Is there any way of writing files to SDCard with flutter File.writeAsStringSync

Question

I am trying to write to a file that is located in the SDCard, I found out that I need special permission for removable storage something that is not found in any known permission handler plugin for flutter (i tried simple_permission and permission_handler with no use).

I tried to acquire those permissions using the android side of things, so I wrote a simple function that would show the dialog and the user would allow the app to modify the content of the SDCard.

even after acquiring the rights to the SDCARD, I still get the same permissions denied error when trying to save files to the SDCard when using File.writeAsStringSync method.

I want to know if there is any known way/hack/workaround to save files in SDCards in flutter.

The android code i used is the same from this answer : https://stackoverflow.com/a/55024683/6641693

NOTE : I am targetting android 7 and beyond but not android 11.

Answer 1

I solved This, by ditching the dart file saving and using the android SAF. First, what I did was try to get the sdCard modification permissions. After that, I get to save the files I need.

here is the code I used to get the permissions ( aka the "allow this app to modify content on your sdCard" dialog )

public void takeCardUriPermission(String sdCardRootPath) {
    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
      File sdCard = new File(sdCardRootPath);
      StorageManager storageManager = (StorageManager) getSystemService(Context.STORAGE_SERVICE);
      StorageVolume storageVolume = storageManager.getStorageVolume(sdCard);
      Intent intent = storageVolume.createAccessIntent(null);
      try {
        startActivityForResult(intent, 4010);
      } catch (ActivityNotFoundException e) {
        Log.e("TUNE-IN ANDROID", "takeCardUriPermission: "+e);
      }
    }
  }

  protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == 4010) {

      Uri uri = data.getData();

      grantUriPermission(getPackageName(), uri, Intent.FLAG_GRANT_WRITE_URI_PERMISSION |
              Intent.FLAG_GRANT_READ_URI_PERMISSION);

      final int takeFlags = data.getFlags() & (Intent.FLAG_GRANT_WRITE_URI_PERMISSION |
              Intent.FLAG_GRANT_READ_URI_PERMISSION);

      getContentResolver().takePersistableUriPermission(uri, takeFlags);
      methodChannel.invokeMethod("resolveWithSDCardUri",getUri().toString());
    }
  }

  public Uri getUri() {
    List<UriPermission> persistedUriPermissions = getContentResolver().getPersistedUriPermissions();
    if (persistedUriPermissions.size() > 0) {
      UriPermission uriPermission = persistedUriPermissions.get(0);
      return uriPermission.getUri();
    }
    return null;
  }

So in order to start the whole permissions acquiring process, you have to first call takeCardUriPermission and passing the URI of the sdCard path.

Note: on my FlutterActivity, i am able to get the sdCardPath directly using getExternalCacheDirs()[1].toString()

After calling takeCardUriPermission and once the allow button is pressed (or the decline) an activity result event will be called and the onActivtyResult method will be called. the requestCode check is useful when you have multiple events and you need to filter this one out.

The activity result code will give the app permissions to modify the files on the sdCard.

The getUri method is the one that we will be using afterwards when trying to save bytes to a file, it returns the URI of the SDCard that we selected (you can have multiple sdCards).

Saving Files

What I used to save a file is a straightforward method. First we need to get the URI of the sdCard and create a Documentfile out of it, then we go through the hierarchy of that directory (DocumentFile can reference files and directories) to find the needed file based on it's URI.

We do this search by splitting the file URI into parts and then navigating the hierarchy by testing if each part exists or not. Once we test all the parts we would have reached our file, if it exists, or we were stuck at the last directory we got to.

the resulting of this iteration is a DocumentFile that we can execute operations on and with.

the following is the full file saving code :

String filepath = (String) arguments.get("filepath");
          final byte[] bytes = methodCall.argument("bytes");

          try{
            if(filepath==null || bytes==null)throw new Exception("Arguments Not found");
            DocumentFile documentFile = DocumentFile.fromTreeUri(getApplicationContext(), getUri());
            String[] parts = filepath.split("/");
            for (int i = 0; i < parts.length; i++) {
              DocumentFile nextfile = documentFile.findFile(parts[i]); 
              if(nextfile!=null){
                documentFile=nextfile;
              }
            }
            if(documentFile!=null && documentFile.isFile()){
              OutputStream out = getContentResolver().openOutputStream(documentFile.getUri());
              out.write(bytes);
              out.close();
            }else{
              throw new Exception("File Not Found");
            }
          }catch (Exception e){
            result.error("400",e.getMessage(),e);
            return;
          }
          result.success(true);

Note: in my code, I am calling this under the MethodChannel's MethodCallHandler which will give me the argument I need: filePath which is the String URI of the file I want to write to and the bytes byte array representing the data I want to save. The same can be said for the result.success

The file writing code is simple: open the file, write the data and close the file.