Flutter图片下载相册权限关闭导致下载失败

1、引入permission_handler获取系统权限，引入image_gallery_saver存储图片

2、iOS与Android访问相册权限差异性，iOS获取photos权限，Android获取storage权限

思路：通过检测相册权限时是否开启，如果关闭跳转到应用系统权限设置，引导用户开启相册访问权限

downLoadImage(
    {BuildContext context, String source = '案例详情', String url}) async {
  bool data;
  if (Platform.isAndroid) {
    data = await Permission.storage.request().isGranted;
  } else {
    data = await Permission.photos.request().isGranted;
  }
  debugPrint('权限：$data');

  if (!data) {
    Navigator.of(context).maybePop();
    showCustomDialog(
        context: context,
        title: '',
        content: 'dHome需要存储权限才能下载，请点击开启',
        buttons: ['暂不开启', '开启'],
        onTap: (index) {
          if (index == 0) {
            Navigator.of(context).pop();
          } else {
            var hasOpened = openAppSettings();
            debugPrint('App Settings opened: ' + hasOpened.toString());
          }
        });
    return false;
  }
  /// 保存的图片数据
  var response =
      await Dio().get(url, options: Options(responseType: ResponseType.bytes));
  final result = await ImageGallerySaver.saveImage(
    Uint8List.fromList(response.data),
    quality: 100,
  );
  debugPrint('下载$result');
  Navigator.of(context).maybePop();
  showDesignToast('下载成功');
}