ts-custom-component-lifecycle-callbacks.md

# Custom Component Lifecycle Callbacks


The lifecycle callbacks of a custom component are used to notify users of the lifecycle of the component. These callbacks are private and are invoked by the development framework at a specified time at runtime. They cannot be manually invoked from applications.


## Lifecycle Callback Definition

  | Function | Description | 
| -------- | -------- |
| aboutToAppear | Invoked after a new instance of the custom component is created and before its build function is executed. You can change state variables in the aboutToAppear function. The change will take effect when you execute the build function next time. | 
| aboutToDisappear | Invoked before the destructor of the custom component is consumed. Do not change state variables in the aboutToDisappear function as doing this can cause unexpected errors. For example, the modification of the @Link decorated variable may cause unstable application running. | 
| onPageShow | Invoked when a page is displayed. This callback is used in the routing process or scenarios where the application is switched to the foreground or background. Only the custom components decorated by @Entry take effect. | 
| onPageHide | Invoked when a page is hidden. This callback is used in the routing process or scenarios where the application is switched to the foreground or background. Only the custom components decorated by @Entry take effect. | 
| onBackPress | Invoked when a user clicks the back button. Only the custom components decorated by @Entry take effect.<br/>- The value true is returned if the page processes the return logic instead of performing page routing.<br/>- The value false is returned if the default return logic is used.<br/>- If no value is returned, the default return logic is used. | 


## Example

  
```
@Component
struct CountDownTimerComponent {
    @State countDownFrom: number = 10
    private timerId: number = -1

    private aboutToAppear(): void  {
        this.timerId = setInterval(() => {
            if (this.countDownFrom <= 1) {
                clearTimeout(this.timerId)
            }
            this.countDownFrom -= 1
        }, 1000) // decr counter by 1 every second
    }

    private aboutToDisappear(): void {
        if (this.timerId > 0) {
            clearTimeout(this.timerId)
            this.timerId = -1
        }
    }

    build() {
        Text(`${this.countDownFrom} sec left`)
    }
}
```

The example above shows that lifecycle functions are critical for CountDownTimerComponent to manage its timer resources. Similar functions include loading resources asynchronously from the network.


> ![icon-note.gif](public_sys-resources/icon-note.gif) **NOTE**:
> - Promise and asynchronous callback functions can be used in lifecycle functions, for example, network resource getters and timer setters.
> 
> - Do not use async await in lifecycle functions.