mkdir rust-for-android-ios-flutter
cd rust-for-android-ios-flutter
mkdir ios android rust flutter

cd rust
cargo init --name rustylib --lib

# Android targets
rustup target add aarch64-linux-android armv7-linux-androideabi i686-linux-android x86_64-linux-android

# iOS targets
rustup target add aarch64-apple-ios armv7-apple-ios armv7s-apple-ios x86_64-apple-ios i386-apple-ios

# install the Xcode build tools.
xcode-select --install
# this cargo subcommand will help you create a universal library for use with iOS.
cargo install cargo-lipo
# this tool will let you automatically create the C/C++11 headers of the library.
cargo install cbindgen

cargo install cargo-ndk

[package]
name = "rustylib"
version = "0.1.0"
authors = ["Roberto Huertas <roberto.huertas@outlook.com>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[lib]
name = "rustylib"
# this is needed to build for iOS and Android.
crate-type = ["staticlib", "cdylib"]

# this dependency is only needed for Android.
[target.'cfg(target_os = "android")'.dependencies]
jni = { version = "0.13.1", default-features = false }

use std::ffi::{CStr, CString};
use std::os::raw::c_char;

#[no_mangle]
pub unsafe extern "C" fn hello(to: *const c_char) -> *mut c_char {
    let c_str = CStr::from_ptr(to);
    let recipient = match c_str.to_str() {
        Ok(s) => s,
        Err(_) => "you",
    };

    CString::new(format!("Hello from Rust: {}", recipient))
        .unwrap()
        .into_raw()
}

#[no_mangle]
pub unsafe extern "C" fn hello_release(s: *mut c_char) {
    if s.is_null() {
        return;
    }
    CString::from_raw(s);
}

cd rust
cbindgen src/lib.rs -l c > rustylib.h

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

char *hello(const char *to);

void hello_release(char *s);

# it's important to not forget the release flag.
cargo lipo --release

# we're still in the `rust` folder so...
inc=../ios/include
libs=../ios/libs

mkdir ${inc}
mkdir ${libs}

cp rustylib.h ${inc}
cp target/universal/release/librustylib.a ${libs}

import SwiftUI

struct ContentView: View {
    let s = getName()
    var body: some View {
        Text(s)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

func getName() -> String {
    let result = hello("Rob")
    let sr = String(cString: result!)
    // IMPORTANT: once we get the result we have to release the pointer.
    hello_release(UnsafeMutablePointer(mutating: result))
    return sr
}

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        let result = hello("Rob")
        let s_result = String(cString: result!)
        // IMPORTANT: once we get the result we have to release the pointer.
        hello_release(UnsafeMutablePointer(mutating: result))
        print(s_result)
    }
}

cd rust/src
echo > android.rs

#![cfg(target_os = "android")]
#![allow(non_snake_case)]

use crate::hello;
use jni::objects::{JClass, JString};
use jni::sys::jstring;
use jni::JNIEnv;
use std::ffi::CString;

// NOTE: RustKt references the name rusty.kt, which will be the kotlin file exposing the functions below.
// Remember the JNI naming conventions.

#[no_mangle]
pub extern "system" fn Java_com_robertohuertas_rusty_1android_1lib_RustyKt_helloDirect(
  env: JNIEnv,
  _: JClass,
  input: JString,
) -> jstring {
  let input: String = env
    .get_string(input)
    .expect("Couldn't get Java string!")
    .into();
  let output = env
    .new_string(format!("Hello from Rust: {}", input))
    .expect("Couldn't create a Java string!");
  output.into_inner()
}

#[allow(clippy::similar_names)]
#[no_mangle]
pub extern "system" fn Java_com_robertohuertas_rusty_1android_1lib_RustyKt_hello(
  env: JNIEnv,
  _: JClass,
  input: JString,
) -> jstring {
  let java_str = env.get_string(input).expect("Couldn't get Java string!");
  // we call our generic func for iOS
  let java_str_ptr = java_str.as_ptr();
  let result = unsafe { hello(java_str_ptr) };
  // freeing memory from CString in ios function
  // if we call hello_release we won't have access to the result
  let result_ptr = unsafe { CString::from_raw(result) };
  let result_str = result_ptr.to_str().unwrap();
  let output = env
    .new_string(result_str)
    .expect("Couldn't create a Java string!");
  output.into_inner()
}

#![cfg(target_os = "android")]
#![allow(non_snake_case)]

// add it below the use declarations.
#[cfg(target_os = "android")]
mod android;

#!/usr/bin/env bash
# set the version to use the library
min_ver=22
# verify before executing this that you have the proper targets installed
cargo ndk --target aarch64-linux-android --android-platform ${min_ver} -- build --release
cargo ndk --target armv7-linux-androideabi --android-platform ${min_ver} -- build --release
cargo ndk --target i686-linux-android --android-platform ${min_ver} -- build --release
cargo ndk --target x86_64-linux-android --android-platform ${min_ver} -- build --release

# moving libraries to the android project
jniLibs=../android/rusty-android/rusty-android-lib/src/main/jniLibs
libName=libdevicers.so

rm -rf ${jniLibs}

mkdir ${jniLibs}
mkdir ${jniLibs}/arm64-v8a
mkdir ${jniLibs}/armeabi-v7a
mkdir ${jniLibs}/x86
mkdir ${jniLibs}/x86_64

cp target/aarch64-linux-android/release/${libName} ${jniLibs}/arm64-v8a/${libName}
cp target/armv7-linux-androideabi/release/${libName} ${jniLibs}/armeabi-v7a/${libName}
cp target/i686-linux-android/release/${libName} ${jniLibs}/x86/${libName}
cp target/x86_64-linux-android/release/${libName} ${jniLibs}/x86_64/${libName}

package com.robertohuertas.rusty_android_lib

external fun hello(to: String): String
external fun helloDirect(to: String): String

fun loadRustyLib() {
    System.loadLibrary("rustylib")
}

dependencies {
    implementation project(':rusty-android-lib')
}

android:id="@+id/txt"

package com.robertohuertas.rusty_android

import android.os.Bundle
import com.google.android.material.snackbar.Snackbar
import androidx.appcompat.app.AppCompatActivity
import android.view.Menu
import android.view.MenuItem
import android.widget.TextView

import kotlinx.android.synthetic.main.activity_main.*
import com.robertohuertas.rusty_android_lib.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(toolbar)

        loadRustyLib()

        findViewById<TextView>(R.id.txt).let {
            it?.text = hello("Rob")
        }

        var greeting2 = helloDirect("Rob Direct")

        fab.setOnClickListener { view ->
            Snackbar.make(view, greeting2, Snackbar.LENGTH_LONG)
                .setAction("Action", null).show()
        }
    }

    override fun onCreateOptionsMenu(menu: Menu): Boolean {
        // Inflate the menu; this adds items to the action bar if it is present.
        menuInflater.inflate(R.menu.menu_main, menu)
        return true
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        return when (item.itemId) {
            R.id.action_settings -> true
            else -> super.onOptionsItemSelected(item)
        }
    }
}

# let's use the flutter folder
cd flutter
# create a plugin project, set its namespace and its name
flutter create --template=plugin --org com.robertohuertas rusty_flutter_lib
# now you'll have a folder called rusty_flutter_lib inside the flutter folder
# for convenience, we'll move everything to the parent directory (flutter)
# this last step is completely optional.
mv rusty_flutter_lib/{.,}* .
rm -rf rusty_flutter_lib

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    // this is the line to add including the directory holding our .aar package:
    implementation fileTree(include: '*.aar', dir: 'rusty-android-lib-release')
}

package com.robertohuertas.rusty_flutter_lib

// importing the Android library
import com.robertohuertas.rusty_android_lib.*

import io.flutter.plugin.common.MethodCall
import io.flutter.plugin.common.MethodChannel
import io.flutter.plugin.common.MethodChannel.MethodCallHandler
import io.flutter.plugin.common.MethodChannel.Result
import io.flutter.plugin.common.PluginRegistry.Registrar

class RustyFlutterLibPlugin: MethodCallHandler {
  companion object {
    @JvmStatic
    fun registerWith(registrar: Registrar) {
      val channel = MethodChannel(registrar.messenger(), "rusty_flutter_lib")
      channel.setMethodCallHandler(RustyFlutterLibPlugin())
      // dynamically loading the android library
      loadRustyLib()
    }
  }

  override fun onMethodCall(call: MethodCall, result: Result) {
    when {
      call.method == "getPlatformVersion" -> result.success("Android ${android.os.Build.VERSION.RELEASE}")
      call.method == "getHello" -> {
        val to = call.argument<String>("to")
          if (to == null) {
            result.success("No to parameter found")
          } else {
            // we're using the helloDirect function here
            // but you could also use the hello function, too.
            val res = helloDirect(to)
            result.success(res)
          }
      }
      else -> result.notImplemented()
    }
  }
}

# copy the header to the Classes folder
cp ../rust/rustylib.h ios/Classes
# create a new libs folder
mkdir ios/libs
# copy the universal library into the libs folder
cp ../rust/target/universal/release/librustylib.a ios/libs/

s.ios.vendored_library = 'libs/librustylib.a'

import Flutter
import UIKit

public class SwiftRustyFlutterLibPlugin: NSObject, FlutterPlugin {
  public static func register(with registrar: FlutterPluginRegistrar) {
    let channel = FlutterMethodChannel(name: "rusty_flutter_lib", binaryMessenger: registrar.messenger())
    let instance = SwiftRustyFlutterLibPlugin()
    registrar.addMethodCallDelegate(instance, channel: channel)
  }

  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    if (call.method == "getPlatformVersion") {
      result("iOS " + UIDevice.current.systemVersion)
    } else if (call.method == "hello") {
      let res = hello("Rob")
      let sr = String(cString: res!)
      hello_release(UnsafeMutablePointer(mutating: res))
      result(sr)
    } else {
      result("No method found")
    }
  }
}

static Future<String> hello({to: String}) async {
  final String greetings = await _channel.invokeMethod('hello', {'to': to});
  return greetings;
}

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

import 'package:flutter/services.dart';
import 'package:rusty_flutter_lib/rusty_flutter_lib.dart';

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _platformVersion = 'Unknown';
  String _greeting = '';

  @override
  void initState() {
    super.initState();
    initPlatformState();
  }

  // Platform messages are asynchronous, so we initialize in an async method.
  Future<void> initPlatformState() async {
    String platformVersion;
    String greeting;
    // Platform messages may fail, so we use a try/catch PlatformException.
    try {
      platformVersion = await RustyFlutterLib.platformVersion;
      greeting = await RustyFlutterLib.hello(to: 'Rob');
    } on PlatformException {
      platformVersion = 'Failed to get platform version.';
      greeting = 'Failed to get hello';
    }

    // If the widget was removed from the tree while the asynchronous platform
    // message was in flight, we want to discard the reply rather than calling
    // setState to update our non-existent appearance.
    if (!mounted) return;

    setState(() {
      _platformVersion = platformVersion;
      _greeting = greeting;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Plugin example app'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text(
                _greeting,
                style: TextStyle(fontSize: 20),
              ),
              Text('Running on: $_platformVersion\n'),
            ],
          ),
        ),
      ),
    );
  }
}

pod install

version: 1.0.0+1