# @ohos.app.ability.EnvironmentCallback (EnvironmentCallback)

The **EnvironmentCallback** module provides APIs for the application context to listen for system environment changes.

> **NOTE**
> 
> The initial APIs of this module are supported since API version 9. Newly added APIs will be marked with a superscript to indicate their earliest API version.
>
> The APIs of this module can be used only in the stage model.


## Modules to Import

```ts
import { EnvironmentCallback } from '@kit.AbilityKit';
```


## EnvironmentCallback.onConfigurationUpdated

onConfigurationUpdated(config: Configuration): void

Called when the system environment changes.

**Atomic service API**: This API can be used in atomic services since API version 11.

**System capability**: SystemCapability.Ability.AbilityRuntime.AbilityCore

**Parameters**

  | Name| Type| Mandatory| Description| 
  | -------- | -------- | -------- | -------- |
  | config | [Configuration](js-apis-app-ability-configuration.md) | Yes| **Configuration** object after the change.|

**Example**

See [Usage of EnvironmentCallback](#usage-of-environmentcallback).

## EnvironmentCallback.onMemoryLevel

onMemoryLevel(level: AbilityConstant.MemoryLevel): void

Called when the system memory level changes.

**Atomic service API**: This API can be used in atomic services since API version 11.

**System capability**: SystemCapability.Ability.AbilityRuntime.AbilityCore

**Parameters**

  | Name| Type| Mandatory| Description| 
  | -------- | -------- | -------- | -------- |
  | level | [AbilityConstant.MemoryLevel](js-apis-app-ability-abilityConstant.md#memorylevel) | Yes| Memory level that indicates the memory usage status. When the specified memory level is reached, a callback will be invoked and the system will start adjustment.|

**Example**

See [Usage of EnvironmentCallback](#usage-of-environmentcallback).

## Usage of EnvironmentCallback

**Example**

```ts
import { UIAbility, EnvironmentCallback } from '@kit.AbilityKit';
import { BusinessError } from '@kit.BasicServicesKit';

let callbackId: number;

export default class MyAbility extends UIAbility {
  onCreate() {
    console.log('MyAbility onCreate');
    let environmentCallback: EnvironmentCallback  =  {
      onConfigurationUpdated(config){
        console.log(`onConfigurationUpdated config: ${JSON.stringify(config)}`);
      },

      onMemoryLevel(level){
        console.log(`onMemoryLevel level: ${JSON.stringify(level)}`);
      }
    };
    // 1. Obtain an applicationContext object.
    let applicationContext = this.context.getApplicationContext();
    try {
      // 2. Register a listener for the environment changes through the applicationContext object.
      callbackId = applicationContext.on('environment', environmentCallback);
    } catch (paramError) {
      console.error(`error: ${(paramError as BusinessError).code}, ${(paramError as BusinessError).message}`);
    }
    console.log(`registerEnvironmentCallback number: ${JSON.stringify(callbackId)}`);
  }

  onDestroy() {
    let applicationContext = this.context.getApplicationContext();
    try {
      applicationContext.off('environment', callbackId, (error, data) => {
        if (error && error.code !== 0) {
          console.error(`unregisterEnvironmentCallback fail, error: ${JSON.stringify(error)}`);
        } else {
          console.log(`unregisterEnvironmentCallback success, data: ${JSON.stringify(data)}`);
        }
      });
    } catch (paramError) {
      console.error(`error: ${(paramError as BusinessError).code}, ${(paramError as BusinessError).message}`);
    }
  }
}
```