如何使用 return 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 returns 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) 我怎样才能运行 do_something_synchronous 异步，这样 nodejs 事件循环就不会被阻塞和 return 承诺？

Answer 1

下面的代码片段是这个异步操作的关键组成部分。 napi_create_async_work() 函数分配一个工作对象，我们可以在其中指定工作处理程序函数，比如在我们的例子中 ExecuteMyPromise1() （ long 运行或进程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并且它将与node.js主事件循环线程并行异步执行。

到目前为止一切正常，当 ExecuteMyPromise1 函数计划直接从工作线程与 JavaScript 层交换结果时就会出现问题。 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。这就是我们使用 CompleteMyPromise1 函数处理程序的原因（如果任何最终清理 activity 也是从该函数完成的）。当异步逻辑完成或取消时，将从主事件循环线程调用此本机函数。

// 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;
}

// 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);
}

// Handle the completion of the asynchronous work.
void CompleteMyPromise1(napi_env env, napi_status status, void *data)
{
    napi_value rcValue;

    prom_data_ex_t *promDataEx = (prom_data_ex_t *)data;

    napi_create_int32(env, promDataEx->PrimeCount, &rcValue);

    // Resolve or reject the promise associated with the deferred depending on
    // whether the asynchronous action succeeded.
    if ( promDataEx->asynchronous_action_status == 0) // Success
    {
        status = napi_resolve_deferred(env, promDataEx->deferred, rcValue);
    }
    else
    {
        napi_value undefined;

        status = napi_get_undefined(env, &undefined);
        status = napi_reject_deferred(env, promDataEx->deferred, undefined );
    }
    if (status != napi_ok)
    {
        napi_throw_error(env, NULL, "Unable to create promise result.");
    }

    napi_delete_async_work(env, promDataEx->work);
    free(promDataEx);
}

这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

如何使用 return 承诺的 NAPI 创建异步函数

How to create async function using NAPI that return Promises

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