# @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 '@ohos.app.ability.EnvironmentCallback';
```


## EnvironmentCallback.onConfigurationUpdated

onConfigurationUpdated(config: Configuration): void

Called when the system environment changes.

**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.

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

**Parameters**

  | Name| Type| Mandatory| Description| 
  | -------- | -------- | -------- | -------- |
  | level | [AbilityConstant.MemoryLevel](js-apis-app-ability-abilityConstant.md#abilityconstantmemorylevel) | 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 from '@ohos.app.ability.UIAbility';
import EnvironmentCallback from '@ohos.app.ability.EnvironmentCallback';

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();
        // 2. Register a listener for the environment changes through the applicationContext object.
        callbackId = applicationContext.on('environment', environmentCallback);
        console.log(`registerEnvironmentCallback number: ${JSON.stringify(callbackId)}`);
    }

    onDestroy() {
        let applicationContext = this.context.getApplicationContext();
        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)}`);
            }
        });
    }
}
```