CWAC-Provider: Helping to Make Content Providers Sane

UPDATE 2021-05-08: This project is discontinued. This repository will be removed from public access on or after 1 December 2021.

This project offers a StreamProvider, based on Google's FileProvider. Like FileProvider, StreamProvider is designed to serve up files, for reading and writing, through the ContentProvider interface (content:// Uri values). StreamProvider offers:

Serving files from assets and raw resources
Serving files from getExternalFilesDir() and getExternalCacheDir()

in addition to FileProvider's support for serving files from getFilesDir(), getCacheDir(), and Environment.getExternalStoragePublicDirectory().

You can simply use the StreamProvider directly, without creating your own subclass — everything can be handled through configuration rather than coding.

This Android library project is available as a JAR or as an artifact for use with Gradle. To use that, add the following blocks to your build.gradle file:

repositories {
    maven {
        url "https://s3.amazonaws.com/repo.commonsware.com"
    }
}

dependencies {
    compile 'com.commonsware.cwac:provider:0.5.3'
}

Or, if you cannot use SSL, use http://repo.commonsware.com for the repository URL.

NOTE: The JAR name, as of v0.2.1, has a cwac- prefix, to help distinguish it from other JARs.

Usage: StreamProvider

Once you add the JAR or artifact to your project, it works much along the lines of FileProvider:

Define an XML metadata file with a <paths> root element, containing one or more elements describing what you want the provider to serve (described in greater detail below)
Define a <provider> in your manifest as follows:

<provider
	android:name="com.commonsware.cwac.provider.StreamProvider"
	android:authorities="..."
	android:exported="false"
	android:grantUriPermissions="true">
	<meta-data
		android:name="com.commonsware.cwac.provider.STREAM_PROVIDER_PATHS"
		android:resource="@xml/..."/>
</provider>

(where you fill in your desired authority name and reference to your XML metadata from step #1)

Notably, the provider must have the <meta-data> element pointing to your XML metadata.

Use FLAG_GRANT_READ_URI_PERMISSION and FLAG_GRANT_WRITE_URI_PERMISSION in Intent objects you use to have third parties use the files the StreamProvider serves, to allow those apps selective, temporary access to the file.

Exporting and Usage Patterns

If your StreamProvider is exported, all of your streams will be considered read-only, regardless of any other configuration. Mostly, this mode is here for cases where you need a streaming provider and cannot grant Uri permissions (e.g., implementing a ChooserTargetService).

If your StreamProvider is not exported, and it has android:grantUriPermissions set, then you can control, on a per-Uri basis, which clients get access to your streams. This works identically to how FileProvider works. Whether a particular source of streams is read-only or read-write will depend on whether the stream is a file and your metadata configuration.

Wherever possible, elect to not export the provider and use FLAG_GRANT_READ_URI_PERMISSIONS or similar techniques to selectively grant access to your content.

Note that the exported-and-read-only rule is on a per-provider basis. If you have some content that needs to be published globally and others that are not:

Use StreamProvider and one <provider> element for one set of content, with one authority and android:exported setting
Subclass StreamProvider and have a separate <provider> element for the other set of content, with a separate authority and android:exported setting

Metadata Elements

Google's FileProvider supports:

<files-path> for serving files from your app's getFilesDir()
<external-path> for serving files from Environment.getExternalStoragePublicDirectory()
<cache-path> for serving files from your app's getCacheDir()
<external-files-path> for serving files from getExternalFilesDir()
<external-cache-path> for serving files from getExternalCacheDir()

Each of those take a name attribute, indicating the first path segment of the Uri that should identify this particular source of files. For example, a name of foo would mean that content://your.authority.here/foo/... would look for a ... file in that particular element's source of files.

Each of those optionally take a path attribute, indicating a subdirectory under the element-defined root to use as the source of files, rather than the root itself. So, a <files-path> with a path="stuff" attribute would serve files from the stuff/ subdirectory within getFilesDir(). Note that path can point to a file as well, to limit access to a single file rather than a directory. Note that path is required for <files-path>, so you do not accidentally serve everything under getFilesDir().

Also, each can optionally take a readOnly attribute. If this is set to true, then the files will be readable, but not writeable.

<external-files-path> also can take an optional dir attribute. If missing, the files are served from getExternalFilesDir(). If a valid value of dir is supplied, that value is passed into getExternalFilesDir(). As such, dir is limited to be one of the Environment.DIRECTORY_* constants:

Alarms
DCIM
Documents
Download
Movies
Music
Notifications
Pictures
Podcasts
Ringtones

However, you cannot have both <external-files-path> with no dir (indicating that you are serving from getExternalFilesDir(null)) and one or more <external-files-path> elements with dir values, as they will conflict.

StreamProvider adds support for:

<raw-resource> for serving a particular raw resource, where the path is the name of the raw resource (without file extension)
<asset> for serving files from assets/
<dir-path>, for serving files from locations identified by getDir()
<external-public-path>, for serving files from locations identified by Environment.getExternalStoragePublicDirectory()

In the case of <dir-path>, two attributes are required:

dir, which indicates what directory to serve (this is passed into getDir())
path, which serves its normal role, to determine what to serve from the directory identified by dir

In the case of <external-public-path>, dir is required. It needs to be the string value of one of the Environment.DIRECTORY_* constants, listed above.

Of course, your metadata can have one or more of each of these types as needed to declare what you want to be served.

Supporting Legacy Apps

Some apps assume that any content:// Uri that they get must be from the MediaStore or otherwise have a MediaStore.MediaColumns.DATA column that can be queried. This, of course, was never the case, and is less the case nowadays. But, it sometimes takes firms a while to get with the program, and in the meantime, StreamProvider could have issues working with such apps.

Adding another <meta-data> element to the <provider> can help improve compatibility:

<meta-data
  android:name="com.commonsware.cwac.provider.USE_LEGACY_CURSOR_WRAPPER"
  android:value="true"/>

This tells StreamProvider to include a fake MediaStore.MediaColumns.DATA in the result set, with a null value, to try to cajole these legacy apps into using the Uri as they are supposed to: via ContentResolver and openInputStream().

Similarly, you can add this <meta-data> element to the <provider>:

<meta-data
  android:name="com.commonsware.cwac.provider.USE_URI_FOR_DATA_COLUMN"
  android:value="true"/>

Clients of a streaming ContentProvider should not be assuming that they can query() for a _DATA column. Alas, some developers still do, thinking that all content: Uri values come from the MediaStore. By default, StreamProvider returns null for the _DATA column, should somebody query() for it. However, with the above <meta-data> element, StreamProvider will return the Uri used for the query() as the value for _DATA.

Gradle Settings

Starting with version 0.3.0 of the library, for files you are looking to share from your app's assets/, you will need to teach the build system to avoid compressing those files. On the plus side, we can now use AssetFileDescriptor for those, and greatly improve compatibility with apps using our streams.

To do this, add the following closure to your android closure in your module's build.gradle file:

aaptOptions {
    noCompress 'pdf', 'mp4', 'ogg'
}

(here, the file extensions are from the demo app — you would list the file extensions that you are looking to share)

Getting Uri Values

For files served through StreamProvider (as opposed to assets or raw resources), StreamProvider offers a static getUriForFile() method that works akin to its equivalent on FileProvider. It takes two parameters:

The authority name of the provider you are interested in
The File object that you want to serve

It returns a Uri pointing to that file or null if the File does not seem to be served by that provider.

For anything else, you have to assemble the Uri yourself:

private static final String AUTHORITY=
    "com.commonsware.cwac.provider.demo";
private static final Uri PROVIDER=
      Uri.parse("content://"+AUTHORITY);

private Uri buildUri(String path) {
    return(PROVIDER
          .buildUpon()
          .appendPath(StreamProvider.getUriPrefix(AUTHORITY))
          .appendPath(path)
          .build());
}

Uri Prefixes

To help defeat some security attacks, StreamProvider, starting with 0.4.0, by default, puts a per-install UUID into every Uri, as the first path segment after the authority name. So, for example, in the following Uri, some-prefix is the prefix:

content://com.commonsware.hithere/some-prefix/foo/bar.txt

If you are constructing a Uri supported by a StreamProvider — and you cannot use getUriForFile() (e.g., you are serving assets or raw resources) — call the static getUriPrefix() method, passing in the authority name of the provider. If it returns a non-null value, that is the prefix to put into the Uri. If getUriPrefix() returns null, there is no prefix.

Extending StreamProvider

You are welcome to create custom subclasses of StreamProvider, to handle cases that are not covered by StreamProvider itself. This process is covered in a separate documentation page.

Limitations

Compared to FileProvider, StreamProvider has the following limitations:

FileProvider has support for an additional, undocumented metadata element; StreamProvider drops support for that element.
StreamProvider no longer allows you to serve everything from getFilesDir(), for security reasons. The path attribute is required.

Upgrading to 0.5.0+ From Earlier Versions

If you created your own subclass of StreamProvider and overrode buildStrategy(), note that the method signature has changed and is now:

protected StreamStrategy buildStrategy(Context context,
                                     String tag, String name,
                                     String path, boolean readOnly,
                                     HashMap<String, String> attrs)

readOnly is a boolean indicating if this content should be treated as read-only (true) or read-write (false).

Also, if you are using LegacyCompatCursorWrapper, it now has an additional, three-parameter constructor:

public LegacyCompatCursorWrapper(Cursor cursor, String mimeType,
                                   Uri uriForDataColumn)

That third parameter should be the Uri to use for the value of the _DATA column, should somebody attempt to request that column from this Cursor. The default value is null. A likely alternative would be whatever Uri generated this Cursor (e.g., from the provider's query() implementation).

Also, if you are using LegacyCompatCursorWrapper, its fields are now marked final private. If you had been referencing those fields, and this now breaks your code, please file an issue and explain your use case, so an appropriate API can be added to LegacyCompatCursorWrapper.

Upgrading to 0.4.0+ From Earlier Versions

If you are upgrading an existing StreamProvider implementation to 0.4.0 or higher, please note the new Uri prefix discussed earlier in the documentation. Your provider's Uri values will have this prefix by default, and you need to include the prefix in any Uri values that you publish.

Usage: LegacyCompatCursorWrapper

Some consumers of content:// Uri values make unfortunate assumptions, that they can query() on that Uri and get columns back other than those in OpenableColumns. Of note, one or more popular consumers request:

MediaStore.MediaColumns.DATA (erroneously thinking that the Uri must be known to the MediaStore)
MediaStore.MediaColumns.MIME_TYPE (rather than calling getType() on a ContentResolver, the way talented developers would)

For Google's FileProvider, or other ContentProvider implementations that also have a need for these fake columns in the query() result, this library offers LegacyCompatCursorWrapper. Just wrap your Cursor in the LegacyCompatCursorWrapper (e.g., new LegacyCompatCursorWrapper(cursor)), and return the LegacyCompatCursorWrapper. It will automatically add the fake columns for queries that request them, delegating all other requests to the underlying Cursor.

Many thanks to Stefan Rusek for pioneering the basic approach.

Dependencies

This project has no dependencies.

Version

This is version v0.5.3 of this module, meaning it is pretty new.

Demo

In the demo/ sub-project you will find a sample project demonstrating the use of StreamProvider.

Additional Documentation

JavaDocs are available.

The Busy Coder's Guide to Android Development contains a section dedicated to StreamProvider. It also uses LegacyCompatCursorWrapper in all of its FileProvider samples.

License

The code in this project is licensed under the Apache Software License 2.0, per the terms of the included LICENSE file.

Questions

If you have questions regarding the use of this code, please post a question on Stack Overflow tagged with commonsware-cwac and android after searching to see if there already is an answer. Be sure to indicate what CWAC module you are having issues with, and be sure to include source code and stack traces if you are encountering crashes.

If you have encountered what is clearly a bug, or if you have a feature request, please post an issue. Be certain to include complete steps for reproducing the issue. The contribution guidelines provide some suggestions for how to create a bug report that will get the problem fixed the fastest.

You are also welcome to join the CommonsWare Community and post questions and ideas to the CWAC category.

Do not ask for help via social media.

Also, if you plan on hacking on the code with an eye for contributing something back, please open an issue that we can use for discussing implementation details. Just lobbing a pull request over the fence may work, but it may not. Again, the contribution guidelines provide a bit of guidance here.

Release Notes

v0.5.3: upgraded to Android Gradle Plugin 3.0.0
v0.5.2: added exported read-only support, published JavaDoc/source JARs in repo
v0.5.1: fixed bug blocking use of multiple authorities in a single provider
v0.5.0:
- Added support for Environment.getExternalStoragePublicDirectory()
- Added support for getDir()
- Added support for non-null versions of getExternalFilesDir()
- Added com.commonsware.cwac.provider.USE_URI_FOR_DATA_COLUMN option per issue #20
v0.4.4: fix for CompositeStreamStrategy bug
v0.4.3: exposed yet another method for extending StreamProvider
v0.4.2: exposed a few more methods for extending StreamProvider
v0.4.1: fix for CompositeStreamStrategy bug
v0.4.0: added Uri prefix, clearer subclassing support, refactored into Android Studio project structure, etc.
v0.3.1: fixed local path bug, added support for MediaStore.MediaColumns.MIME_TYPE to LegacyCompatCursorWrapper
v0.3.0: switched to openAssetFile() where possible for better compatibility
v0.2.5: pulled out permissions check into separate method
v0.2.4: added LegacyCompatCursorWrapper and USE_LEGACY_CURSOR_WRAPPER
v0.2.3: resolved issue #8, supporting actual length for assets and raw resources
v0.2.2: updated for Android Studio 1.0 and new AAR publishing system
v0.2.1: updated Gradle, fixed manifest for merger, added cwac- prefix to JAR
v0.2.0: migrated to Gradle, published AAR
v0.1.0: initial release

commonsguy/cwac-provider

commonsguy

Reviews

Repository Details