How to Publish Files Via a content Uri

As noted on Monday, it appears that [the file scheme is being banned, by and large]((https://commonsware.com/blog/2016/03/14/psa-file-scheme-ban-n-developer-preview.html).

Yesterday, I covered how to consume content from a Uri, particularly a Uri using the content scheme, which is our primary way of sharing content between apps going forward.

Today, let’s look at the other side: how to publish “files” via a content Uri.

Option #1: `FileProvider`

Google will steer you in the direction of FileProvider, from the support-v4 library. This provides a canned implementation of a ContentProvider that can be used to serve files from internal storage and external storage. You configure it with an XML resource, indicating what directories (or individual files) can be served, add the <provider> element to your manifest, and you are (in theory) set.

Note that there is a long-standing documentation bug that, at this rate, will never get fixed.

Option #2: `FileProvider` with `LegacyCompatCursorWrapper`

The other problem with FileProvider is that it works properly… which conflicts with various apps that assume that content Uri values come from MediaStore and try to query() for things that FileProvider does not support.

To help with this, I put together a LegacyCompatCursorWrapper that supplies the “missing” values, to try to improve compatibility with broken clients.

To use it with FileProvider, you create a tiny subclass of FileProvider that wraps the query() result in a LegacyCompatCursorWrapper, then use your subclass in your <provider> element:

public class LegacyCompatFileProvider extends FileProvider {
  @Override
  public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
    return(new LegacyCompatCursorWrapper(super.query(uri, projection, selection, selectionArgs, sortOrder)));
  }
}

(from this sample project, from this book that you really ought to read, if I do say so myself :-)

Option #3: `StreamProvider`

FileProvider, though, only works with files in a few locations. Among other things, it does not support things like assets or raw resources.

I wrote a StreamProvider that handles those cases.

Option #4: Your Own Custom `ContentProvider`

If your needs are more esoteric — for example, you need to serve streams coming from a BLOB column in a SQLite database — you can roll your own streaming ContentProvider. You create a custom subclass of ContentProvider that handles:

getType() to return the MIME type for the content pointed to by one of your Uri values
one of the open...() series of methods, such as openFile() or openPipeHelper()
query(), to support the OpenableColumns (returning the size and “display name” of the content)

Compatibility becomes a problem if the stream is not backed by a local file on the filesystem, though. Any open...() method that winds up using a pipe-based ParcelFileDescriptor seems to result in a stream that is not seekable, and various clients will expect to be able to seek forwards and backwards in the stream (e.g., media players). I am not aware of a workaround for this for content Uri values — if somebody knows of one, please publicize it!

Option #5: `MediaStore`

I see some developers on Stack Overflow using methods like insertImage() on MediaStore.Images.Media as a way of getting a Uri to an image. These methods are ill-documented, and I have never used them, so I do not know whether they are a great idea, a terrible idea, or something in between.