public class ExoResourcesRewriter extends Object
Some background: Android's resources are packaged into the APK primarily in the resources.arsc file with some references out to other files in the APK's res/ directory (.png/.xml mostly). The resources.arsc file is a binary file in a format that isn't really well documented, but you can get a good idea of its structure by looking at ResourceTable and other classes in this package. At runtime, Android constructs an AssetManager from the APK and resource lookups go through that. While this is primarily done for an app's own resources, the framework may construct one to access an app's resources directly (e.g. for driving animations, for intent pickers, etc) and apps can access resources of other apps (e.g. a launcher app will access names/icons).
For exo-for-resources, we determine a minimal set of resources (including referenced files) that need to be in the main APK. This set includes all resources referenced from the AndroidManifest.xml or from any animation. We construct a resources.arsc for these resources and then the primary apk includes those resources. To avoid odd issues, we rewrite the full resources.arsc and all compiled .xml files such that references match between the primary apk and the exo resources (and without this, we have run into issues).
For assets, we don't package any into the main apk. For exo resources, assets are put into a separate zip from the exo .arsc (and resource files).
TODO(cjhopman): The underlying c++ resource handling supports some things that we should take advantage of. First, we are able to easily have multiple resource apks (including multiple .arsc with the same package id) as long as we don't have the same package-id/type-id pair in different .arsc files. Second, there's no restriction that all resources of the same type actually have the same type id (e.g. we could have strings with type id 0x01, 0x02, and 0x03). I'm pretty sure that aapt's --feature-of/--feature-after work by using different type ids for the same type.
Using these, we should be able to split resources across some larger number of .zips in such a way that users will typically only need to install a small number of resources (for example, in a particular large app that I've looked at, a vast majority of resource size is spent on just the 'strings' type). It's probably also possible for us to construct multiple top-level aapt targets that handle smaller subsets of the resources (e.g. construct a separate 'strings' top-level aapt rule). While it would be hard to construct multiple aapt rules for a particular type (adding restrictions on resource overriding would make it easier) we could still easily split a particular type into multiple exo resources zips (using different type ids).
It might be possible to get even more ids to work by using a different package id (other than 0x7f). I believe the package id space is partitioned like so:
As the normal Android build system doesn't use anything other than 0x7f, and that's always been the only package id used by applications, and that we have some leeway in the type id space, I didn't think it was worth it now to further investigate the feasibility/difficulty of using different package ids.
|Modifier and Type||Method and Description|