[Android:Fix] Fix an issue where saveFile would return an incorrect path

[Dogbonee]

Currently saveFile always returns a path to downloads with the filename appended on the end. This doesn't seem like intended behavior, but the path seemed to be hardcoded to the downloads directory environment variable. This PR just changes the path to the activity's uri real path. Also fixed a minor issue in the example where the bytes argument was required to save, but was not included in the example.

Related to issue #1636

[navaronbracke]

@Dogbonee Do you have some before/after samples to verify your fix?

[Dogbonee]

I used the unmodified base example to test my fix (with the added bytes argument in saveFile). The example will output the path of the chosen directory when "save file" is picked. This directory would previously only show as the Downloads directory, but after this change it will show the user's chosen directory. This seems to be a reasonable validation of the fix, as it now shows meaningful output.

As a side note, the merge that you committed changed the bytes argument in the example from Uint8List(0) to _paths?.first.bytes, which caused a Invalid Argument exception when I tested it with the example.

[navaronbracke]

As a side note, the merge that you committed changed the bytes argument in the example from Uint8List(0) to _paths?.first.bytes, which caused a Invalid Argument exception when I tested it with the example.

Hmm, that error probably has to do with a recent change for saveFile. I'll take a look at it.

[navaronbracke]

@vicajilau With the recent Kotlin migration, can you look into porting this fix into the new Kotlin sources?

[vicajilau]

@navaronbracke this has been resolved as you comment. I attach evidence:

[navaronbracke]

@vicajilau The returned path is probably not valid for the dart:io File API, and only works with Android's ContentResolver API? (due to the : in the path)

[vicajilau]

@navaronbracke Android does not expose the actual file path behind a content://, for security and storage encapsulation reasons (especially from Android 10 onwards with Scoped Storage). So our Android Uri won't work with a simple File of dart:io unless we move it to a temporary folder, which I'm not particularly happy about... I'm still analyzing alternatives

[vicajilau]

@navaronbracke After analysis, the conclusion is that there are 3 options to solve this.

1. Return the path to the temporary file instead of the written file even though it will not be the actual path to the file.
2. Return the filename without the full path.
3. Return the status of the operation without returning a path.

The disadvantage of working with or returning the temporary path is that we do not control or decide how long Android will decide that the file exists. For that reason, I think the API should aim to ensure that the saveFile does not return the path.