Android: how to create file in the directory opened with ACTION_OPEN_DOCUMENT_TREE

Question

Setup: I get an URI of the directory in shared storage using procedure described here: Access documents and other files from shared storage / Grant access to a directory's contents.

Question: How to create a file inside this directory?

More details:

The guide mentioned above explains, how to list files in this directory, open some file for reading or modification and also how to delete it. All this using ContentResolver. (Edit: actually guide does not explain it also, but just how to delete or modify files obtained interactively with ACTION_OPEN_DOCUMENT). But I cannot see, how to create some file inside this directory.

Two candidates, which I can imagine, could help me with the job are: ContentResolver.insert() or ParcelFileDescriptor(file, MODE_CREATE), but I cannot figure out how I could use them in this situation.

I should also mention, that I know about the possibility to create the file, when user interactively selects the file name and location, but it is not good solution for me. In my context I need to create several files inside one folder. All files should be accessible/modifiable/deletable by user from outside of my application, thus "shared storage". And the files are named and located within their "main" directory according to some conventions, so I don't want the user to explicitly provide me the name for each file, but just the location of "main directory".

Answer 1

Point 1: blackapps is right in the comments to the original question and this question is duplicate of e.g. those:

Create new file in the directory returned by Intent.ACTION_OPEN_DOCUMENT_TREE

or

Write file to directory using Intent.ACTION_OPEN_DOCUMENT_TREE

Point 2 (Quick answer): One should use in this case DocumentFile.fromTreeUri(this, uri) with the URI returned from the Intent.ACTION_OPEN_DOCUMENT_TREE. This worked for me out of the box.

Point 3: One thing was still confusing for me:

If I tried to use something like this (from the original guide, which I used) - to query metadata of the URI:

Cursor cursor = getContentResolver().query(uri, null, null, null, null, null);

Or if I tried to call something like this, hoping to create new document without use of DocumentFile, but instead using directly DocumentsContract:

Uri createdDocumentUri = DocumentsContract.createDocument(
    getContentResolver(),
    uri,
    "text/plain",
    "someDocument.txt");

Then I always got an exception. In the first case it was:

java.lang.UnsupportedOperationException: Unsupported Uri content://com.android.externalstorage.documents...

And in the second case:

java.lang.IllegalArgumentException: Invalid URI: content://com.android.externalstorage.documents...

Different exceptions, but in both cases the message is "Unupported URI".

For a while I could not understand, why is it the case. But when I looked inside DocumentFile.fromTreeUri() and discovered, that instead of using URI returned by Intent.ACTION_OPEN_DOCUMENT_TREE directly, one needs to process it in a way like this (the code is taken from DocumentFile.fromTreeUri()):

    String documentId = DocumentsContract.getTreeDocumentId(uri);
    Uri newUri = DocumentsContract.buildDocumentUriUsingTree(uri, documentId);

The documentation of DocumentsContract.buildDocumentUriUsingTree(), explains little bit, what is going on:

... However, instead of directly accessing the target document, the returned URI will leverage access granted through a subtree URI, typically returned by Intent#ACTION_OPEN_DOCUMENT_TREE. The target document must be a descendant (child, grandchild, etc) of the subtree.

This is typically used to access documents under a user-selected directory tree, since it doesn't require the user to separately confirm each new document access.

I hope, that this answer will help someone to save a bit of confusion, which I had, while I was trying to clarify this topic...