如何订阅系统环境变量的变化

import common from '@ohos.app.ability.common';
import EnvironmentCallback from '@ohos.app.ability.EnvironmentCallback';
import { BusinessError } from '@ohos.base';
import hilog from '@ohos.hilog';
import { Configuration } from '@ohos.app.ability.Configuration';
const TAG: string = '[CollaborateAbility]';
const DOMAIN_NUMBER: number = 0xFF00;
@Entry
@Component
struct Index {
  private context = getContext(this) as common.UIAbilityContext;

subscribeConfigurationUpdate(): void {

let applicationContext = this.context.getApplicationContext();

let environmentCallback: EnvironmentCallback = {
      onConfigurationUpdated(newConfig: Configuration) {
        hilog.info(DOMAIN_NUMBER, TAG, `onConfigurationUpdated systemLanguage is ${systemLanguage}, newConfig: ${JSON.stringify(newConfig)}`);
        if (this.systemLanguage !== newConfig.language) {
          hilog.info(DOMAIN_NUMBER, TAG, `systemLanguage from ${systemLanguage} changed to ${newConfig.language}`);

}
      },
      onMemoryLevel(level) {
        hilog.info(DOMAIN_NUMBER, TAG, `onMemoryLevel level: ${level}`);
      }
    }
    try {
      this.callbackId = applicationContext.on('environment', environmentCallback);
    } catch (err) {
      let code = (err as BusinessError).code;
      let message = (err as BusinessError).message;
      hilog.error(DOMAIN_NUMBER, TAG, `Failed to register applicationContext. Code is ${code}, message is ${message}`);
    };
  }

build() {
    //...
  }
}