improt {Platform，BackHandler} from react native

componentDidMount() {

if (Platform.OS === ‘android‘){
  BackHandler.addEventListener(‘hardwareBackPress‘, this.onBackAndroid);
}
}

componentWillUnmount() {

if (Platform.OS === ‘android‘) {
  BackHandler.removeEventListener(‘hardwareBackPress‘, this.onBackAndroid);
}
}

onBackAndroid = () => {

//禁用返回键
if(this.props.navigation.isFocused()) {//判断   该页面是否处于聚焦状态
    if (this.lastBackPressed && this.lastBackPressed + 2000 >= Date.now()) {
      //最近2秒内按过back键，可以退出应用。
      // return false;
      BackHandler.exitApp();//直接退出APP
    }else{
      this.lastBackPressed = Date.now();
      ToastAndroid.show(‘再按一次退出应用‘, 1000);//提示
      return true;
    }
}
}