | 10          |
  | [BindContentCover](../reference/arkui-ts/ts-universal-attributes-modal-transition.md) | isShow | 10          |

- When the variable bound to $$ changes, the UI is re-rendered synchronously.


## Example

This example uses the **show** parameter of the [bindPopup](../reference/arkui-ts/ts-universal-attributes-popup.md) attribute method.


```ts
// xxx.ets
@Entry
@Component
struct bindPopupPage {
  @State customPopup: boolean = false;

  build() {
    Column() {
      Button('Popup')
        .margin(20)
        .onClick(() => {
          this.customPopup = !this.customPopup
        })
        .bindPopup($$this.customPopup, {
          message: 'showPopup'
        })
    }
  }
}
```


![popup](figures/popup.gif)