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.

Theory

Think about acceleration sensors : you want to get the acceleration values in

python nothing is available natively, but 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.

http://code.google.com/p/android-scripting/source/browse/android/BluetoothFacade/src/com/googlecode/android_scripting/facade/BluetoothFacade.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 :)

Jni gottchas

- package must be org.renpy.android, don't change it.

extend.rst