Xamarin Forms图片下载至设备问题：Android12及iOS适配异常

Android 12+ 下载失败修复 #

• 适配Scoped Storage机制：Android 10及以上启用了Scoped Storage，12+对文件访问限制更严格，不能再用旧方式直接写入公共目录，必须通过MediaStore操作媒体文件。
• 权限适配：Android 13+需申请READ_MEDIA_IMAGES权限，12及以下保留WRITE_EXTERNAL_STORAGE权限；android:requestLegacyExternalStorage="true"仅兼容到Android 11，12+必须弃用。
• 修改AndroidDownloader实现：

  public async Task DownloadImageAsync(string url, string fileName)
  {
      using (var httpClient = new HttpClient())
      {
          var imageBytes = await httpClient.GetByteArrayAsync(url);
          var contentResolver = Android.App.Application.Context.ContentResolver;
          var imageUri = MediaStore.Images.Media.ExternalContentUri;
  
          var values = new ContentValues();
          values.Put(MediaStore.Images.Media.Columns.DisplayName, fileName);
          values.Put(MediaStore.Images.Media.Columns.MimeType, "image/jpeg");
          values.Put(MediaStore.Images.Media.Columns.RelativePath, "Pictures/YourAppFolder");
  
          var targetUri = contentResolver.Insert(imageUri, values);
          using (var stream = contentResolver.OpenOutputStream(targetUri))
          {
              await stream.WriteAsync(imageBytes, 0, imageBytes.Length);
          }
  
          // 通知系统扫描文件，让图库立即识别
          MediaScannerConnection.ScanFile(Android.App.Application.Context, new[] { targetUri.Path }, null, null);
      }
  }
  

• 动态权限申请：下载前需根据系统版本动态申请对应权限，确保权限通过后再执行下载逻辑。

iOS 下载失败修复 #

• 路径与权限调整：iOS需将图片保存到系统相册才能在图库显示，需确保Info.plist已配置NSPhotoLibraryAddUsageDescription权限描述。
• 修改IosDownloader实现：

  public async Task DownloadImageAsync(string url, string fileName)
  {
      using (var httpClient = new HttpClient())
      {
          var imageBytes = await httpClient.GetByteArrayAsync(url);
          var tempPath = Path.Combine(Path.GetTempPath(), fileName);
          File.WriteAllBytes(tempPath, imageBytes);
  
          // 请求相册权限并保存图片
          var authStatus = await PHPhotoLibrary.RequestAuthorizationAsync();
          if (authStatus == PHAuthorizationStatus.Authorized)
          {
              PHPhotoLibrary.SharedPhotoLibrary.PerformChanges(() =>
              {
                  var creationRequest = PHAssetCreationRequest.CreateAsset();
                  creationRequest.AddResource(PHAssetResourceType.Photo, NSData.FromFile(tempPath), null);
              }, (success, error) =>
              {
                  if (!success)
                  {
                      // 处理保存失败逻辑
                  }
                  // 删除临时文件
                  File.Delete(tempPath);
              });
          }
      }
  }
  

确保图片在图库/文件管理器可见 #

• Android：通过MediaScannerConnection.ScanFile通知系统扫描新增文件，强制图库刷新显示。
• iOS：直接保存到系统相册，图库默认会显示；若需在文件管理器访问，可开启UIFileSharingEnabled（Info.plist中设为YES），将图片保存到Documents目录，但此方式图库不会识别，建议优先保存到相册。
• Xamarin Forms端权限校验优化：

  private async void DownloadBtn_Clicked(object sender, EventArgs e)
  {
      bool hasPermission = false;
      if (DeviceInfo.Platform == DevicePlatform.Android)
      {
          if (Android.OS.Build.VERSION.SdkInt >= Android.OS.BuildVersionCodes.Tiramisu)
          {
              hasPermission = await Permissions.CheckStatusAsync<Permissions.MediaWrite>() == PermissionStatus.Granted;
              if (!hasPermission) hasPermission = await Permissions.RequestAsync<Permissions.MediaWrite>() == PermissionStatus.Granted;
          }
          else
          {
              hasPermission = await Permissions.CheckStatusAsync<Permissions.StorageWrite>() == PermissionStatus.Granted;
              if (!hasPermission) hasPermission = await Permissions.RequestAsync<Permissions.StorageWrite>() == PermissionStatus.Granted;
          }
      }
      else if (DeviceInfo.Platform == DevicePlatform.iOS)
      {
          var status = await Permissions.CheckStatusAsync<Permissions.PhotosAddOnly>();
          hasPermission = status == PermissionStatus.Granted;
          if (!hasPermission) hasPermission = await Permissions.RequestAsync<Permissions.PhotosAddOnly>() == PermissionStatus.Granted;
      }
  
      if (hasPermission)
      {
          var downloader = DependencyService.Get<IDownloader>();
          await downloader.DownloadImageAsync("https://your-image-url.jpg", "saved-image.jpg");
      }
      else
      {
          await DisplayAlert("提示", "需要存储/相册权限才能完成下载", "确定");
      }
  }
  

内容的提问来源于stack exchange，提问作者Sreejith Sree