如何使用返回承诺的 NAPI 创建异步函数

Question

我正在尝试使用 NAPI 创建节点模块。我必须创建返回承诺的异步函数。我不希望 testasynfunction 会阻塞 NodeJS 事件循环。do_something_asynchronous 是一个同步函数。

napi_deferred do_something_synchronous(napi_env env,napi_deferred deferred){
  printf("\n3) Function called");
  //napi_deferred deferred;
  napi_value undefined;
  napi_status status;

  // Create a value with which to conclude the deferred.
  status = napi_get_undefined(env, &undefined);
  if (status != napi_ok) return NULL;
  sleep(5);
  // Resolve or reject the promise associated with the deferred depending on
  // whether the asynchronous action succeeded.
  if (false) {
    printf("\n5) Success\nXXXXXXX");
    status = napi_resolve_deferred(env, deferred, undefined);
  } else {
    printf("\nReject");
    status = napi_reject_deferred(env, deferred, undefined);
  }
  if (status != napi_ok) return NULL;

  // At this point the deferred has been freed, so we should assign NULL to it.
  deferred = NULL;
}

//Function will be called from the js 
napi_value testasynfunction(napi_env env, napi_callback_info info){
  printf("XXXXX Hello \n");
  napi_deferred deferred;
  napi_value promise;
  napi_status status;
  // Create the promise.
  status = napi_create_promise(env, &deferred, &promise);
  if (status != napi_ok) return NULL;
  printf("\n1) Calling function to do something");
  do_something_synchronous(env,deferred);
  //std::async(do_something_asynchronous,env,deferred);
  printf("\n2) Returning Promise");
  return promise;
}
napi_property_descriptor testasync = DECLARE_NAPI_METHOD("testasyn", testasynfunction);
  status = napi_define_properties(env, exports, 1, &testasync);
  assert(status == napi_ok);

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

问题：1）如何异步运行 do_something_synchronous 以便 nodejs 事件循环不会被阻塞并返回承诺？

Answer 1

以下代码片段是此异步操作的关键组件。该napi_create_async_work（）函数分配工作对象，我们可以指定工人处理函数，比如在我们的例子ExecuteMyPromise1（）（长时间运行或处理繁重的任务可以用它来部署）。此函数将排队等待工作池线程，它将与 node.js 主事件循环线程并行异步执行。

到目前为止一切看起来都很好，当 ExecuteMyPromise1 函数计划直接从工作线程与 JavaScript 层交换结果时，问题就会出现。 任何 JavaScript 操作通常只能从本机插件的主线程调用。然后对于这个结果交换，我们必须使用另一个将从主线程调用的本机函数。这就是为什么我们使用CompleteMyPromise1函数处理程序（如果任何最终清理活动也是从这个函数完成的）。当异步逻辑完成或取消时，将从主事件循环线程调用该原生函数。

// The function called by javascript to get a promise returned.
napi_value MyPromise1(napi_env env, napi_callback_info info)
{

  // -- -- -- --
  // -- -- -- --

  // Create a promise object.
  status = napi_create_promise(env, &promDataEx->deferred, &promise);
  if (status != napi_ok)
  {
    napi_throw_error(env, NULL, "Unable to create promise.");
  }


  // -- -- -- --
  // -- -- -- --

  {
    // Create the async function.
    napi_value resource_name;
    napi_create_string_utf8(env, "MyPromise1", -1, &resource_name);
    napi_create_async_work(env, NULL, resource_name, 
        ExecuteMyPromise1, CompleteMyPromise1, 
        promDataEx, &promDataEx->work);
    napi_queue_async_work(env, promDataEx->work);
  }


  return promise;
}

// The function called by javascript to get a promise returned.
napi_value MyPromise1(napi_env env, napi_callback_info info)
{

  // -- -- -- --
  // -- -- -- --

  // Create a promise object.
  status = napi_create_promise(env, &promDataEx->deferred, &promise);
  if (status != napi_ok)
  {
    napi_throw_error(env, NULL, "Unable to create promise.");
  }


  // -- -- -- --
  // -- -- -- --

  {
    // Create the async function.
    napi_value resource_name;
    napi_create_string_utf8(env, "MyPromise1", -1, &resource_name);
    napi_create_async_work(env, NULL, resource_name, 
        ExecuteMyPromise1, CompleteMyPromise1, 
        promDataEx, &promDataEx->work);
    napi_queue_async_work(env, promDataEx->work);
  }


  return promise;
}

// Execute the asynchronous work.
void ExecuteMyPromise1(napi_env env, void *data)
{
    prom_data_ex_t *promDataEx = (prom_data_ex_t *)data;

    // Calculate prime count
    promDataEx->PrimeCount = CPrimeCount( promDataEx->x, promDataEx->y );

    // Set the status as asynchronous_action is success
    promDataEx->asynchronous_action_status = 0;
    //sleep(3);
}

这里是它的完整示例代码 https://github.com/msatyan/MyNodeC/blob/master/src/mync1/MyPromise1.cpp

JavaScrip 调用是 TestPromiseWithAsync() https://github.com/msatyan/MyNodeC/blob/master/test/TestExtensions.js