除了包装和返回 C++ 对象,也可以通过使用 Node.js 的辅助函数 node::ObjectWrap::Unwrap
进行去包装来传递包装的对象。
以下例子展示了一个 add()
函数,它可以把两个 MyObject
对象作为输入参数:
// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"
namespace demo {
using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
MyObject::NewInstance(args);
}
void Add(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
args[0]->ToObject(context).ToLocalChecked());
MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
args[1]->ToObject(context).ToLocalChecked());
double sum = obj1->value() + obj2->value();
args.GetReturnValue().Set(Number::New(isolate, sum));
}
void InitAll(Local<Object> exports) {
MyObject::Init(exports->GetIsolate());
NODE_SET_METHOD(exports, "createObject", CreateObject);
NODE_SET_METHOD(exports, "add", Add);
}
NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)
} // namespace demo
在 myobject.h
中,新增了一个新的公共方法用于在去包装对象后访问私有值。
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Isolate* isolate);
static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
inline double value() const { return value_; }
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static v8::Global<v8::Function> constructor;
double value_;
};
} // namespace demo
#endif
myobject.cc
中的实现类似之前的例子:
// myobject.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
// 注意!这不是线程安全的,此插件不能用于工作线程。
Global<Function> MyObject::constructor;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Isolate* isolate) {
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
tpl->InstanceTemplate()->SetInternalFieldCount(1);
Local<Context> context = isolate->GetCurrentContext();
constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());
AddEnvironmentCleanupHook(isolate, [](void*) {
constructor.Reset();
}, nullptr);
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Context> context = isolate->GetCurrentContext();
if (args.IsConstructCall()) {
// 像构造函数一样调用:`new MyObject(...)`
double value = args[0]->IsUndefined() ?
0 : args[0]->NumberValue(context).FromMaybe(0);
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// 像普通方法 `MyObject(...)` 一样调用,转为构造调用。
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
}
void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
const unsigned argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Context> context = isolate->GetCurrentContext();
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
} // namespace demo
测试:
// test.js
const addon = require('./build/Release/addon');
const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);
console.log(result);
// 打印: 30