PoC for a translation layer that would execute the abolute minimum of android-specific code necessary in order to run an android app on a Linux system

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build instructions:
make: compile everything (except dalvik, which is precompiled from https://gitlab.com/Mis012/dalvik_standalone)

usage:
./launch_activity.sh <path_to_apk> <path_to_activity>
example: ./launch_activity.sh test_apks/org.happysanta.gd_29.apk org/happysanta/gd/GDActivity
note: some apps don't like runtime changes to resolution, and currently GLSurfaceView will stretch instead of changing resolution
example with custom width/height: ./launch_activity.sh test_apks/org.happysanta.gd_29.apk 'org/happysanta/gd/GDActivity -w 540 -h 960'

NOTE: you might need to copy some files out from the apk under data/, e.g the assets folder; additionally, resources (res) currently need to be "decompiled" (e.g. by apktool, though this additionally replaces hex IDs with string names, which needs to be reversed)
NOTE: on X11, Gtk might decide to use GLX, which completely messes up our EGL-dependent code. If you have a debug build of Gtk, you can use GDK_DEBUG=gl-egl to force the use of EGL

when it doesn't work:
if you are trying to launch a random app, chances are that we are missing implementations for some
stuff that it needs, and we also don't have (sufficiently real looking) stubs for the stuff it says
it needs but doesn't really.
the workflow is basically to see where it fails (usually a Class or Method was not found) and to create
stubs which sufficiently satisfy the app so that it continues trying to launch.
once the app launches, you may find that some functionality (UI elements, ...) is missing; to enable
such functionality, you need to convert the relevant stubs to actual implementation.
you can look at simple widgets (e.g. TextView, or ImageView which is extremely simple since it's
currently displaying an image-not-found icon instead of the actual image) to see how to implement a
widget such that it shows up as a Gtk Widget. for layout widgets, you can cheat by duplicating
LinearLayout or RelativeLayout, which are currently implemented in an extremely simplistic manner
(it might not look correctly, but it should work)

for more specific instructions, see doc/QuickHelp.md for general description of the architecure, see doc/Architecture.md

screenshot:

note: running two or more different apps at the same time needs a tiny bit of luck (no conflicting files), since the data directory path is currently hardcoded to ./data/ no matter the app

FAQ:

Q:
did you ever notice that most apps nowadays use at least some .so libraries?
some apps even use them for 99% of their functionality!

A:
recently, support for loading bionic-linked libraries was added, and two games which use
GLSurfaceView (rendered to from native code) as their main UI element - Angry Birds 3.2.0
and Worms 2 Armageddon - are currently fully playable

Q:
this seems pretty basic, most apps don't work at all...
A:
we have three apps working for now, how fast that number grows depends on how many
people work on reimplementing stuff :)
there are probably more than enough people in the world with the required skills who don't have
anything better to do for the number of implemented APIs to grow appreciably fast, maybe you are
one of them? ;)

Q:
ok, so this can sorta run three apps. What's the catch?

A:
well, first things first, technically I compiled Gravity Defied myself and removed
some bug-reporting-init-related code which I got frustrated with stubbing out. however,
adding more stubs should make that unnecessary.
now for the second issue: ./data/ contains some stuff that should instead be read from the apk,
and some of this stuff is also externally converted (e.g Xml from binary form to actual Xml).
obviously this is not ideal for user experience. NOTE: it seems that the binary form might be
protobuf-based, which would make reading it directly easier.
and the third issue: Gravity Defied is still extremely simple compared to most android apps,
doesn't acknowledge compat layers, and the most intricate UI element is completely custom drawn
using the canvas interface, in a manner that makes it easy to implement with cairo.
angry birds (old version) and worms 2 armageddon were chosen because they similarly don't use
compat layers, and basically the entire UI is custom drawn with OpenGL calls from native code.

Q:
why are only 32bit architectures supported? A:
we are currently using good old dalvik vm (probably newer commit than ever shipped though), which
is so hopelessly hardcoded to assume 32bit ints and pointers that it probably made google's cost/benefit
analyses of writing art from scratch a lot easier.
Eventually, we should probably move to ART, but it's an open question whether that will bring issues for our
usecase (is there a way to use ART without zygote? is it straightforward to port the few modifications that we did to dalvik?)

Roadmap:

• fix issues mention above

• fix ugly hacks

  • don't assume apps only have one (relevant) activity
  • figure out a way to dynamically change the resolution of a GLSurfaceView

• implement more stuff (there is a lot of it, and it won't get done if nobody helps... ideally pick a simple-ish application and stub/implement stuff until it works)

• especially implement the alternatives to GLSurfaceView (using SurfaceView to get an EGL surface, native activity, not sure if there are others?) which would allow us to support a few more 99%-native applications with relative ease

• explore switching to ART in the interest of supporting 64bit architectures

• explore using apparmor to enforce the security policies that google helpfully forces apps to comply with (and our own security policies, like no internet access for apps which really shouldn't need it and are not scummy enough to refuse to launch without it)