Lots of good questions, let's dig in. :)
How do you use it?
Here's a great tutorial for interacting with the Storage Access Framework in KitKat:
https://developer.android.com/guide/topics/providers/document-provider.html#client
Interacting with the new APIs in Lollipop is very similar. To prompt the user to pick a directory tree, you can launch an intent like this:
Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
startActivityForResult(intent, 42);
Then in your onActivityResult(), you can pass the user-picked Uri to the new DocumentFile helper class. Here's a quick example that lists the files in the picked directory, and then creates a new file:
public void onActivityResult(int requestCode, int resultCode, Intent resultData) {
if (resultCode == RESULT_OK) {
Uri treeUri = resultData.getData();
DocumentFile pickedDir = DocumentFile.fromTreeUri(this, treeUri);
// List all existing files inside picked directory
for (DocumentFile file : pickedDir.listFiles()) {
Log.d(TAG, "Found file " + file.getName() + " with size " + file.length());
}
// Create a new file and write into it
DocumentFile newFile = pickedDir.createFile("text/plain", "My Novel");
OutputStream out = getContentResolver().openOutputStream(newFile.getUri());
out.write("A long time ago...".getBytes());
out.close();
}
}
The Uri returned by DocumentFile.getUri() is flexible enough to use with may different platform APIs. For example, you could share it using Intent.setData() with Intent.FLAG_GRANT_READ_URI_PERMISSION.
If you want to access that Uri from native code, you can call ContentResolver.openFileDescriptor() and then use ParcelFileDescriptor.getFd() or detachFd() to obtain a traditional POSIX file descriptor integer.
How do you check if you can access the files/folders?
By default, the Uris returned through Storage Access Frameworks intents are not persisted across reboots. The platform "offers" the ability to persist the permission, but you still need to "take" the permission if you want it. In our example above, you'd call:
getContentResolver().takePersistableUriPermission(treeUri,
Intent.FLAG_GRANT_READ_URI_PERMISSION |
Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
You can always figure out what persisted grants your app has access to through the ContentResolver.getPersistedUriPermissions() API. If you no longer need access to a persisted Uri, you can release it with ContentResolver.releasePersistableUriPermission().
Is this available on KitKat?
No, we can't retroactively add new functionality to older versions of the platform.
Can I see what apps have access to files/folders?
There's currently no UI that shows this, but you can find the details in the "Granted Uri Permissions" section of adb shell dumpsys activity providers output.
What happens if an app is installed for multiple users on the same device?
Uri permission grants are isolated on a per-user basis, just like all other multi-user platform functionality. That is, the same app running under two different users has no overlaping or shared Uri permission grants.
Can the permissions be revoked?
The backing DocumentProvider can revoke permission at any time, such as when a cloud-based document is deleted. The most common way to discover these revoked permissions is when they disappear from ContentResolver.getPersistedUriPermissions() mentioned above.
Permissions are also revoked whenever app data is cleared for either app involved in the grant.
Would asking for the permission work recursively on a selected folder?
Yep, the ACTION_OPEN_DOCUMENT_TREE intent gives you recursive access to both existing and newly created files and directories.
Does this allow multiple selection?
Yep, multiple selection has been supported since KitKat, and you can allow it by setting EXTRA_ALLOW_MULTIPLE when starting your ACTION_OPEN_DOCUMENT intent. You can use Intent.setType() or EXTRA_MIME_TYPES to narrow the types of files that can be picked:
http://developer.android.com/reference/android/content/Intent.html#ACTION_OPEN_DOCUMENT
Is there a way on the emulator to try the new API?
Yep, the primary shared storage device should appear in the picker, even on the emulator. If your app only uses the Storage Access Framework for accessing shared storage, you no longer need the READ/WRITE_EXTERNAL_STORAGE permissions at all and can remove them or use the android:maxSdkVersion feature to only request them on older platform versions.
What happens when the user replaces the SD-card with another one?
When physical media is involved, the UUID (such as FAT serial number) of the underlying media is always burned into the returned Uri. The system uses this to connect you to the media that the user originally selected, even if the user swaps the media around between multiple slots.
If the user swaps in a second card, you'll need to prompt to gain access to the new card. Since the system remembers grants on a per-UUID basis, you'll continue to have previously-granted access to the original card if the user reinserts it later.
http://en.wikipedia.org/wiki/Volume_serial_number
