Node-API Development Process

To implement cross-language interaction using Node-API, you need to register and load modules based on the Node-API mechanism first.

ArkTS/JS: Call C++ methods by importing the required .so library.
Native: Implement module registration via a .cpp file. You need to declare the name of the library to register and define the mappings between the native and JS/ArkTS APIs in the callbacks registered.

The following demonstrates how to implement cross-language interaction by calling callNative() in ArkTS/JS code and implementing CallNative() in native code.

Creating a Native C++ Project

In DevEco Studio, choose New > Create Project, select the Native C++ template, click Next, select the API version, set the project name, and click Finish.
The main code of the project created consists of two parts: cpp and ets. For details about the project structure, see C++ Project Directory Structure.

Implementing Native APIs

Set module registration information.

When a native module is imported in ArkTS, the .so file will be loaded. During the loading process, the napi_module_register method is called to register the module with the system and call the module initialization function.

napi_module has two key properties: .nm_register_func and .nm_modname. .nm_register_func defines the module initialization function, and .nm_modname defines the module name, that is, the name of the .so file imported by ArkTS.

// entry/src/main/cpp/napi_init.cpp

// Information about the module. Record information such as the Init() function and module name.
static napi_module demoModule = {
    .nm_version = 1,
    .nm_flags = 0,
    .nm_filename = nullptr,
    .nm_register_func = Init,
    .nm_modname = "entry",
    .nm_priv = ((void*)0),
    .reserved = {0},
};

// When the .so file is loaded, this function is automatically called to register the demoModule module with the system.
extern "C" __attribute__((constructor)) void RegisterDemoModule() { 
    napi_module_register(&demoModule);
}

NOTE
You do not need to copy the above code as it is pre-configured in the napi_init.cpp file when the native C++ project is created.

Initialize the module.

Implement the mappings between the ArkTS and C++ APIs.

// entry/src/main/cpp/napi_init.cpp
EXTERN_C_START
// Initialize the module.
static napi_value Init(napi_env env, napi_value exports) {
    // Implement the mappings between the ArkTS and C++ APIs.
    napi_property_descriptor desc[] = {
        // You only need to copy the following two lines of code. Init has been configured in napi_init.cpp when the native C++ project is created.
        {"callNative", nullptr, CallNative, nullptr, nullptr, nullptr, napi_default, nullptr},
        {"nativeCallArkTS", nullptr, NativeCallArkTS, nullptr, nullptr, nullptr, napi_default, nullptr}
    };
    // Hook the CallNative and NativeCallArkTS APIs to the exports object.
    napi_define_properties(env, exports, sizeof(desc) / sizeof(desc[0]), desc);
    return exports;
}
EXTERN_C_END

Add JS APIs in the index.d.ts file.

// entry/src/main/cpp/types/libentry/index.d.ts
export const callNative: (a: number, b: number) => number;
export const nativeCallArkTS: (cb: (a: number) => number) => number;

Associate index.d.ts with .cpp in the oh-package.json5 file.

// entry/src/main/cpp/types/libentry/oh-package.json5
{
  "name": "libentry.so",
  "types": "./index.d.ts",
  "version": "",
  "description": "Please describe the basic information."
}

Set CMake packaging parameters in the CMakeLists.txt file.

# entry/src/main/cpp/CMakeLists.txt
cmake_minimum_required(VERSION 3.4.1)
project(MyApplication2)

set(NATIVERENDER_ROOT_PATH ${CMAKE_CURRENT_SOURCE_DIR})

include_directories(${NATIVERENDER_ROOT_PATH}
                    ${NATIVERENDER_ROOT_PATH}/include)

# Add a library named entry.
add_library(entry SHARED napi_init.cpp)
# Build the library to be linked to this executable.
target_link_libraries(entry PUBLIC libace_napi.z.so)

Implement CallNative and NativeCallArkTS. The code is as follows:

// entry/src/main/cpp/napi_init.cpp
static napi_value CallNative(napi_env env, napi_callback_info info)
{
    size_t argc = 2;
    // Declare the parameter array.
    napi_value args[2] = {nullptr};

    // Obtain input parameters and put them into the parameter array in sequence.
    napi_get_cb_info(env, info, &argc, args, nullptr, nullptr);

    // Obtain the parameters in sequence.
    double value0;
    napi_get_value_double(env, args[0], &value0);
    double value1;
    napi_get_value_double(env, args[1], &value1);

    // Return the sum of the two numbers.
    napi_value sum;
    napi_create_double(env, value0 + value1, &sum);
    return sum;
}

static napi_value NativeCallArkTS(napi_env env, napi_callback_info info)
{    
    size_t argc = 1;
    // Declare the parameter array.
    napi_value args[1] = {nullptr};

    // Obtain input parameters and put them into the parameter array in sequence.
    napi_get_cb_info(env, info, &argc, args, nullptr, nullptr);

    // Create an int() as the input parameter of ArkTS.
    napi_value argv = nullptr;    
    napi_create_int32(env, 2, &argv);

    // Invoke the callback that is passed in, and return the result.
    napi_value result = nullptr;
    napi_call_function(env, nullptr, args[0], 1, &argv, &result);
    return result;
}

Calling C/C++ APIs on ArkTS

On ArkTS, import the .so file that contains the processing logic on the native side to use C/C++ APIs.

// entry/src/main/ets/pages/Index.ets
// Import the native APIs.
import nativeModule from 'libentry.so'

@Entry
@Component
struct Index {
  @State message: string = 'Test Node-API callNative result: ';
  @State message2: string = 'Test Node-API nativeCallArkTS result: ';
  build() {
    Row() {
      Column() {
        // Pressing the first button calls callNative(), which uses CallNative() in the native code to add the two numbers.
        Text(this.message)
          .fontSize(50)
          .fontWeight(FontWeight.Bold)
          .onClick(() => {
            this.message += nativeModule.callNative(2, 3);
            })
        // Pressing the second button calls nativeCallArkTS, which corresponds to NativeCallArkTS in the native code. The ArkTS function is called on the native side.
        Text(this.message2)
          .fontSize(50)
          .fontWeight(FontWeight.Bold)
          .onClick(() => {
            this.message2 += nativeModule.nativeCallArkTS((a: number)=> {
                return a * 2;
            });
          })
      }
      .width('100%')
    }
    .height('100%')
  }
}

Node-API Constraints

Naming Rules of .so Files

The case of the module name to import must be the same as that registered. For example, if the module name is entry, the .so file name must be libentry.so, and the nm_modname field in napi_module must be entry. When importing the module in ArkTS, use **import **xxx from 'libentry.so'.

Registration

To prevent conflicts with symbols in other .so files, add static to the function (for example, the Init function) corresponding to nm_register_func.
The name of the module registration entry, that is, the function modified by attribute((constructor)) must be unique. For example, the RegisterDemoModule function in this document.

Multithread Processing

Each engine instance corresponds to an ArkTS thread. The objects of an instance cannot be operated across threads. Otherwise, the application crashes. Observe the following rules:

The Node-API can be used only by ArkTS threads.
The input parameter env of a native API can be bound to an ArkTS thread and can be used only by the thread that creates the env.
Data created using Node-APIs must be released before env is completely destroyed. Otherwise, memory leaks may be caused. In addition, accessing or using the data after napi_env is destroyed may cause the process to crash.

Debugging Device Selection

You are advised to use a real device for code debugging. If no real device is available or you do not have the permission to use a real device, use an emulator for debugging. For details about the problems that may occur during emulator debugging, see Bundle Manager.

The previewer is mainly used to debug UI components. Do not use it for function debugging. Otherwise, the following error message may be displayed:

TypeError: undefined is not callable

Maintenance and testing measures have been added to address common errors. For details, see Analyzing Exceptions and Crashes Using Node-API.