Using Codegen

This guide teaches how to:

• Configure Codegen.
• Invoke it manually for each platform.

It also describes the generated code.

Prerequisites

You always need a React Native app to generate the code properly, even when invoking the Codegen manually.

The Codegen process is tightly coupled with the build of the app, and the scripts are located in the react-native NPM package.

For the sake of this guide, create a project using the React Native CLI as follows:

npx @react-native-community/cli@latest init SampleApp --version 0.76.0

Codegen is used to generate the glue-code for your custom modules or components. See the guides for Turbo Native Modules and Fabric Native Components for more details on how to create them.

Configuring Codegen

Codegen can be configured in your app by modifying the package.json file. Codegen is controlled by a custom field called codegenConfig.

package.json

  "codegenConfig": {
    "name": "<SpecName>",
    "type": "<types>",
    "jsSrcsDir": "<source_dir>",
    "android": {
      "javaPackageName": "<java.package.name>"
    },
    "ios": {
      "modulesConformingToProtocol": {
        "RCTImageURLLoader": [
          "<iOS-class-conforming-to-RCTImageURLLoader>",
          // example from react-native-camera-roll: https://github.com/react-native-cameraroll/react-native-cameraroll/blob/8a6d1b4279c76e5682a4b443e7a4e111e774ec0a/package.json#L118-L127
          // "RNCPHAssetLoader",
        ],
        "RCTURLRequestHandler": [
          "<iOS-class-conforming-to-RCTURLRequestHandler>",
          // example from react-native-camera-roll: https://github.com/react-native-cameraroll/react-native-cameraroll/blob/8a6d1b4279c76e5682a4b443e7a4e111e774ec0a/package.json#L118-L127
          // "RNCPHAssetUploader",
        ],
        "RCTImageDataDecoder": [
          "<iOS-class-conforming-to-RCTImageDataDecoder>",
          // we don't have a good example for this, but it works in the same way. Pass the name of the class that implements the RCTImageDataDecoder. It must be a Native Modules.
        ]
      }
    }
  },

You can add this snippet to your app and customize the various fields:

• name: This is the name that will be used to create files containing your specs. As a convention, It should have the suffix Spec, but this is not mandatory.
• type: the type of code we need to generate. Allowed values are modules, components, all.
  • modules: use this value if you only need to generate code for Turbo Native Modules.
  • components: use this value if you only need to generate code for Native Fabric Components.
  • all: use this value if you have a mixture of components and modules.
• jsSrcsDir: this is the root folder where all your specs live.
• android.javaPackageName: this is an Android specific setting to let Codegen generate the files in a custom package.
• ios: the ios field is an object that can be used by app developers and library maintainers to provide some advanced functionalities. All the following fields are optional.
  • ios.modulesConformingToProtocol: React Native offers some protocols that native modules can implement to customize some behaviors. These fields allow you to define the list of modules that conform to those protocols. These modules will be injected in the React Native runtime when the app starts.
    • ios.modulesConformingToProtocol.RCTImageURLLoader: list of iOS native module that implements the RCTImageURLLoader protocol. You need to pass the class names of iOS classes that implements the RCTImageURLLoader. They must be Native Modules.
    • ios.modulesConformingToProtocol.RCTURLRequestHandler: list of iOS native module that implements the RCTURLRequestHandler protocol. You need to pass the class names of iOS classes that implements the RCTURLRequestHandler. They must be Native Modules.
    • ios.modulesConformingToProtocol.RCTImageDataDecoder: list of iOS native module that implements the RCTImageDataDecoder protocol. You need to pass the class names of iOS classes that implements the RCTImageDataDecoder. They must be Native Modules.

When Codegen runs, it searches among all the dependencies of the app, looking for JS files that respects some specific conventions, and it generates the required code:

• Turbo Native Modules require that the spec files are prefixed with Native. For example, NativeLocalStorage.ts is a valid name for a spec file.
• Native Fabric Components require that the spec files are suffixed with NativeComponent. For example, WebViewNativeComponent.ts is a valid name for a spec file.

Running Codegen

The rest of this guide assumes that you have a Native Turbo Module, a Native Fabric Component or both already set up in your project. We also assume that you have valid specification files in the jsSrcsDir specified in the package.json.

Android

Codegen for Android is integrated with the React Native Gradle Plugin (RNGP). The RNGP contains a task that can be invoked that reads the configurations defined in the package.json file and execute Codegen. To run the gradle task, first navigate inside the android folder of your project. Then run:

./gradlew generateCodegenArtifactsFromSchema

This task invokes the generateCodegenArtifactsFromSchema command on all the imported projects of the app (the app and all the node modules which are linked to it). It generates the code in the corresponding node_modules/<dependency> folder. For example, if you have a Fabric Native Component whose Node module is called my-fabric-component, the generated code is located in the SampleApp/node_modules/my-fabric-component/android/build/generated/source/codegen path. For the app, the code is generated in the android/app/build/generated/source/codegen folder.

The Generated Code

After running the gradle command above, you will find the codegen code in the SampleApp/android/app/build folder. The structure will look like this:

build
└── generated
    └── source
        └── codegen
            ├── java
            │   └── com
            │       ├── facebook
            │       │   └── react
            │       │       └── viewmanagers
            │       │           ├── <nativeComponent>ManagerDelegate.java
            │       │           └── <nativeComponent>ManagerInterface.java
            │       └── sampleapp
            │           └── NativeLocalStorageSpec.java
            ├── jni
            │   ├── <codegenConfig.name>-generated.cpp
            │   ├── <codegenConfig.name>.h
            │   ├── CMakeLists.txt
            │   └── react
            │       └── renderer
            │           └── components
            │               └── <codegenConfig.name>
            │                   ├── <codegenConfig.name>JSI-generated.cpp
            │                   ├── <codegenConfig.name>.h
            │                   ├── ComponentDescriptors.cpp
            │                   ├── ComponentDescriptors.h
            │                   ├── EventEmitters.cpp
            │                   ├── EventEmitters.h
            │                   ├── Props.cpp
            │                   ├── Props.h
            │                   ├── ShadowNodes.cpp
            │                   ├── ShadowNodes.h
            │                   ├── States.cpp
            │                   └── States.h
            └── schema.json

The generated code is split in two folders:

• java which contains the platform specific code
• jni which contains the C++ code required to let JS and Java interact correctly.

In the java folder, you can find the Fabric Native component generated code in the com/facebook/viewmanagers subfolder.

• the <nativeComponent>ManagerDelegate.java contains the methods that the ViewManager can call on the custom Native Component
• the <nativeComponent>ManagerInterface.java contains the interface of the ViewManager.

In the folder whose name was set up in the codegenConfig.android.javaPackageName, instead, you can find the abstract class that a Turbo Native Module has to implement to carry out its tasks.

In the jni folder, finally, there is all the boilerplate code to connect JS to Android.

• <codegenConfig.name>.h this contains the interface of your custom C++ Turbo Native Modules.
• <codegenConfig.name>-generated.cpp this contains the glue code of your custom custom C++ Turbo Native Modules.
• react/renderer/components/<codegenConfig.name>: this folder contains all the glue-code required by your custom component.

This structure has been generated by using the value all for the codegenConfig.type field. If you use the value modules, expect to see no react/renderer/components/ folder. If you use the value components, expect not to see any of the other files.

iOS

Codegen for iOS relies on some Node scripts that are invoked during the build process. The scripts are located in the SampleApp/node_modules/react-native/scripts/ folder.

The main script is the generate-codegen-artifacts.js script. To invoke the script, you can run this command from the root folder of your app:

node node_modules/react-native/scripts/generate-codegen-artifacts.js

Usage: generate-codegen-artifacts.js -p [path to app] -t [target platform] -o [output path]

Options:
      --help            Show help                                      [boolean]
      --version         Show version number                            [boolean]
  -p, --path            Path to the React Native project root.        [required]
  -t, --targetPlatform  Target platform. Supported values: "android", "ios",
                        "all".                                        [required]
  -o, --outputPath      Path where generated artifacts will be output to.

where:

• --path is the path to the root folder of your app.
• --outputPath is the destination where Codegen will write the generated files.
• --targetPlatform is the platform you'd like to generate the code for.

The Generated Code

Running the script with these arguments:

node node_modules/react-native/scripts/generate-codegen-artifacts.js \
    --path . \
    --outputPath ios/ \
    --targetPlatform ios

Will generate these files in the ios/build folder:

build
└── generated
    └── ios
        ├── <codegenConfig.name>
        │   ├── <codegenConfig.name>-generated.mm
        │   └── <codegenConfig.name>.h
        ├── <codegenConfig.name>JSI-generated.cpp
        ├── <codegenConfig.name>JSI.h
        ├── FBReactNativeSpec
        │   ├── FBReactNativeSpec-generated.mm
        │   └── FBReactNativeSpec.h
        ├── FBReactNativeSpecJSI-generated.cpp
        ├── FBReactNativeSpecJSI.h
        ├── RCTModulesConformingToProtocolsProvider.h
        ├── RCTModulesConformingToProtocolsProvider.mm
        └── react
            └── renderer
                └── components
                    └── <codegenConfig.name>
                        ├── ComponentDescriptors.cpp
                        ├── ComponentDescriptors.h
                        ├── EventEmitters.cpp
                        ├── EventEmitters.h
                        ├── Props.cpp
                        ├── Props.h
                        ├── RCTComponentViewHelpers.h
                        ├── ShadowNodes.cpp
                        ├── ShadowNodes.h
                        ├── States.cpp
                        └── States.h

Part of these generated files are used by React Native in the Core. Then there is a set of files which contains the same name you specified in the package.json codegenConfig.name field.

• <codegenConfig.name>/<codegenConfig.name>.h: this contains the interface of your custom iOS Turbo Native Modules.
• <codegenConfig.name>/<codegenConfig.name>-generated.mm: this contains the glue code of your custom iOS Turbo Native Modules.
• <codegenConfig.name>JSI.h: this contains the interface of your custom C++ Turbo Native Modules.
• <codegenConfig.name>JSI-generated.h: this contains the glue code of your custom custom C++ Turbo Native Modules.
• react/renderer/components/<codegenConfig.name>: this folder contains all the glue-code required by your custom component.

This structure has been generated by using the value all for the codegenConfig.type field. If you use the value modules, expect to see no react/renderer/components/ folder. If you use the value components, expect not to see any of the other files.