尝试检索时，通过nan传递数组会返回undefined

Question

我目前正在使用他们的c++/v8系统为NodeJS编写一个本地插件，并试图从Javascript到c++编写一个数组，然后再检索它。每当我试图从数组中检索值并返回它时，它返回一个空数组-因为我最近正在深入研究c++，我不确定这是我误解的指针/c++基础还是NodeJS c++交互。

我的文件如下：

函数.c

#include "functions.h"
#include <node.h>
#include <nan.h>
using namespace std;
using namespace v8;
NAN_METHOD(nothing) {
}
NAN_METHOD(aString) {
    info.GetReturnValue().Set(Nan::New("This is a thing.").ToLocalChecked());
}
NAN_METHOD(aBoolean) {
    info.GetReturnValue().Set(false);
}
NAN_METHOD(aNumber) {
    info.GetReturnValue().Set(1.75);
}
NAN_METHOD(anObject) {
    v8::Local<v8::Object> obj = Nan::New<v8::Object>();
    Nan::Set(obj, Nan::New("key").ToLocalChecked(), Nan::New("value").ToLocalChecked());
    info.GetReturnValue().Set(obj);
}
NAN_METHOD(anArray) {
    v8::Local<v8::Array> arr = Nan::New<v8::Array>(3);
    Nan::Set(arr, 0, Nan::New(1));
    Nan::Set(arr, 1, Nan::New(2));
    Nan::Set(arr, 2, Nan::New(3));
    info.GetReturnValue().Set(arr);
}
NAN_METHOD(callback) {
    v8::Local<v8::Function> callbackHandle = info[0].As<v8::Function>();
    Nan::MakeCallback(Nan::GetCurrentContext()->Global(), callbackHandle, 0, 0);
}
// Wrapper Impl
Nan::Persistent<v8::Function> MyObject::constructor;
NAN_MODULE_INIT(MyObject::Init) {
  v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
  tpl->SetClassName(Nan::New("MyObject").ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);
  //Nan::SetPrototypeMethod(tpl, "plusOne", PlusOne);
  Nan::SetPrototypeMethod(tpl, "getArr", arrY);
  Nan::SetPrototypeMethod(tpl, "PassArray", passArray);
  Nan::SetPrototypeMethod(tpl, "setArray", SetArray);
  constructor.Reset(Nan::GetFunction(tpl).ToLocalChecked());
  Nan::Set(target, Nan::New("MyObject").ToLocalChecked(), Nan::GetFunction(tpl).ToLocalChecked());
}
MyObject::MyObject(v8::Local<v8::Array> value) : value_(value) {
}
MyObject::~MyObject() {
}
NAN_METHOD(MyObject::New) {
  if (info.IsConstructCall()) {
    v8::Local<v8::Array> arr = Nan::New<v8::Array>();
    v8::Local<v8::Array> value = v8::Handle<v8::Array>::Cast(arr);
    MyObject *obj = new MyObject(value);
    obj->Wrap(info.This());
    info.GetReturnValue().Set(info.This());
  } else {
    const int argc = 1; 
    v8::Local<v8::Value> argv[argc] = {info[0]};
    v8::Local<v8::Function> cons = Nan::New(constructor);
    info.GetReturnValue().Set(cons->NewInstance(argc, argv));
  }
}
// NAN_METHOD(MyObject::PlusOne) {
//   MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.This());
//   obj->value_ += 1;
//   info.GetReturnValue().Set(obj->value_);
// }
NAN_METHOD(MyObject::SetArray){
    Nan::HandleScope scope;
    vector<string> result;
    Handle<Value> val;
    Local<Array> arr = Nan::New<Array>();
    MyObject* obj = ObjectWrap::Unwrap<MyObject>(info.Holder());
    if (info[0]->IsArray()) {
      Handle<Array> jsArray = Handle<Array>::Cast(info[0]);
      for (unsigned int i = 0; i < jsArray->Length(); i++) {
        val = jsArray->Get(i);
        result.push_back(string(*String::Utf8Value(val)));
        Nan::Set(arr, i, val);
      }
    }
    info.GetReturnValue().Set(obj->value_);
}
NAN_METHOD(MyObject::arrY){
  MyObject* obj = ObjectWrap::Unwrap<MyObject>(info.Holder());
  info.GetReturnValue().Set(Local<Array>::Cast(obj->value_));
}
NAN_METHOD(MyObject::passArray) {
    Nan::HandleScope scope;
    vector<string> result;
    Handle<Value> val;
    Local<Array> arr = Nan::New<Array>();
    if (info[0]->IsArray()) {
      Handle<Array> jsArray = Handle<Array>::Cast(info[0]);
      for (unsigned int i = 0; i < jsArray->Length(); i++) {
        val = jsArray->Get(i);
        result.push_back(string(*String::Utf8Value(val)));
        Nan::Set(arr, i, val);
      }
    }
    MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.This());
    obj->value_ = arr;
    info.GetReturnValue().Set(obj->value_);
}

函数.h

#ifndef NATIVE_EXTENSION_GRAB_H
#define NATIVE_EXTENSION_GRAB_H
#include <nan.h>
// Example top-level functions. These functions demonstrate how to return various js types.
// Implementations are in functions.cc
NAN_METHOD(nothing);
NAN_METHOD(aString);
NAN_METHOD(aBoolean);
NAN_METHOD(aNumber);
NAN_METHOD(anObject);
NAN_METHOD(anArray);
NAN_METHOD(callback);
// Example with node ObjectWrap
// Based on https://nodejs.org/api/addons.html#addons_wrapping_c_objects but using NAN
class MyObject : public Nan::ObjectWrap {
  public:
    static NAN_MODULE_INIT(Init);
  private:
    explicit MyObject(v8::Local<v8::Array> value);
    ~MyObject();
    static NAN_METHOD(New);
    static NAN_METHOD(PlusOne);
    static NAN_METHOD(SetArray);
    static NAN_METHOD(arrY);
    static NAN_METHOD(passArray);
    static Nan::Persistent<v8::Function> constructor;
    v8::Local<v8::Array> value_;
};
#endif

index.js

var NativeExtension = require('bindings')('NativeExtension');
module.exports = NativeExtension;

console.log(NativeExtension.aString());
var obj = NativeExtension.MyObject([1,2,3]);
console.log(NativeExtension.aString());
console.log(obj.PassArray([1, 1, 3]));
console.log(obj.getArr());
console.log(obj.setArray())

出于测试目的，代码绝对不是DRY，它基于通过github提供的nan样板代码的非精简版本，可以在这里找到。

在重读我自己的代码时，我可以肯定地判断出我的方法命名是bad，所以PassArr应该设置一个数组，getArr应该返回"PassArr"传递的数组。SetArray是我留下的一次黑客尝试。

重申一下，我试图实现的功能是：

Javascript Array -> C++ variable
C++ variable -> Javascript Array

但是当前的功能并没有持久化我的变量。运行index.js的结果如下：

This is a thing.
This is a thing.
[ 1, 1, 3 ]
Console {
  log: [Function: bound ],
  info: [Function: bound ],
  warn: [Function: bound ],
  error: [Function: bound ],
  dir: [Function: bound ],
  time: [Function: bound ],
  timeEnd: [Function: bound ],
  trace: [Function: bound trace],
  assert: [Function: bound ],
  Console: [Function: Console] }
[]

Answer 1

迟做总比不做好。。。这里有两个问题：

第一个是MyObject::value_是一个本地句柄。它应该是一个持久句柄，您需要在~MyObject()中重置它，否则它会泄漏。

第二个问题是MyObject::passArray中的数组arr的长度为0，您应该使用正确的长度来构造它。