Extending Python for android native support¶
So, you want to get into python-for-android and extend what’s available to Python on Android ?
Turns out it’s not very complicated, here is a little introduction on how to go about it. Without Pyjnius, the schema to access the Java API from Cython is:
 Cython ->  C JNI ->  Java
Think about acceleration sensors: you want to get the acceleration values in Python, but nothing is available natively. Lukcily you have a Java API for that : the Google API is available here http://developer.android.com/reference/android/hardware/Sensor.html
You can’t use it directly, you need to do your own API to use it in python, this is done in 3 steps
Step 1: write the java code to create very simple functions to use¶
like : accelerometer Enable/Reading In our project, this is done in the Hardware.java: https://github.com/kivy/python-for-android/blob/master/src/src/org/renpy/android/Hardware.java you can see how it’s implemented
Step 2 : write a jni wrapper¶
This is a C file to be able to invoke/call Java functions from C, in our case, step 2 (and 3) are done in the android python module. The JNI part is done in the android_jni.c: https://github.com/kivy/python-for-android/blob/master/recipes/android/src/android_jni.c
Step 3 : you have the java part, that you can call from the C¶
You can now do the Python extension around it, all the android python part is done in https://github.com/kivy/python-for-android/blob/master/recipes/android/src/android.pyx
→ [python] android.accelerometer_reading ⇒ [C] android_accelerometer_reading ⇒ [Java] Hardware.accelerometer_reading()
The jni part is really a C api to call java methods. a little bit hard to get it with the syntax, but working with current example should be ok
Example with bluetooth¶
Start directly from a fork of https://github.com/kivy/python-for-android
The first step is to identify where and how they are doing it in sl4a, it’s really easy, because everything is already done as a client/server client/consumer approach, for bluetooth, they have a “Bluetooth facade” in java.
You can learn from it, and see how is it’s can be used as is, or if you can simplify / remove stuff you don’t want.
From this point, create a bluetooth file in python-for-android/tree/master/src/src/org/renpy/android in Java.
Do a good API (enough simple to be able to write the jni in a very easy manner, like, don’t pass any custom java object in argument).
Then write the JNI, and then the python part.
3 steps, once you get it, the real difficult part is to write the java part :)
- package must be org.renpy.android, don’t change it.
Create your own recipes¶
A recipe is a script that contains the “definition” of a module to compile. The directory layout of a recipe for a <modulename> is something like:
python-for-android/recipes/<modulename>/recipe.sh python-for-android/recipes/<modulename>/patches/ python-for-android/recipes/<modulename>/patches/fix-path.patch
When building, all the recipe builds must go to:
For example, if you want to create a recipe for sdl, do:
cd python-for-android/recipes mkdir sdl cp recipe.sh.tmpl sdl/recipe.sh sed -i 's#XXX#sdl#' sdl/recipe.sh
Then, edit the sdl/recipe.sh to adjust other information (version, url) and complete the build function.