Integrating Rust + Flutter + Kotlin for Mobile Applications

Question

As next week will have importat launch for Rust 2018 and Flutter 1.0, I thought to build an app using Rust for the business logic and Flutter for the user interface, that can run at both Android and iOS, I built one and tested it at Android and it is working fine.

I just wonder how to measure the performance and compare it with native Android/iOS app.

The app flow is:

1. Main is in Flutter, that is calling native function through platform_channel
2. The native function is calling rust library through JNI (JNI wrapper is required to be call the rust library)

The structure is as below:

The code used is:

main.dart:

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  static const platform = const MethodChannel('samples.flutter.io/battery');

  String _batteryLevel = 'Unknown battery level.';

  Future<void> _getBatteryLevel() async {
    String batteryLevel;
    try {
      final String hello = await platform.invokeMethod('getText');
      final int result = await platform.invokeMethod('getBatteryLevel');

      batteryLevel = '$hello Battery level at $result %.';
    } on PlatformException catch (e) {
      batteryLevel = "Failed to get battery level: '${e.message}'.";
    }

    setState(() {
      _batteryLevel = batteryLevel;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Material(
      child: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            RaisedButton(
              child: Text('Get Battery Level'),
              onPressed: _getBatteryLevel,
            ),
            Text(_batteryLevel),
          ],
        ),
      ),
    );
  }
}

JNI wrapper - RustGreetings.kt

package com.mozilla.greetings

class RustGreetings {
    companion object {
        init {
            System.loadLibrary("greetings")
        }
    }

    private external fun greeting(pattern: String): String

    fun sayHello(to: String): String = greeting(to)
}

And the Main Android activity is:

package com.example.batterylevel

import android.os.Bundle
import io.flutter.app.FlutterActivity
import io.flutter.plugins.GeneratedPluginRegistrant
import io.flutter.plugin.common.MethodChannel

import android.content.Context
import android.content.ContextWrapper
import android.content.Intent
import android.content.IntentFilter
import android.os.BatteryManager
import android.os.Build.VERSION
import android.os.Build.VERSION_CODES

import lib.Library
import com.mozilla.greetings.RustGreetings

class MainActivity: FlutterActivity() {
  private val CHANNEL = "samples.flutter.io/battery"

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    GeneratedPluginRegistrant.registerWith(this)

    MethodChannel(flutterView, CHANNEL).setMethodCallHandler { call, result ->
      if (call.method == "getText") {
        result.success(getText())
      } else if (call.method == "getBatteryLevel") {
       // result.success(getText())
        val batteryLevel = getBatteryLevel()

        if (batteryLevel != -1) {
          result.success(batteryLevel)
        } else {
          result.error("UNAVAILABLE", "Battery level not available.", null)
        }
      }
      else {

        result.notImplemented()
      }
    }
  }

  private fun getBatteryLevel(): Int {
    val batteryLevel: Int
    if (VERSION.SDK_INT >= VERSION_CODES.LOLLIPOP) {
      val batteryManager = getSystemService(Context.BATTERY_SERVICE) as BatteryManager
      batteryLevel = batteryManager.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY)
    } else {
      val intent = ContextWrapper(applicationContext).registerReceiver(null, IntentFilter(Intent.ACTION_BATTERY_CHANGED))
      batteryLevel = intent!!.getIntExtra(BatteryManager.EXTRA_LEVEL, -1) * 100 / intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1)
    }
    return batteryLevel
  }

  private fun getText(): String {
    val x = Library().someLibraryMethod()
    val g = RustGreetings()
    val r = g.sayHello("My $x Rust")
    return r
  }
}

In the Android gradle.build I just added the below, as I'm interested to check also the impact of adding kotlin JVM library and getting it interacted with the Rust library within the mobile application:

dependencies {
    implementation(files("src/main/libs/lib.jar"))
}

My question is: How can check the performance and impact of each process when it is executed or called by another process

Answer 1

With the introduction of ffi in Dart, things became more smoother now, with a better performance as the interction now is Dart/Rust directly, without a need for Dart/Kotlin/Rust or Dart/Swift/Rust cycle, below a simple example:

First src/lib.rs

#[no_mangle]
pub extern fn rust_fn(x: i32) -> i32 {   
    println!("Hello from rust\nI'll return: {}", x.pow(2));
    x.pow(2)
 }

and Cargo.toml

[package]
name = "Double_in_Rost"
version = "0.1.0"
authors = ["Hasan Yousef"]
edition = "2018"

[lib]
name = "rust_lib"
crate-type = ["dylib"] # could be `staticlib` as well

[dependencies]

Running cargo build --release will generate target\release\rust_lib.dll copy/paste it into Dart application root directory

Write Dart code as below:

import 'dart:ffi';
import 'dart:io' show Platform;

// FFI signature of the hello_world C function
typedef ffi_func = Int32 Function(Int32 x);  //pub extern fn rust_fn(x: i32) -> i32
// Dart type definition for calling the C foreign function
typedef dart_func = int Function(int x);

void main() {
  // Open the dynamic library
  var path = './rust_lib.so';
  if (Platform.isMacOS) path = './rust_lib.dylib';
  if (Platform.isWindows) path = 'rust_lib.dll';
  final dylib = DynamicLibrary.open(path);
  // Look up the Rust/C function
  final my_func =
      dylib.lookup<NativeFunction<ffi_func>>('rust_fn').asFunction<dart_func>();
  print('Double of 3 is ${my_func(3)}');
}