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 (numpy/scipy/pandas/tflite-runtime, etc).

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

Example project

Flutter + Python example

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

# 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)
# NOTE: sticky services are persistent and will not close when the main app closes
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, ""); 
       }
   
   //needed if service is not foreground/sticky
   @Override
   protected void onResume () {
       ServiceSrv.start(this, "");
       super.onResume();
   }
} 

Lastly, there is ServiceSrv.stop(this) API to stop the service manually. If sticky is not specified in buildozer.spec, service is automatically stopped when the app exits.

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 logcat , while the flutter demo is running. Options for communicating between python and flutter include:

• flask server
• zeromq
• nanomsg-ng
• raw UDP/TCP sockets

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.** {
    *;
}