Nitrogen

Nitrogen is Nitro's code-generator. It parses TypeScript code using an AST parser to generate native interfaces from TypeScript definitions.

Math.nitro.ts

interface Math extends HybridObject {
  readonly pi: number
  add(a: number, b: number): number
}

HybridMathSpec.swift (generated)

protocol HybridMathSpec: HybridObjectSpec {
  var pi: Double { get }
  func add(a: Double, b: Double) -> Double
}

When HybridMathSpec is not implemented properly on the native side (e.g. if add(..) is missing, or if a type is incorrect), the app will not compile, which ensures full type-safety and null-safety at compile-time.

Nitrogen is optional

Nitrogen is a fully optional CLI that does some of the work for you. You can also build Nitro Modules and create Hybrid Objects without nitrogen, by just calling the registerHybrids method yourself.

Who uses Nitrogen?

Nitrogen should be used by library-authors, and generated specs should be committed to the repository/package.

If you build an app that uses libraries built with Nitro, you do not need to run nitrogen yourself.

Configuration

Nitrogen should be installed as a dev-dependency in the Nitro Module (library).

npm

npm i nitro-codegen --save-dev

yarn

yarn add nitro-codegen -D

pnpm

pnpm add nitro-codegen -D

bun

bun i nitro-codegen -d

warning

Nitrogen is currently named nitro-codegen instead of nitrogen on npm.

Each Nitro Module needs to have a nitro.json configuration file.

Create a nitro.json file in the root directory of your Nitro Module (next to package.json), and add the following content:

nitro.json

{
  "cxxNamespace": ["math"],
  "ios": {
    "iosModuleName": "NitroMath"
  },
  "android": {
    "androidNamespace": ["math"],
    "androidCxxLibName": "NitroMath"
  },
  "autolinking": {}
}

Tweak your module name and namespaces as needed.

Usage

Nitrogen parses all TypeScript files that end in .nitro.ts.

1. Write TypeScript specs

For example, let's create Math.nitro.ts:

Math.nitro.ts

import { type HybridObject } from 'react-native-nitro-modules'

interface Math extends HybridObject<{ ios: 'swift', android: 'kotlin' }> {
  add(a: number, b: number): number
}

2. Generate native specs

Now run nitrogen:

npm

npm run nitro-codegen

yarn

yarn nitro-codegen

pnpm

pnpm nitro-codegen

bun

bun nitro-codegen

warning

Nitrogen is currently named nitro-codegen instead of nitrogen on npm.

This will generate a single C++ interface by default, which goes into ./nitrogen/generated/:

🔧  Loading nitro.json config...
🚀  Nitrogen runs at ~/Projects/nitro/example/dummy
    🔍  Nitrogen found 1 spec in ~/Projects/nitro/example/dummy
⏳  Parsing Math.nitro.ts...
    ⚙️  Generating specs for HybridObject "Math"...
        shared: Generating C++ code...
⛓️   Setting up build configs for autolinking...
🎉  Generated 1/1 HybridObject in 0.6s!
💡  Your code is in ./nitrogen/generated
‼️  Added 8 files - you need to run `pod install`/sync gradle to update files!

Note: You should push the files in nitrogen/generated/ to git, and make sure those files are part of your npm package. This way your library will always ship a working package as a whole (including generated interfaces), and the user does not need to do anything else than to install your package.

3. Add generated sources to your library

All the generated sources (nitrogen/generated/) need to be part of your library's code - so we need to add it to the iOS/Android build files.

With the Nitro template

If you created a library using the Nitro Module template, your library already includes nitrogen's generated sources.

Manually

iOS

On iOS, you need to call add_nitrogen_files(...) from your library's .podspec. Put this after any s.source_files = calls:

load 'nitrogen/generated/ios/NitroExample+autolinking.rb'
add_nitrogen_files(s)

Android

On Android, you first need to add the autogenerated Java/Kotlin sources to your build.gradle. Put this at the top of your build.gradle, right after any other apply calls:

apply from: '../nitrogen/generated/android/NitroExample+autolinking.gradle'

Then, add the autogenerated C++ sources to your CMakeLists.txt. Put this somewhere after add_library(...):

include(${CMAKE_SOURCE_DIR}/../nitrogen/generated/android/NitroImage+autolinking.cmake)

4. Implement the Hybrid Objects

To implement Math now, you just need to implement the spec:

Swift

HybridMath.swift

class HybridMath : HybridMathSpec {
  var hybridContext = margelo.nitro.HybridContext()
  var memorySize: Int {
    return getSizeOf(self)
  }

  public func add(a: Double, b: Double) throws -> Double {
    return a + b
  }
}

Kotlin

HybridMath.kt

class HybridMath : HybridMathSpec() {
  override val memorySize: Long
    get() = 0L

  override fun add(a: Double, b: Double): Double {
    return a + b
  }
}

C++

HybridMath.hpp

class HybridMath: public HybridMathSpec {
public:
  double add(double a, double b) override;
};

HybridMath.cpp

double HybridMath::add(double a, double b) {
  return a + b;
}

5. Register the Hybrid Objects

Nitro needs to be able to initialize an instance of your Hybrid Object - so we need to tell it how to do that. In your nitro.json, register HybridMath in the "autolinking" section:

Swift/Kotlin

{
  ...
  "autolinking": {
    "Math": {
      "swift": "HybridMath",
      "kotlin": "HybridMath"
    }
  }
}

C++

{
  ...
  "autolinking": {
    "Math": {
      "cpp": "HybridMath"
    }
  }
}

Make sure HybridMath is default-constructible and scoped inside the correct namespace/package/file, then run Nitrogen.

5.1. Initialize Android (C++)

For Android, you also need to explicitly call initialize() in your JNI OnLoad function (most of the time this is inside cpp-adapter.cpp):

cpp-adapter.cpp

#include <jni.h>
#include "NitroMathOnLoad.hpp"

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void*) {
  return margelo::nitro::math::initialize(vm);
}

5.2. Initialize Android (Java/Kotlin)

Then, to make sure JNI_OnLoad is called, you need to load the C++ library somewhere from your Java/Kotlin code.

Nitro Module (library)

Inside NitroMathPackage.java, add:

public class NitroMathPackage extends TurboReactPackage {
  // ...
  static {
    System.loadLibrary("NitroMath");
  }
}

The NitroMathPackage will be initialized by React Native's own autolinking system. We just create an empty dummy Package which we use to initialize Nitro instead.

App

Inside MainApplication.kt, add:

class MainApplication {
  // ...
  companion object {
    init {
      System.loadLibrary("NitroMath")
    }
  }
}

5.3. (Optional) ProGuard

If you are using ProGuard on Android, make sure to add a @DoNotStrip annotation above your HybridMath class (and constructor) so Nitro can construct it from C++ in release builds.

6. Initialize the Hybrid Objects

And finally, to initialize HybridMath from JS you just need to call createHybridObject:

export const MathModule = NitroModules.createHybridObject<Math>("Math")
const result = MathModule.add(5, 7)