Part 5 - APK Expansion Files

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last updated: 2017-03

Some applications (some games, for instance) require more resources and assets than can be provided in the maximum Android app size limit imposed by Google Play. This limit depends on the version of Android that your APK is targeted for:

  • 100MB for APKs that target Android 4.0 or higher (API level 14 or higher).
  • 50MB for APKs that target Android 3.2 or lower (API level 13 or higher).

To overcome this limitation, Google Play will host and distribute two expansion files to go along with an APK, allowing an application to indirectly exceed this limit.

On most devices, when an application is installed, expansion files will be downloaded along with the APK and will be saved to the shared storage location (the SD card or the USB-mountable partition) on the device. On a few older devices, the expansion files may not automatically install with the APK. In these situations, it is necessary for the application to contain code that will download the expansion files when the user first runs the applications.

Expansion files are treated as opaque binary blobs (obb) and may be up to 2GB in size. Android does not perform any special processing on these files after they are downloaded—the files can be in any format that is appropriate for the application. Conceptually, the recommended approach to expansion files is as follows:

  • Main expansion – This file is the primary expansion file for resources and assets that will not fit in the APK size limit. The main expansion file should contain the primary assets that an application needs and should rarely be updated.
  • Patch expansion – This is intended for small updates to the main expansion file. This file can be updated. It is the responsibility of the application to perform any necessary patches or updates from this file.

The expansion files must be uploaded at the same time as the APK is uploaded. Google play does not allow an expansion file to be uploaded to an existing APK or for existing APKs to be updated. If it is necessary to update an expansion file, then a new APK must be uploaded with the versionCode updated.

Expansion File Storage

When the files are downloaded to a device, they will be stored in the following location:

<shared-stored>/Android/obb/<package-name>
  • <shared-store> – This is the directory specified by Android.OS.Environment.ExternalStorageDirectory .
  • <package-name> – This is the application’s Java-style package name.

Once downloaded, expansion files should not be moved, altered, renamed, or deleted from their location on the device. To do so will cause the expansion files to be downloaded again, and the old file(s) will be deleted. Additionally, the expansion file directory should contain only the expansion pack files.

Expansion files offer no security or protection around their content—other applications or users may access any files saved on the shared storage.

If it is necessary to unpack an expansion file, the unpacked files should be stored in a separate directory, such as one in Android.OS.Environment.ExternalStorageDirectory.

An alternative to extracting files from an expansion file is to read the assets or resources directly from the expansion file. The expansion file is nothing more than a zip file that can be used with an appropriate ContentProvider. The Android.Play.ExpansionLibrary contains an assembly, System.IO.Compression.Zip, which includes a ContentProvider that will allow for direct file access to some media files. If media files are being packaged into a zip file, media playback calls may directly use files in the zip without having to unpack the zip file. The media files should not be compressed when added to the zip file.

FileName Format

When the expansion files are downloaded, Google Play will use the following scheme to name the expansion:

[main|patch].<expansion-version>.<package-name>.obb

The three components of this scheme are:

  • main or patch – This specifies whether this is the main or patch expansion file. There can be only one of each.
  • <expansion-version> – This is an integer that matches the versionCode of the APK that the file was first associated with.
  • <package-name> - This is the application’s Java-style package name.

For example, if the APK version is 21, and the package name is mono.samples.helloworld, the main expansion file will be named:

main.21.mono.samples.helloworld

Download Process

When an application is installed from Google Play, the expansion files should be downloaded and saved along with the APK. In certain situations this may not happen, or expansion files may be deleted. To handle this condition, an app needs to check to see whether the expansion files exist and then download them, if necessary. The following flowchart displays the recommended workflow of this process:

APK expansion flowchart

When an application starts up, it should check to see if the appropriate expansion files exist on the current device. If they do not, then the application must make a request from Google Play’s Application Licensing. This check is made by using the License Verification Library (LVL), and must be made for both free and licensed applications. The LVL is primarily used by paid applications to enforce license restrictions. However, Google has extended the LVL so that it can be used with expansion libraries as well. Free applications have to perform the LVL check, but can ignore the license restrictions. The LVL request is responsible for providing the following information about the expansion files that the application requires:

  • File Size – The file sizes of the expansion files are used as part of the check that determines whether or not the correct expansion files have already been downloaded.
  • Filenames – This is the file name (on the current device) to which the expansion packs must be saved.
  • URL for Download – The URL that should be used to download the expansion packs. This is unique for every download and will expire shortly after it is provided.

After the LVL check has been performed, the application should download the expansion files, taking into consideration the following points as part of the download:

  • The device may not have enough space to store the expansion files.
  • If Wi-Fi is not available, then the user should be allowed to pause or cancel the download to prevent unwanted data charges.
  • The expansion files are downloaded in the background to avoid blocking user interactions.
  • While the download is occurring in the background, a progress indicator should be displayed.
  • Errors that occur during the download are gracefully handled and recoverable.

Architectural Overview

When the main activity starts, it checks to see if the expansion files are downloaded. If the files are downloaded, they must be checked for validity.

If the expansion files have not been downloaded or if the current files are invalid, then new expansion files must be downloaded. A bounded service is created as part of the application. When the main activity of the application is started, it uses the bounded service to perform a check against the Google Licensing services to find out the expansion file names and the URL of the files to download. The bounded service will then download the files on a background thread.

To ease the effort required to integrate expansion files into an application, Google created several libraries in Java. The libraries in question are:

  • Downloader Library – This is a library that reduces the effort required to integrate expansion files in an application. The library will download the expansion files in a background service, display user notifications, handle network connectivity issues, resume downloads, etc.
  • License Verification Library (LVL) – A library for making and processing the calls to the Application Licensing services. It can also be used to perform licensing checks, to see if the application is authorized for use on the device.
  • APK Expansion Zip Library (optional) – If the expansion files are in a zip file, this library will act as a content provider and allow an application to read resources and assets directly from the zip file without having to expand the zip file.

These libraries have been ported to C# and are available under the Apache 2.0 license. To quickly integrate expansion files into an existing application, these libraries can be added to an existing Xamarin.Android application. The code is available at the Android.Play.ExpansionLibrary on GitHub.

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