Cocos2d-x V3.x内存管理分析

妖狐艹你老母 2022-09-23 14:48 20阅读 0赞

原文：http://galoisplusplus.coding.me/blog/2014/07/30/memory-management-in-cocos2d-x-v3/

cocos2d-x移植自Objective C的cocos2d，其内存管理其实也来自于OC。因而对于写过OC程序的朋友来讲，cocos2d-x的内存管理应该是一目了然的，但对于本渣这枚没接触过OC的C++码农来说，或许直接看cocos2d-x源代码才是最直接快捷的方式。

# Node类 #

我们首先来看Node类的代码，Node是cocos2d-x中极重要的基类，许多常用的Scene、Layer、MenuItem等都继承自Node。

Node的创建是通过以下的接口，该函数返回一个Node的静态对象指针：

下面让我们来看这个函数的实现。该函数采用二段式创建的方式——首先用new operator在heap中开辟空间并进行简单的初始化，假如new返回一个合法地址（cocos2d-x没有采用c++的异常处理机制），则接着init函数用于实际初始化Node的成员。只有在这二者都成功后，才把创建的指针设为`autorelease`（关于`autorelease`后面会继续解释）并返回。

对于创建失败的情况，cocos2d-x使用了下面的宏保证该指针被delete且被设为nullptr：

这个二段式的create函数在cocos2d-x中非常常用，因而cocos2d-x用了以下一个叫`CREATE_FUNC`来表示这个函数以便给继承Node的子类使用：

这样，继承Node的子类（例如`ExampleLayer`）只需要在类声明(class declaration)中加入`CREATE_FUNC(类名)`（例如`CREATE_FUNC(ExampleLayer)`），再override下init函数即可。

# Ref类 #

在cocos2d-x中，Node类的父类是Ref类，之前我们所看到的`autorelease`方法实际上就来自于这个父类。

下面我们先来看Ref类的声明，这里为了突出重点，我们忽略script binding的情况：

从上面的代码，我们可以初步了解到：Ref采用引用计数（reference counting）的方法来管理某个指针所指向的某个对象，初始创建时计数是1，当计数变为0时该对象被析构；`retain`方法会增加计数并拿到所有权，而与之对应的，`release`方法会减少计数；`autorelease`是把所有权交给友类（friend class）`AutoreleasePool`，让它来决定何时减少计数，这个类我们后面会继续谈到。

下面我们来看Ref类的实现（definition）：

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</pre> </td> 
 <td style="margin:0px; padding:0px; border:0px; font-style:inherit; font-variant:inherit; line-height:inherit; font-family:inherit; font-size:18.4px; vertical-align:middle; width:1109px"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; border:none; font-style:inherit; font-variant:inherit; font-weight:inherit; line-height:1.45em; font-family:Menlo,Monaco,'Andale Mono','lucida console','Courier New',monospace; font-size:13px; vertical-align:baseline; color:rgb(147,161,161); overflow:auto; background:none"><code style="margin:0px; border:0px; font-style:inherit; font-variant:inherit; font-weight:inherit; line-height:1.45em; vertical-align:baseline; overflow-y:hidden; display:block; overflow-x:auto; padding:0.8em!important; font-family:Menlo,Monaco,'Andale Mono','lucida console','Courier New',monospace!important; background:rgb(0,0,0)">#if CC_USE_MEM_LEAK_DETECTION
static void trackRef(Ref* ref);
static void untrackRef(Ref* ref);
#endif

// 在初始化列表中将计数设为1
Ref::Ref()
: _referenceCount(1) // when the Ref is created, the reference count of it is 1
{
 
// 假如开启内存泄露检测，则追踪该对象指针，将该对象指针放入一个列表（list）中
// 后面的代码我们很快就会看到这个list
#if CC_USE_MEM_LEAK_DETECTION
 trackRef(this);
#endif
}

Ref::~Ref()
{
 
// 假如开启内存泄露检测且引用计数非0，则在追踪列表中找到该对象指针并删除
#if CC_USE_MEM_LEAK_DETECTION
 if (_referenceCount != 0)
 untrackRef(this);
#endif
}

// retain只是单纯将计数递增
void Ref::retain()
{
 
 // CCASSERT是cocos2d-x对C++的assert所封装的宏
 CCASSERT(_referenceCount &gt; 0, "reference count should greater than 0");
 ++_referenceCount;
}

void Ref::release()
{
 
 // 首先计数递减
 CCASSERT(_referenceCount &gt; 0, "reference count should greater than 0");
 --_referenceCount;

 // 计数为0，应当析构对象
 if (_referenceCount == 0)
 {
 
#if defined(COCOS2D_DEBUG) &amp;&amp; (COCOS2D_DEBUG &gt; 0)
 // 得到一个PoolManager单例的对象
 // PoolManager类后面会解释
 auto poolManager = PoolManager::getInstance();
 // 后面会详细解释这段代码
 if (!poolManager-&gt;getCurrentPool()-&gt;isClearing() &amp;&amp; poolManager-&gt;isObjectInPools(this))
 {
 
 // 以下的注释很重要，很快会解释到
 // Trigger an assert if the reference count is 0 but the Ref is still in autorelease pool.
 // This happens when 'autorelease/release' were not used in pairs with 'new/retain'.
 //
 // Wrong usage (1):
 //
 // auto obj = Node::create(); // Ref = 1, but it's an autorelease Ref which means it was in the autorelease pool.
 // obj-&gt;autorelease(); // Wrong: If you wish to invoke autorelease several times, you should retain `obj` first.
 //
 // Wrong usage (2):
 //
 // auto obj = Node::create();
 // obj-&gt;release(); // Wrong: obj is an autorelease Ref, it will be released when clearing current pool.
 //
 // Correct usage (1):
 //
 // auto obj = Node::create();
 // |- new Node(); // `new` is the pair of the `autorelease` of next line
 // |- autorelease(); // The pair of `new Node`.
 //
 // obj-&gt;retain();
 // obj-&gt;autorelease(); // This `autorelease` is the pair of `retain` of previous line.
 //
 // Correct usage (2):
 //
 // auto obj = Node::create();
 // obj-&gt;retain();
 // obj-&gt;release(); // This `release` is the pair of `retain` of previous line.
 CCASSERT(false, "The reference shouldn't be 0 because it is still in autorelease pool.");
 }
#endif

// 假如开启内存泄露检测，则在追踪列表中找到该对象指针并删除
#if CC_USE_MEM_LEAK_DETECTION
 untrackRef(this);
#endif
 // 调用析构函数并释放空间
 delete this;
 }
}

// 把该对象指针交给友类AutoreleasePool（具体来说，是PoolManager单例对象所得到的当前的AutoreleasePool）来管理
Ref* Ref::autorelease()
{
 
 PoolManager::getInstance()-&gt;getCurrentPool()-&gt;addObject(this);
 return this;
}

unsigned int Ref::getReferenceCount() const
{
 
 return _referenceCount;
}

#if CC_USE_MEM_LEAK_DETECTION

// 这里便是存放所追踪的对象指针的列表
static std::list&lt;Ref*&gt; __refAllocationList;

void Ref::printLeaks()
{
 
 // Dump Ref object memory leaks
 if (__refAllocationList.empty())
 {
 
 log("[memory] All Ref objects successfully cleaned up (no leaks detected).\n");
 }
 else
 {
 
 log("[memory] WARNING: %d Ref objects still active in memory.\n", (int)__refAllocationList.size());

 // C++的range-for语法
 // 打印出每个泄露内存的对象指针的类型和引用计数
 for (const auto&amp; ref : __refAllocationList)
 {
 
 CC_ASSERT(ref);
 const char* type = typeid(*ref).name();
 log("[memory] LEAK: Ref object '%s' still active with reference count %d.\n", (type ? type : ""), ref-&gt;getReferenceCount());
 }
 }
}

// 将对象指针放入列表中
static void trackRef(Ref* ref)
{
 
 CCASSERT(ref, "Invalid parameter, ref should not be null!");

 // Create memory allocation record.
 __refAllocationList.push_back(ref);
}

// 在列表中找到该对象指针并删除
static void untrackRef(Ref* ref)
{
 
 auto iter = std::find(__refAllocationList.begin(), __refAllocationList.end(), ref);
 if (iter == __refAllocationList.end())
 {
 
 log("[memory] CORRUPTION: Attempting to free (%s) with invalid ref tracking record.\n", typeid(*ref).name());
 return;
 }

 __refAllocationList.erase(iter);
}

#endif // #if CC_USE_MEM_LEAK_DETECTION
</code></pre> </td> 
 </tr> 
 </tbody> 
</table>

这段源代码对使用者最重要的在于release函数中的注释：

*  当Ref的计数变为0时，它一定不能在AutoreleasePool中。
 *  Ref的计数为0且同时在AutoreleasePool中的错误是由new/retain和autorelease/release没有对应引起的（有木有想起C++中new和delete没对应所引起的内存泄露？）：
    
     *  autorelease缺乏对应的retain。 例如：

- release缺乏对应的retain。

例如：

*  正确的用法是在create后调用autorelease或release方法前先用retain拿到所有权： 例如：

又如：

# AutoreleasePool类 #

现在我们来看Ref类的友类AutoreleasePool。 首先来看类声明：

从类声明中能解读出的最重要的信息是AutoreleasePool类用STL vector来存放它所管理的Ref所指向的对象。要搞清楚原理还需要继续看它的实现：

# PoolManager类 #

下面我们来看PoolManager类，在cocos2d-x中，这个类是典型的单例（singleton）工厂类——及有且只有一个PoolManager对象，该PoolManger有一个存放AutoreleasePool对象指针的stack，该stack是由STL::vector实现的。需要注意的是，cocos2d-x的单例类都不是线程安全的，跟内存管理紧密相关的PoolManager类也不例外，因此在多线程中使用cocos2d-x的接口需要特别注意内存管理的问题。

我们先来看类声明：

再来看类实现：

# 最后的疑问 #

想必各位用惯了c++的看官在看完了以上的代码之后，最有疑问的还是神秘的`Ref::autorelease`函数。我们从AutoreleasePool的源代码看到，事实上被autorelease的对象最后还是通过release函数来减少其引用计数的，只不过release函数不是由使用者来调用，而是AutoreleasePool来调用，调用的地方在`AutoreleasePool::clear()`函数。那么AutoreleasePool如何个「auto」自动管理内存法儿？`AutoreleasePool::clear()`会在哪个地方被调用？

谜底隐藏在`cocos/base/CCDirector.cpp`中：

这里就不纠缠Director类的实现细节了，上面的代码揭示的事实是：在图像渲染的主循环中，如果当前的图形对象是在当前帧，则调用显示函数，并调用`AutoreleasePool::clear()`减少这些对象的引用计数。mainLoop是每一帧都会自动调用的，所以下一帧时这些对象都被当前的AutoreleasePool对象release了一次。这也是AutoreleasePool「自动」的来由。