如何释放包裹的 C++ 当关联的 Javascript 对象在 V8 中被垃圾收集时，对象？

发布于 2024-07-06 22:19:36 字数 232 浏览 7 评论 0原文

V8 的文档介绍了如何创建包装 C++ 对象的 Javascript 对象。 Javascript 对象保存一个指向 C++ 对象实例的指针。我的问题是，假设您在堆上创建了 C++ 对象，当 Javascript 对象被 gc 收集时，如何获得通知，以便释放堆分配的 C++ 对象？

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缱倦旧时光 2024-07-13 22:19:36

诀窍是创建一个持久句柄（链接到的API参考中的第二个要点：“持久句柄不保存在堆栈中，只有当您明确指定时才会删除删除它们...当您需要为多个函数调用保留对对象的引用，或者当句柄生存期与 C++ 作用域不对应时，请使用持久句柄。”），并调用 MakeWeak()< /code> ，传递一个回调函数来进行必要的清理（“可以使用 Persistent::MakeWeak 使持久句柄变弱，以在唯一的情况下触发垃圾收集器的回调）对对象的引用来自弱持久句柄。”——也就是说，当所有“常规”句柄都超出范围并且垃圾收集器即将删除该对象时）。

Persistent::MakeWeak 方法签名为：

void MakeWeak(void* parameters, WeakReferenceCallback callback);

其中 WeakReferenceCallback 被定义为带有两个参数的函数指针：

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

这些参数可在随以下版本分发的 v8.h 头文件中找到： V8 作为公共 API。

您可能希望传递给 MakeWeak 的函数来清理 Persistent

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);

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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