What is the use of Intent.FLAG_GRANT_PREFIX_URI_PERMISSION when requesting access to a folder using Storage access framework?

Question

When requesting access to a folder (document-tree) via StorageAccessFramework, we use this code. I got this code from this repo and also seen in other questions on StackOverflow like this and this.

val intent = Intent(Intent.ACTION_OPEN_DOCUMENT_TREE).apply {
    addFlags(
        Intent.FLAG_GRANT_READ_URI_PERMISSION
                or Intent.FLAG_GRANT_WRITE_URI_PERMISSION
                or Intent.FLAG_GRANT_PERSISTABLE_URI_PERMISSION
                or Intent.FLAG_GRANT_PREFIX_URI_PERMISSION
    )
}

startActivityForResult(intent, 101)

4 flags are added to the intent so that read/write permissions to the folder are given and also so that those permissions are persisted (across device restarts). My question is, what is the purpose of the FLAG_GRANT_PREFIX_URI_PERMISSION flag?

As per the documentation, this is what it does -

When combined with FLAG_GRANT_READ_URI_PERMISSION and/or FLAG_GRANT_WRITE_URI_PERMISSION, the URI permission grant applies to any URI that is a prefix match against the original granted URI. (Without this flag, the URI must match exactly for access to be granted.) Another URI is considered a prefix match only when scheme, authority, and all path segments defined by the prefix are an exact match.

This leaves me with more questions than answers. Like what is a "prefix match" and what originally granted URI is the documentation referring to? Is this a flag which is useful when app requests permissions for a new folder?

Any clarification on this will be helpful. Thanks.

CommonsWare · Accepted Answer · 2021-07-30T16:04:20.127

Get rid of the addFlags() call. None of those are for requesting content. Principally, they are for granting rights to others. For example, you might use FLAG_GRANT_READ_URI_PERMISSION and/or FLAG_GRANT_WRITE_URI_PERMISSION in an ACTION_VIEW Intent, as you are supplying content to another app and need to ensure that it has rights to read and/or write that content.

FLAG_GRANT_PERSISTABLE_URI_PERMISSION and FLAG_GRANT_PREFIX_URI_PERMISSION are used to further describe the permission grant:

Can we request that the permissions be saved by the system?
Are those permissions just for the associated Uri, or is it for the Uri and any descendants (i.e., where the associated Uri is a prefix)

What is the use of Intent.FLAG_GRANT_PREFIX_URI_PERMISSION when requesting access to a folder using Storage access framework?

1 Answers1