Version: 0.2.0

Flutter Setup

This tutorial will guide you through integrating the iOS bindings and Android bindings into an Flutter) project. Before you begin, make sure you’ve completed the "Getting Started - 3. Mopro build" process with selecting iOS platform and Android platform and have the MoproiOSBindings and MoproAndroidBindings folder ready:

Flutter is a framework for building natively compiled, multi-platform applications from a single codebase.

In this tutorial, you will learn how to create a native Mopro module on both Android and iOS simulators/devices.

info

In this example, we use Circom circuits and their corresponding .zkey files. The process is similar for other provers.

0. Prerequisites

Install Flutter

If Flutter is not already installed, you can follow the official Flutter installation guide for your operating system.
Check Flutter Environment

After installing Flutter, verify that your development environment is properly set up by running the following command in your terminal:
```
flutter doctor
```
This command will identify any missing dependencies or required configurations.
Install Flutter Dependencies

Navigate to the root directory of the project in your terminal and run:
```
flutter pub get
```
This will install the necessary dependencies for the project.
Create a Flutter App

If you already have Flutter app, you can skip this step. If you don’t have a one, follow this tutorial to set one up. Or run
```
flutter create <YOUR_FLUTTER_APP>
```

1. Creating a Native Module

Create a plugin to integrate Mopro bindings into your project.

flutter create mopro_flutter_plugin -t plugins

info

To learn more about flutter packages/plugins, please refer to Flutter - Developing packages & plugins

To support both iOS and Android, we need to build native modules for each platform.

Start by adding the necessary platforms to the plugin:

Navigate to the mopro_flutter_plugin directory

cd mopro_flutter_plugin

and run the following command:

flutter create -t plugin --platforms ios,android .

2. Implement the module on iOS

info

Please refer to flutter-app to see the latest update.

2-1 Use a framework

Get the MoproiOSBindings from mopro build.

info
See Getting Started

Place the MoproiOSBindings/mopro.swift file to mopro_flutter_plugin/ios/Classes/mopro.swift
Place the MoproiOSBindings/MoproBindings.xcframework file to mopro_flutter_plugin/ios/MoproBindings.xcframework.
The structure will look like

  flutter-app/
  ├── ...
  └── mopro_flutter_plugin
        └── ios/
        ├── ...
        ├── Classes/
        │   ├── ...
        │   └── mopro.swift
        └── MoproBindings.xcframework/...

Bundle the bindings in mopro_flutter_plugin/ios/mopro_flutter_plugin.podspec

/mopro_flutter_plugin/ios/mopro_flutter_plugin.podspec
    ...
    s.source_files = 'Classes/**/*'
    s.vendored_frameworks = 'MoproBindings.xcframework'
    s.preserve_paths = 'MoproBindings.xcframework/**/*'
    ...

2-2 Create convertible types for Javascript library with swift.

Create these types in the file: mopro_flutter_plugin/ios/Classes/MoproFlutterPlugin.swift

/mopro_flutter_plugin/ios/Classes/MoproFlutterPlugin.swift
class FlutterG1 {
    let x: String
    let y: String
    let z: String

    init(x: String, y: String, z: String) {
       self.x = x
       self.y = y
       self.z = z
    }
}
// ...

note

See the full implementation here: MoproFlutterPlugin.swift

Define helper functions to bridge types between the Mopro bindings and the Flutter framework:

/mopro_flutter_plugin/ios/Classes/MoproFlutterPlugin.swift
 // convert the mopro proofs to be exposed to Flutter framework
 func convertCircomProof(res: CircomProofResult) -> [String: Any] {
    let g1a = FlutterG1(x: res.proof.a.x, y: res.proof.a.y, z: res.proof.a.z)
    // ...
 }

 // convert the Flutter proofs to be used in mopro bindings
 func convertCircomProofResult(proof: [String: Any]) -> CircomProofResult {
    let proofMap = proof["proof"] as! [String: Any]
    let aMap = proofMap["a"] as! [String: String]
    let g1a = G1(x: aMap["x"] ?? "0", y: aMap["y"] ?? "0", z: aMap["z"] ?? "1")
    // ...
 }

note

See the full implementation here: MoproFlutterPlugin.swift

Define the native module API. See the Writing custom platform-specific code for details.

/mopro_flutter_plugin/ios/Classes/MoproFlutterPlugin.swift
public class MoproFlutterPlugin: NSObject, FlutterPlugin {
  public static func register(with registrar: FlutterPluginRegistrar) {
    let channel = FlutterMethodChannel(name: "mopro_flutter_plugin", binaryMessenger: registrar.messenger())
    let instance = MoproFlutterPlugin()
    registrar.addMethodCallDelegate(instance, channel: channel)
  }

  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
      switch call.method {
         case "generateCircomProof":
         guard let args = call.arguments as? [String: Any],
            let zkeyPath = args["zkeyPath"] as? String,
            let inputs = args["inputs"] as? String,
            let proofLib = args["proofLib"] as? ProofLib
         else {
            result(FlutterError(code: "ARGUMENT_ERROR", message: "Missing arguments", details: nil))
            return
         }

         do {
            // Call the function from mopro.swift
            let proofResult = try generateCircomProof(
               zkeyPath: zkeyPath, circuitInputs: inputs, proofLib: proofLib)
            let resultMap = convertCircomProof(res: proofResult)

            // Return the proof and inputs as a map supported by the StandardMethodCodec
            result(resultMap)
         } catch {
            result(
               FlutterError(
                  code: "PROOF_GENERATION_ERROR", message: "Failed to generate proof",
                  details: error.localizedDescription))
         }
      }
   }
   // ...
}

info

See the full implementation here: MoproFlutterPlugin.swift

3. Implement the module on Android

3-1 Add dependency for jna in the file `build.gradle`.

/mopro_flutter_plugin/android/build.gradle
dependencies {
  implementation("net.java.dev.jna:jna:5.13.0@aar")
}

3-2 Include Mopro bindings in the native Android module

Get the MoproAndroidBindings from mopro build.

info
See Getting Started

Move the jniLibs directory to mopro_flutter_plugin/android/src/main.
And move uniffi directory to mopro_flutter_plugin/android/src/main/kotlin.
The folder structure should be as follows:

flutter-app/
├── ...
└── mopro_flutter_plugin
      └── android/
      ├── ...
      └── src/
            ├── ...
            └── main/
                  ├── ...
                  ├── jniLibs/...
                  └── kotlin/
                        ├── ...
                        └── uniffi/mopro/mopro.kt

3-3 Create convertible types for Javascript library with kotlin.

Create these types in the file: mopro_flutter_plugin/android/src/main/kotlin/com/example/mopro_flutter_plugin/MoproFlutterPlugin.kt
/mopro_flutter_plugin/android/src/main/kotlin/com/example/mopro_flutter_plugin/MoproFlutterPlugin.kt
```
class FlutterG1(x: String, y: String, z: String) {
   val x = x
   val y = y
   val z = z
}
// ...
```
note
See the full implementation here: MoproFlutterPlugin.kt

Define helper functions to bridge types between the Mopro bindings and the Flutter framework:

/mopro_flutter_plugin/android/src/main/kotlin/com/example/mopro_flutter_plugin/MoproFlutterPlugin.kt
 // convert the mopro proofs to be exposed to Flutter framework
 fun convertCircomProof(res: CircomProofResult): Map<String, Any> {
    val g1a = FlutterG1(res.proof.a.x, res.proof.a.y, res.proof.a.z)
    // ...
 }

 // convert the Flutter proofs to be used in mopro bindings
 fun convertCircomProofResult(proofResult: Map<String, Any>): CircomProofResult {
    val proofMap = proofResult["proof"] as Map<String, Any>
    val aMap = proofMap["a"] as Map<String, Any>
    val g1a = G1(
       aMap["x"] as String,
       aMap["y"] as String,
       aMap["z"] as String
    )
    // ...
 }

note

See the full implementation here: MoproFlutterPlugin.kt

Define the native module API. See the Writing custom platform-specific code for details.

class MoproFlutterPlugin: FlutterPlugin, MethodCallHandler {
  /// The MethodChannel that will the communication between Flutter and native Android
  ///
  /// This local reference serves to register the plugin with the Flutter Engine and unregister it
  /// when the Flutter Engine is detached from the Activity
  private lateinit var channel : MethodChannel

  override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
    channel = MethodChannel(flutterPluginBinding.binaryMessenger, "mopro_flutter_plugin")
    channel.setMethodCallHandler(this)
  }

  override fun onMethodCall(call: MethodCall, result: Result) {
      if (call.method == "generateCircomProof") {
         val zkeyPath = call.argument<String>("zkeyPath") ?: return result.error(
            "ARGUMENT_ERROR",
            "Missing zkeyPath",
            null
         )

         val inputs =
            call.argument<String>("inputs") ?: return result.error(
               "ARGUMENT_ERROR",
               "Missing inputs",
               null
            )

         val proofLibIndex = call.argument<Int>("proofLib") ?: return result.error(
            "ARGUMENT_ERROR",
            "Missing proofLib",
            null
         )

         val proofLib = if (proofLibIndex == 0) ProofLib.ARKWORKS else ProofLib.RAPIDSNARK

         val res = generateCircomProof(zkeyPath, inputs, proofLib)
         val resultMap = convertCircomProof(res)


         result.success(resultMap)
      // ...
      } else {
         result.notImplemented()
      }
  }
   // ...
}

note

See the full implementation here: MoproFlutterPlugin.kt

4. Define Dart APIs

info

Please refer to flutter-app to see the latest update.

Define the types for the native module. Add the following types in the file mopro_flutter_plugin/lib/mopro_types.dart:

/mopro_flutter_plugin/lib/mopro_types.dart
import 'dart:typed_data';

class G1Point {
   final String x;
   final String y;
   final String z;

   G1Point(this.x, this.y, this.z);

   @override
   String toString() {
      return "G1Point(\nx: $x, \ny: $y, \nz: $z)";
   }
}

enum ProofLib { arkworks, rapidsnark }

// ...

note

See the full implementation here: mopro_types.dart

Add the native module's API functions in these files.

mopro_flutter_plugin/lib/mopro_flutter_plugin_method_channel.dart
class MethodChannelMoproFlutterPlugin extends MoproFlutterPluginPlatform {
   /// The method channel used to interact with the native platform.
   @visibleForTesting
   final methodChannel = const MethodChannel('mopro_flutter_plugin');

   @override
   Future<CircomProofResult?> generateCircomProof(
      String zkeyPath, String inputs, ProofLib proofLib) async {
      final proofResult = await methodChannel
        .invokeMethod<Map<Object?, Object?>>('generateCircomProof', {
            'zkeyPath': zkeyPath,
            'inputs': inputs,
            'proofLib': proofLib.index,
         });

      if (proofResult == null) {
         return null;
      }

      var circomProofResult = CircomProofResult.fromMap(proofResult);
      return circomProofResult;
   }
   // ...
}

note

See the full implementation here: mopro_flutter_method_channel.dart

mopro_flutter_plugin/lib/mopro_flutter_plugin_platform_interface.dart
abstract class MoproFlutterPluginPlatform extends PlatformInterface {
   //...
   Future<CircomProofResult?> generateCircomProof(
      String zkeyPath, String inputs, ProofLib proofLib) {
         throw UnimplementedError('generateCircomProof() has not been implemented.');
   }
   //...
}

note

See the full implementation here: mopro_flutter_platform_interface.dart

mopro_flutter_plugin/lib/mopro_flutter_plugin.dart
class MoproFlutterPlugin {
   Future<String> copyAssetToFileSystem(String assetPath) async {
      // Load the asset as bytes
      final byteData = await rootBundle.load(assetPath);
      // Get the app's document directory (or other accessible directory)
      final directory = await getApplicationDocumentsDirectory();
      //Strip off the initial dirs from the filename
      assetPath = assetPath.split('/').last;

      final file = File('${directory.path}/$assetPath');

      // Write the bytes to a file in the file system
      await file.writeAsBytes(byteData.buffer.asUint8List());

      return file.path; // Return the file path
   }

   Future<CircomProofResult?> generateCircomProof(
      String zkeyFile, String inputs, ProofLib proofLib) async {
      return await copyAssetToFileSystem(zkeyFile).then((path) async {
         return await MoproFlutterPlatform.instance
            .generateCircomProof(path, inputs, proofLib);
      });
   }
   //...
}

note

See the full implementation here: mopro_flutter.dart

5. Use the plugin

Follow the steps below to integrate mopro plugin.

Add the plugin to pubspec.yaml as a dependency:

---
dependencies:
flutter:
    sdk: flutter
mopro_flutter_plugin:
    path: ./mopro_flutter_plugin

Copy keys in the assets folder like this

flutter-app/
├── ...
├── assets/multiplier2_final.zkey
└── lib/main.dart

and update pubspec.yaml

flutter:
    assets:
        - assets/multiplier2_final.zkey

Generate proofs in the app

import 'package:mopro_flutter_plugin/mopro_flutter_plugin.dart';
import 'package:mopro_flutter_plugin/mopro_types.dart';

final _moproFlutterPlugin = MoproFlutterPlugin();

var inputs = '{"a":["3"],"b":["5"]}';
var proofResult = await _moproFlutterPlugin.generateCircomProof(
   "assets/multiplier2_final.zkey",
   inputs,
   ProofLib.arkworks
);

6. Customizing the zKey

Place your .zkey file in your app's assets folder and remove the example file assets/multiplier2_final.zkey. If your .zkey has a different file name, don't forget to update the asset definition in your app's pubspec.yaml:
```
assets:
-  - assets/multiplier2_final.zkey
+  - assets/your_new_zkey_file.zkey
```

Load the new .zkey file in your Dart code by updating the file path in lib/main.dart:

var inputs = '{"a":["3"],"b":["5"]}';
- proofResult = await _moproFlutterPlugin.generateCircomProof("assets/multiplier2_final.zkey", inputs, ProofLib.arkworks);
+ proofResult = await _moproFlutterPlugin.generateCircomProof("assets/your_new_zkey_file.zkey", inputs, ProofLib.arkworks);

Don't forget to modify the input values for your specific case!

7. What's next

Update your ZK circuits as needed. After making changes, be sure to run:
```
mopro build
mopro update --dest ../MyFlutterApp
```
warning
mopro update only works if the Android project was created within the Rust project directory during mopro init. Otherwise, you can manually update the bindings by following Step 2-1 and Step 3-2.
This ensures the bindings are regenerated and reflect your latest updates.
Build your mobile app frontend according to your business logic and user flow.
Expose additional Rust functionality: If a function is missing in Swift, Kotlin, React Native, or Flutter, you can:
- Add the required Rust crate in Cargo.toml
- Annotate your function with #[uniffi::export] (See the Rust setup guide for details).
  Once exported, the function will be available across all supported platforms.

⚠️ Error when running `flutter run --release`

If you see an error like this:

E/AndroidRuntime(17363): java.lang.UnsatisfiedLinkError: Can't obtain peer field ID for class com.sun.jna.Pointer
...

This happens because UniFFI relies on JNA, which is not compatible with code shrinking or obfuscation enabled in Android release builds.

See: Android code shrinking
See: UniFFI Kotlin JNA requirements

✅ Solution

To fix this, disable code shrinking in your Android build.gradle for release builds:

android {
    buildTypes {
        release {
            minifyEnabled false       // Disable code shrinking & obfuscation
            shrinkResources false     // Optional: also disable resource shrinking
        }
    }
}

After applying this change, you should be able to run:

flutter run --release

without hitting the JNA-related crash.

Flutter Setup

0. Prerequisites​

1. Creating a Native Module​

2. Implement the module on iOS​

2-1 Use a framework​

2-2 Create convertible types for Javascript library with swift.​

3. Implement the module on Android​

3-1 Add dependency for jna in the file build.gradle.​

3-2 Include Mopro bindings in the native Android module​

3-3 Create convertible types for Javascript library with kotlin.​

4. Define Dart APIs​

5. Use the plugin​

6. Customizing the zKey​

7. What's next​

⚠️ Error when running flutter run --release​

0. Prerequisites

1. Creating a Native Module

2. Implement the module on iOS

2-1 Use a framework

2-2 Create convertible types for Javascript library with swift.

3. Implement the module on Android

3-1 Add dependency for jna in the file `build.gradle`.

3-2 Include Mopro bindings in the native Android module

3-3 Create convertible types for Javascript library with kotlin.

4. Define Dart APIs

5. Use the plugin

6. Customizing the zKey

7. What's next

⚠️ Error when running `flutter run --release`