Getting started with Nitro

November 6, 2024 • 15 min read

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Hey there! 👋🤓

If you're a React Native developer and you follow enough RN engineers on X, you've probably heard about Nitro. Nitro Modules are a new way for building cross-platform libraries for React Native. It's an alternative to the pre-existing approachs for building libraries using Native modules, Turbo modules or Expo modules.

It recently became stable and I thought it would be a good time to experiment with.

What are the benefits?

  • Statically typed: TypeScript for the win
  • JSI-based: it's Britney Bridgeless, bitch 💁‍♀️
  • C++ codegen: thin runtime layer close to zero overhead
  • Fast performance: multiply the speed of execution by 5~15 folds 🏎️💨

Check out the Nitro benchmarks to see how it compares to the other approaches.

What are we building?

I want to build a module that's simple yet useful.

Note

I'm not an experienced library author. This is gonna be my first time building this kind of project, so I'll be learning along the way. I apologize in advance for any mistakes.

I've chosen to build a module that opens a website in an in-app-browser experience, heavily inspired by the react-native-inappbrowser-reborn package, commonly referred by the IAB acronym.

🍎 For iOS, we'll use SFSafariViewController primitive.

🤖 For Android, we'll use ChromeCustomTabs primitive.

Setting up the project

You can use Nitro in an existing app or within a library. I'll be using a new project for this experiment. Luckily, there's a template that makes it easier to get started.

Make sure you replace all <<foo>> with the correct values. Remember to also rename folders.

Defining specs

We start by defining the specs for our module. These are simple TypeScript interfaces that describe the native module and its methods. Nitrogen will generate the native and C++ code for autolinking based on these specs.

For iOS, you'll have src/specs/SFSafariViewController.nitro.ts

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

export interface SFSafariViewController 
    extends HybridObject<{ ios: 'swift' }> {
  present(params: SFSafariViewControllerPresentParams): void
}

export interface SFSafariViewControllerPresentParams {
  url: string
}

For Android, you'll have src/specs/ChromeCustomTabs.nitro.ts

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

export interface ChromeCustomTabs 
    extends HybridObject<{ android: 'kotlin' }> {
  launch(params: ChromeCustomTabsLaunchParams): void
}
export interface ChromeCustomTabsLaunchParams {
  url: string
}

Update nitro.json

Once specs are defined, you can update nitro.json to include them.

{
  "cxxNamespace": ["inappbrowser"],
  "ios": {
    "iosModulename": "NitroInAppBrowser"
  },
  "android": {
    "androidNamespace": ["inappbrowser"],
    "androidCxxLibName": "NitroInAppBrowser"
  },
  "autolinking": {
    "SFSafariViewController": { // name used to register the module within registry, must match the name in the specs
      "swift": "HybridSFSafariViewController" // name here must match the class name in the swift module/file
    },
    "ChromeCustomTabs": { // name used to register the module within registry, must match the name in the specs
      "kotlin": "HybridChromeCustomTabs" // name here must match the class name in the kotlin module/file
    }
  },
  "ignorePaths": ["node_modules"]
}

Example

To test your module, you will need a runnable application. You can create one in an /example folder using either react-native-community/cli or expo-cli.

# using react-native-community/cli
npx @react-native-community/cli@latest init NitroInAppBrowserExample

# using expo-cli
npx create-expo-app@latest NitroInAppBrowserExample

Do some basic setup for the app to run (install pods, etc).

Install the local module in the example app. You can either use link:../ or file:../ version in package.json. Some links about using local modules:

I later found out that using link was causing issues with the autolinking process — so I recommend using the file strategy combined with a postinstall rm -rf strategy to avoid circular dependencies.

example/package.json

"scripts": {
  ...
  "postinstall": "./scripts/postinstall.sh"
},
"dependencies": {
  "react": "18.3.1",
  "react-native": "0.75.4",
  "react-native-in-app-browser": "file:../",
  "react-native-nitro-modules": "*"
},

example/scripts/postinstall.sh

#!/bin/bash

folders=(
  "./node_modules/react-native-in-app-browser/example"
  "./node_modules/react-native-in-app-browser/node_modules"
)

for folder in "${folders[@]}"; do
  if [ -d "$folder" ]; then
    echo "Removing $folder"
    rm -rf "$folder"
  else
    echo "Folder $folder does not exist, skipping"
  fi
done

echo "Post-install completed."
exit 0

Oh, you'll also need a custom metro.config.js to correctly resolve the local module.

example/metro.config.js

const { getDefaultConfig, mergeConfig } = require('@react-native/metro-config')
const path = require('path')
const escape = require('escape-string-regexp')
const exclusionList = require('metro-config/src/defaults/exclusionList')
const pak = require('../package.json')

const root = path.resolve(__dirname, '..')
const modules = Object.keys({ ...pak.peerDependencies })

/**
 * Metro configuration
 * https://facebook.github.io/metro/docs/configuration
 *
 * @type {import('metro-config').MetroConfig}
 */
const config = {
  watchFolders: [root],

  // We need to make sure that only one version is loaded for peerDependencies
  // So we block them at the root, and alias them to the versions in example's node_modules
  resolver: {
    blacklistRE: exclusionList(
      modules.map(
        (m) =>
          new RegExp(`^${escape(path.join(root, 'node_modules', m))}\\/.*$`)
      )
    ),

    extraNodeModules: modules.reduce((acc, name) => {
      acc[name] = path.join(__dirname, 'node_modules', name)
      return acc
    }, {}),
  },

  transformer: {
    getTransformOptions: async () => ({
      transform: {
        experimentalImportSupport: false,
        inlineRequires: true,
      },
    }),
  },
}

module.exports = mergeConfig(getDefaultConfig(__dirname), config)

That should be it! 🎉

Test it by running a local build of the example app and importing a mocked module from your local dependency.

Implement your native module

Again, I'm not a Swift or Kotlin expert 🤓

I'm getting help from Cursor, ChatGPT and some GitHub search to implement the native code. learning the language and concepts during the process...

Swift

It's time to dive into Apple's modern language. 🍎

I like to open the /example/ios folder in Xcode and run the app from there. You won't need to link the native module after doing changes because /examples/node_modules/react-native-in-app-browser is a symbolic link. 😉

Implement HybridSFSafariViewController.swift by adding inheritance from the C++ generated specs. XCode helps a lot with the autocompletion of missing methods.

You can see the spec enforces these two properties:

  • hybridContext: a property that handles the context in which this hybrid object operates. This is particularly useful when you need to maintain a consistent state or perform operations that require both JavaScript and native capabilities. It helps in bridging the gap between the two environments, ensuring that the object behaves correctly regardless of where it is being used.
  • memorySize: a property that defines its memory usage. Since Hybrid Objects are implemented in native code, the JavaScript runtime does not inherently know their memory footprint. By specifying memorySize, you can inform the JavaScript garbage collector about the actual memory usage of these objects. Can also be implemented as a function getMemorySize() -> Int.

Tip

It's safe to return 0 if you don't know the size, but it's recommended to estimate the actual size of native object if possible. On Swift, you can use the getSizeOf function to get the size of the object.

ios/HybridSFSafariViewController.swift

import Foundation
import SafariServices

class HybridSFSafariViewController: HybridSFSafariViewControllerSpec {
  var hybridContext = margelo.nitro.HybridContext()
  
  var memorySize: Int {
    return getSizeOf(self)
  }
  
  func present(params: SFSafariViewControllerPresentParams) throws -> Void {
    NSLog("HybridSFSafariViewController.present(url:%@) is being called", params.url)
    
    guard let nativeUrl = URL(string: params.url) else {
      throw NSError(domain: "HybridSFSafariViewController", code: 0, userInfo: [NSLocalizedDescriptionKey: "Invalid URL"])
    }
    
    let safariViewController = SFSafariViewController(url: nativeUrl)
    
    DispatchQueue.main.async {
      if let windowScene = UIApplication.shared.connectedScenes.first as? UIWindowScene,
         let window = windowScene.windows.first,
         let rootViewController = window.rootViewController {
        let topViewController = rootViewController.topMostViewController()
        topViewController.present(safariViewController, animated: true, completion: nil)
      } else {
        NSLog("Failed to find top view controller to present SFSafariViewController")
      }
    }
  }
}

In order for that to work, you'll also need some helper recursive extensions.

ios/UIViewController+Extensions.swift

import UIKit
import SafariServices

extension UIViewController {
  func topMostViewController() -> UIViewController {
    if let presented = self.presentedViewController {
      return presented.topMostViewController()
    }
    
    if let navigation = self as? UINavigationController {
      return navigation.visibleViewController?.topMostViewController() ?? navigation
    }
    
    if let tab = self as? UITabBarController {
      return tab.selectedViewController?.topMostViewController() ?? tab
    }
    
    return self
  }
}

Kotlin

Now, it's time to implement the Android side. 🤖

Implement HybridChromeCustomTabs.kt in a similar fashion, extending the C++ generated specs, and complying with the interface.

android/src/main/java/com/margelo/nitro/inappbrowser/HybridChromeCustomTabs.kt

package com.margelo.nitro.inappbrowser

import android.net.Uri
import android.util.Log
import androidx.browser.customtabs.CustomTabsIntent
import com.margelo.nitro.NitroModules

class HybridChromeCustomTabs : HybridChromeCustomTabsSpec() {
  companion object {
    const val TAG = "HybridChromeCustomTabs"
  }

  override val memorySize: Long
    get() = 0L

  private val applicationContext = NitroModules.applicationContext

  override fun launch(params: ChromeCustomTabsLaunchParams) {
    Log.d(TAG, "launch: ${params.url}")

    val customTabsIntent = CustomTabsIntent.Builder().build()

    try {
      val context = applicationContext?.currentActivity
      if (context == null) {
        Log.e(TAG, "Error launching Custom Tab: Context is null")
        return
      }

      val uri = Uri.parse(params.url)
      if (uri == null) {
        Log.e(TAG, "Error launching Custom Tab: Invalid URL")
        return
      }

      customTabsIntent.launchUrl(context, uri)
    } catch (e: Exception) {
      Log.e(TAG, "Error launching Custom Tab: ${e.message}", e)
    }
  }
}

Note that you can access the applicationContext from the NitroModules class. You don't need to register the kotlin class in NitroInAppBrowserPackage.java, that's automatically done by nitrogen autolinking combined with a static java call to System.loadLibrary("NitroInAppBrowser").

Use the module on the app

Let's turn our attention back to the JavaScript world.

We now can use the native functions within the React Native codebase by calling NitroModules.createHybridObject.

src/SFSafariViewController.ts

import { NitroModules } from 'react-native-nitro-modules'
import type { SFSafariViewController as SFSafariViewControllerInterface } from './specs/SFSafariViewController.nitro'

export const SFSafariViewController =
  NitroModules.createHybridObject<SFSafariViewControllerInterface>(
    'SFSafariViewController' // name used to register the module within registry, check `nitro.json`
  )

src/ChromeCustomTabs.ts

import { NitroModules } from 'react-native-nitro-modules'
import type { ChromeCustomTabs as ChromeCustomTabsInterface } from './specs/ChromeCustomTabs.nitro'

export const ChromeCustomTabs =
  NitroModules.createHybridObject<ChromeCustomTabsInterface>(
    'ChromeCustomTabs' // name used to register the module within registry, check `nitro.json`
  )

src/InAppBrowser.ts

import { Platform } from 'react-native'

import { SFSafariViewController } from './SFSafariViewController'
import { ChromeCustomTabs } from './ChromeCustomTabs'

interface InAppBrowserType {
  open: (url: string) => void
}

export const InAppBrowser: InAppBrowserType = {
  open: (url: string) => {
    const openNative = Platform.select({
      ios: () => SFSafariViewController.present({ url }),
      android: () => ChromeCustomTabs.launch({ url }),
    })

    if (!openNative) {
      throw new Error('InAppBrowser is not supported on this platform')
    }

    openNative()
  },
}

With that TypeScript implementation, you should be able to use the module in your app.

example/App.tsx

import { InAppBrowser } from 'react-native-in-app-browser'

function App() {
  return (
    <Button 
      title="Open in-app browser" 
      onPress={() => {
        InAppBrowser.open('https://reactnative.dev')
      }}
    />
  )
}

Here's a quick demo of the module in action:

Old arch vs new arch

While this post was being written, Meta released a new version of React Native (0.76) that ships with the new architecture by default. Nitro modules are compatible with both the new and old architecture. You can test it out by running the example app with the new arch enabled.

On Android, modify the android/gradle.properties file and turn off the newArchEnabled flag:

-newArchEnabled=true
+newArchEnabled=false

On iOS, you can reinstall the dependencies by running the command:

RCT_NEW_ARCH_ENABLED=0 bundle exec pod install

Next steps

Things I'd like to try:

  • Learn about properties getters and setters
  • Learn about events emitters and subscribers
  • Learn about promises and async/await
  • Build a Hybrid view component WKWebView

Here's the source code for the module if you want to check it out. Also available as an NPM package:

npx install @renanmav/react-native-in-app-browser

Resources

Other nitro modules in the wild:

Podcasts and videos about Nitro: