Flutter with Python on Android

Instructions on how to generate one APK, containing Flutter UI as a frontend, and a Python Service as a backend.

Using the p4a project for bundling python scripts and cross-compiling python libraries with native components (like numpy/scipy/pandas/tflite-runtime).

NOTE: This tutorial requres at minimum p4a version v2022.07.20 and buildozer version 1.4.0

Install buildozer:

https://buildozer.readthedocs.io/en/latest/installation.html

Generate empty buildozer config

cd libapp
buildozer init

Edit a sample python file called srv.py

import time
while True:
    time.sleep(1)
    print('hello world')

Edit buildozer.spec

package.domain = org.domain

# kivy introduces conflicting dependencies and is not needed
requirements = python3

# select service bootstrap 
p4a.bootstrap = service_library 

# ask p4a to output aar instead of apk 
android.release_artifact = aar

# specify your runtime python package dependencies here (comma separated)
requirements = python3

# foreground : so that OS does not kill us (optional)
# sticky : so that OS restarts us on exit/crash  (optional)
services = Srv:srv.py:foreground:sticky  

# pick ABI(s) - NOTE: listing more than needed here grows the final .apk size
android.archs = arm64-v8a, x86

Compile the aar

buildozer android release

Setup a flutter project

cd ..\
flutter create -i objc -a java testapp

Edit testapp/android/app/build.gradle

# change minSdkVersion to match p4a aar library. 

minSdkVersion 21 

# add aar as a dependency 

dependencies { 
   implementation files('../../../libapp/bin/myapp-0.1-arm64-v8a_x86-release.aar') 
} 

Start services on app startup

Here we assume you wish to start the service when your app starts and you are using a sticky foreground service.

edit testapp/android/app/src/main/java/com/example/testapp/MainActivity.java

package com.example.testapp; 
import io.flutter.embedding.android.FlutterActivity; 
import android.os.Bundle; 
import  org.domain.myapp.ServiceSrv; 

public class MainActivity extends FlutterActivity { 

   @Override 
   protected void onCreate(Bundle savedInstanceState) { 
       super.onCreate(savedInstanceState); 
       ServiceSrv.prepare(this); 
       ServiceSrv.start(this, ""); 
       } 
} 

Optionally, you can add a hook when the app exits to stop the python service with ServiceSrv.stop(this) API

Add Foreground Service Permission

Optional if you selected foreground when specifying the service in buildozer.

Edit testapp/android/app/src/main/AndroidManifest.xml write to <manifest>

<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />

Build and run the flutter app

flutter run

At this point, you should see 'hello world' printed if you adb locat , while the flutter demo app running. Communicating between the two is an excercse to the user, you can use raw sockets, ZeroMQ, Nanomsg-NG libraries to name a few.

Some useful projects:

• Access Android Java API from Python: pyjnius
• Cross platform API for common tasks: plyer

Proguard rules for pyjnius

-keep public class org.kivy.android.** {
    *;
}
-keep public class org.jnius.** {
    *;
}