NDK Usage

Where can I find the libc++ library? Is it packed into the HAP? (API version 10)

Solution

You can find libc++_shared.so in the application directory. Each application has an independent libc++_shared.so file in /data/storage/el1/bundle/libs/${arch}.

How do I run a third-party open source C/C++ library on the system? (API version 10)

Solution

Currently, the official SDK supports only build with CMake. It also provides guides for GN build.

Reference

1. GN build:

   Building a Third-Party Library Based on gn_example

   Porting OHOS Based on a Third-Party Build Framework

2. CMake build:

   linux: HOW TO USE NDK (linux))

   windows: HOW TO USE NDK (windows))

What are the common attributes to be used and how do I implement the APIs when using Node-API to extend TS interfaces? How do I obtain env, implement callback- and promise-based APIs, and use libuv? (API version 10)

Solution

1. env uses the modular programming of Node-API. After the module is registered, the APIs in the callbacks are invoked with the callbacks.

   static napi_value CallNapi(napi_env env, napi_callback_info info)
   {
       size_t argc = 1;
       napi_value object = nullptr;
       napi_status status;
       status = napi_get_cb_info(env, info, &argc, &object, nullptr, nullptr);
       return object;
   }
   NAPI_MODULE_INIT()
   {
       napi_property_descriptor desc[] = {
           {" callNapi ", nullptr, CallNapi, nullptr, nullptr, nullptr, napi_default, nullptr}};
       napi_define_properties(env, exports, sizeof(desc) / sizeof(desc[0]), desc);
       return exports;
   }
   

2. Callback implementation:

   #include "napi/native_api.h"
   #include <assert.h>
   static napi_value NativeCall(napi_env env, napi_callback_info info)
   {
       size_t argc = 1;
       napi_value args[1] = {nullptr};
       napi_status status;
       status = napi_get_cb_info(env, info, &argc, args, nullptr, nullptr);
       assert(status == napi_ok);
   
   
       napi_valuetype valuetype;
       napi_typeof(env, args[0], &valuetype);
       if (valuetype != napi_valuetype::napi_function)
       {
           napi_throw_type_error(env, nullptr, "napi_function is expected");
       }
       napi_value cb = args[0];
   
   
       napi_value undefined;
       status = napi_get_undefined(env, &undefined);
       assert(status == napi_ok);
   
   
       napi_value argv[2] = {nullptr};
       status = napi_create_int32(env, 1, &argv[0]);
       assert(status == napi_ok);
       status = napi_create_int32(env, 2, &argv[1]);
       assert(status == napi_ok);
   
   
       napi_value result;
       status = napi_call_function(env, undefined, cb, 2, argv, &result);
       assert(status == napi_ok);
   
   
       return nullptr;
   }
   EXTERN_C_START 
   static napi_value Init(napi_env env, napi_value exports)
   {
       napi_property_descriptor desc[] = {
           {"nativeCall", nullptr, NativeCall, nullptr, nullptr, nullptr, napi_default, nullptr}};
       napi_define_properties(env, exports, sizeof(desc) / sizeof(desc[0]), desc);
       return exports;
   }
   EXTERN_C_END
   static napi_module module = {
       .nm_version = 1,
       .nm_flags = 0,
       .nm_filename = nullptr,
       .nm_register_func = Init,
       .nm_modname = "callback",
       .nm_priv = nullptr,
       .reserved = {0},
   };
   extern "C" __attribute__((constructor)) void RegisterCallbackModule(void)
   {
       napi_module_register(&module);
   }
   

3. Promise implementation:

   #include "napi/native_api.h"
   
   
   // Empty value so that macros here are able to return NULL or void
   #define NAPI_RETVAL_NOTHING // Intentionally blank
   
   
   #define GET_AND_THROW_LAST_ERROR(env)
   do
   {
       const napi_extended_error_info *errorInfo = nullptr;
       napi_get_last_error_info((env), &errorInfo);
       bool isPending = false;
       napi_is_exception_pending((env), &isPending);
       if (!isPending && errorInfo != nullptr)
       {
           const char *errorMessage = 
               errorInfo->error_message != nullptr ? errorInfo->error_message : "empty error message";
           napi_throw_error((env), nullptr, errorMessage);
       }
   } while (0)
   
   
   #define NAPI_ASSERT_BASE(env, assertion, message, retVal)
   do {
       if (!(assertion))
       {
           napi_throw_error((env), nullptr, "assertion(" #assertion ") failed : " message);
           return retVal;
       }
   } while (0)
   
   
   #define NAPI_ASSERT(env, assertion, message) NAPI_ASSERT_BASE(env, assertion, message, nullptr)
   
   
   #define NAPI_ASSERT_RETURN_VOID(env, assertion, message) 
       NAPI_ASSERT_BASE(env, assertion, message, NAPI_RETVAL_NOTHING)
   
   
   #define NAPI_CALL_BASE(env, theCall, retVal)
       do
       {
           if ((theCall) != napi_ok)
           {
               GET_AND_THROW_LAST_ERROR((env));
               return retVal;
           }
       } while (0)
   
   
   #define NAPI_CALL(env, theCall) NAPI_CALL_BASE(env, theCall, nullptr)
   
   
   #define NAPI_CALL_RETURN_VOID(env, theCall) NAPI_CALL_BASE(env, theCall, NAPI_RETVAL_NOTHING)
   
   
   struct AsyncData{
       napi_deferred deferred;
       napi_async_work work;
   
   
       int32_t arg;
       double retVal;
   };
   
   
   double DoSomething(int32_t val)
   {
       if (val != 0)
       {
           return 1.0 / val;
       }
       return 0;
   }
   
   
   void ExecuteCallback(napi_env env, void *data)
   {
       AsyncData* asyncData = reinterpret_cast<AsyncData*>(data);
       asyncData->retVal = DoSomething(asyncData->arg);
   }
   
   
   void CompleteCallback(napi_env env, napi_status status, void *data)
   {
       AsyncData* asyncData = reinterpret_cast<AsyncData*>(data);
   
   
       napi_value retVal;
       if (asyncData->retVal == 0)
       {
           NAPI_CALL_RETURN_VOID(env, napi_create_string_utf8(env, "arg can't be zero", NAPI_AUTO_LENGTH, &retVal));
           NAPI_CALL_RETURN_VOID(env, napi_reject_deferred(env, asyncData->deferred, retVal));
       }
       else
       {
           NAPI_CALL_RETURN_VOID(env, napi_create_double(env, asyncData->retVal, &retVal));
           NAPI_CALL_RETURN_VOID(env, napi_resolve_deferred(env, asyncData->deferred, retVal));
       }
   
   
       NAPI_CALL_RETURN_VOID(env, napi_delete_async_work(env, asyncData->work));
       asyncData->work = nullptr;
       asyncData->deferred = nullptr;
       delete asyncData;
   }
   
   
   static napi_value NativeCall(napi_env env, napi_callback_info info)
   {
       size_t argc = 1;
       napi_value args[1] = {nullptr};
       NAPI_CALL(env, napi_get_cb_info(env, info, &argc, args, nullptr, nullptr));
   
   
       int32_t arg;
       NAPI_CALL(env, napi_get_value_int32(env, args[0], &arg));
   
   
       // Create promise
       napi_deferred deferred;
       napi_value promise;
       NAPI_CALL(env, napi_create_promise(env, &deferred, &promise));
   
   
       AsyncData *data = new AsyncData;
       data->deferred = deferred;
       data->arg = arg;
   
   
       napi_async_work work;
       napi_value workName;
       napi_create_string_utf8(env, "promise", NAPI_AUTO_LENGTH, &workName);
       NAPI_CALL(env, napi_create_async_work(env, nullptr, workName,
           ExecuteCallback, CompleteCallback, data, &work));
   
   
       data->work = work;
       NAPI_CALL(env, napi_queue_async_work(env, work));
   
   
       return promise;
   }
   
   
   EXTERN_C_START
   static napi_value Init(napi_env env, napi_value exports)
   {
       napi_property_descriptor desc[] = {
           {"nativeCall", nullptr, NativeCall, nullptr, nullptr, nullptr, napi_default, nullptr}
       };
       napi_define_properties(env, exports, sizeof(desc) / sizeof(desc[0]), desc);
       return exports;
   }
   EXTERN_C_END
   
   
   static napi_module demoModule = {
       .nm_version = 1,
       .nm_flags = 0,
       .nm_filename = nullptr,
       .nm_register_func = Init,
       .nm_modname = "promise",
       .nm_priv = nullptr,
       .reserved = {0},
   };
   
   
   extern "C" __attribute__((constructor)) void RegisterPromiseModule(void)
   {
       napi_module_register(&demoModule);
   }
   

4. To use libuv, import the libuv library.

How is a raw file read by the thread created by pthread? (API version 10)

Solution

The raw file can be read by a thread-safe function: 1. Obtain and save a resource file object via the UI main thread. 2. Create a thread-safe function. 3. Call the thread-safe function in a non-main thread. 4. Read the file resources in rawfile via the thread-safe function.

How is the processing result of a c++ thread created by an ArkTS thread using Node-API returned to the ArkTS thread? (API version 10)

Solution

Use napi_create_threadsafe_function to create a function that can be called by any thread in the ArkTS thread. Call napi_call_threadsafe_function in the C++ thread to return the result to the main thread.

References

Thread Safety Development Using Node-API

What should I do to permanently hold a JS object created by napi_create_object or passed in as a parameter? (API version 10)

Question

What should I do if I need to permanently hold a JS object created by napi_create_object or passed in as a parameter, and destroy, increment, and decrement the reference count?

Solution

To hold an object persistently, use napi_create_reference to create a strong reference and then save the reference for use. To destroy a reference, use napi_delete_reference. To increment the reference count, use napi_reference_ref; to decrement the reference count, use api_reference_unref.

Can messages from C++ be passed to an ArkTS callback or object wrapped in a native object and synced to the application? (API version 11)

Question

Can messages from C++ be synced to an application via an ArkTS callback or object wrapped in a native object? Can napi_env of the ArkTS object or function be held permanently? Does a specific thread is required for syncing messages to an application?

Solution

An ArkTS object or function cannot be wrapped to a native object. You need to implement the related processing in C++. env can be held for a long period of time, but must be used in the ArkTS thread that creates env.

Reference

1. Wrapping a Native Object in an ArkTS Object

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