当关联的Javascript对象在V8中被垃圾收集时，如何释放包装的C ++对象？ [英] How do you free a wrapped C++ object when associated Javascript object is garbage collected in V8?

问题描述

V8的文档说明了如何创建包装C ++对象的Javascript对象。 Javascript对象持有指向C ++对象实例的指针。我的问题是，让我们说你在堆上创建C ++对象，当你的gc收集Javascript对象时如何获得通知，所以你可以释放堆分配的C ++对象？

<诀窍是创建一个 Persistent 句柄（第二个来自链接的API引用的参考： 持久化句柄不保留在堆栈上，只有当你特别删除它们时才被删除...当你需要保持一个对象的引用更多比如一个函数调用，或者当句柄的生存时间不对应于C ++范围），并调用 MakeWeak（），传递一个回调函数， （当一个对象的唯一引用来自弱持久句柄时，使用 Persistent :: MakeWeak 可以使永久句柄变弱，从垃圾回收器触发回调。 - 即所有常规句柄超出范围，当垃圾收集器要删除对象时）。

Persistent :: MakeWeak 方法签名是：

  void MakeWeak（void * parameters，WeakReferenceCallback callback ）; 
 

其中 WeakReferenceCallback 具有两个参数：

  typedef void（* WeakReferenceCallback）（Persistent< Object&参数）; 
 

这些都是在V8作为公共API分发的头文件中找到的。

您需要传递给 MakeWeak 的函数来清除 Persistent< Object& code>对象参数，当它被调用作为回调时将被传递给它。可以忽略 void * parameter 参数（或 void * parameter 可以指向包含对象的C ++结构需要清除）：

  void CleanupV8Point（Persistent< Object> object，void *）
 {
 //对任何你想要的对象清理
 object.destroyCppObjects（）; 
} 
 
参数< ObjectTemplate> my_obj（ObjectTemplate :: New（））; 
 
 //当my_obj的Javascript部分即将被收集时
 //我们将有V8调用CleanupV8Point（my_obj）
 my_obj.MakeWeak（NULL，& CleanupV8Point ）; 
  




V8's documentation explains how to create a Javascript object that wraps a C++ object. The Javascript object holds on to a pointer to a C++ object instance. My question is, let's say you create the C++ object on the heap, how can you get a notification when the Javascript object is collected by the gc, so you can free the heap allocated C++ object?

解决方案

The trick is to create a Persistent handle (second bullet point from the linked-to API reference: "Persistent handles are not held on a stack and are deleted only when you specifically remove them. ... Use a persistent handle when you need to keep a reference to an object for more than one function call, or when handle lifetimes do not correspond to C++ scopes."), and call MakeWeak() on it, passing a callback function that will do the necessary cleanup ("A persistent handle can be made weak, using Persistent::MakeWeak, to trigger a callback from the garbage collector when the only references to an object are from weak persistent handles." -- that is, when all "regular" handles have gone out of scope and when the garbage collector is about to delete the object).

The Persistent::MakeWeak method signature is:

void MakeWeak(void* parameters, WeakReferenceCallback callback);

Where WeakReferenceCallback is defined as a pointer-to-function taking two parameters:

typedef void (*WeakReferenceCallback)(Persistent<Object> object,
                                      void* parameter);

These are found in the v8.h header file distributed with V8 as the public API.

You would want the function you pass to MakeWeak to clean up the Persistent<Object> object parameter that will get passed to it when it's called as a callback. The void* parameter parameter can be ignored (or the void* parameter can point to a C++ structure that holds the objects that need cleaning up):

void CleanupV8Point(Persistent<Object> object, void*)
{
    // do whatever cleanup on object that you're looking for
    object.destroyCppObjects();
}

Parameter<ObjectTemplate> my_obj(ObjectTemplate::New());

// when the Javascript part of my_obj is about to be collected
// we'll have V8 call CleanupV8Point(my_obj)
my_obj.MakeWeak(NULL, &CleanupV8Point);

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