cocos2dx 3.x 源码阅读之一引用计数原理

之前看过很多的文章，都是讲解cocos2dx的引用计数原理。有好有坏，今天我也自己来写一篇引用计数的文章，并且开始尝试阅读cocos2dx的源码，把自己的收获更新到这个博客，不为多少人看。只为自己记录这一路的点点滴滴。

讲了这么多那么开始吧！

首先就是贴上cocos2dx的万物之源 Ref类，这个类是Node的基类，而所有的显示节点类都是继承自Node类，所以他算是cocos2dx的根。

CCRef.h

class CC_DLL Ref
{
public:
    void retain();

    void release();

    Ref* autorelease();

    unsigned int getReferenceCount() const;

protected:
    Ref();

public:
    virtual ~Ref();

protected:
    //用于计数的变量
    unsigned int _referenceCount;

    friend class AutoreleasePool;

#if CC_ENABLE_SCRIPT_BINDING
public:

    unsigned int        _ID;

    int                 _luaID;

    void* _scriptObject;
#endif

    
#if CC_USE_MEM_LEAK_DETECTION
public:
    static void printLeaks();
#endif
};

CCRef.cpp

#include "base/CCRef.h"
#include "base/CCAutoreleasePool.h"
#include "base/ccMacros.h"
#include "base/CCScriptSupport.h"

#if CC_USE_MEM_LEAK_DETECTION
#include <algorithm>    // std::find
#endif

NS_CC_BEGIN

#if CC_USE_MEM_LEAK_DETECTION
static void trackRef(Ref* ref);
static void untrackRef(Ref* ref);
#endif

Ref::Ref()
: _referenceCount(1) // when the Ref is created,the reference count of it is 1
{
#if CC_ENABLE_SCRIPT_BINDING
    static unsigned int uObjectCount = 0;
    _luaID = 0;
    _ID = ++uObjectCount;
    _scriptObject = nullptr;
#endif
    
#if CC_USE_MEM_LEAK_DETECTION
    trackRef(this);
#endif
}

Ref::~Ref()
{
#if CC_ENABLE_SCRIPT_BINDING
    // if the object is referenced by Lua engine,remove it
    if (_luaID)
    {
        ScriptEngineManager::getInstance()->getScriptEngine()->removeScriptObjectByObject(this);
    }
    else
    {
        ScriptEngineProtocol* pEngine = ScriptEngineManager::getInstance()->getScriptEngine();
        if (pEngine != nullptr && pEngine->getScriptType() == kScriptTypeJavascript)
        {
            pEngine->removeScriptObjectByObject(this);
        }
    }
#endif


#if CC_USE_MEM_LEAK_DETECTION
    if (_referenceCount != 0)
        untrackRef(this);
#endif
}

void Ref::retain()
{
    CCASSERT(_referenceCount > 0,"reference count should greater than 0");
    ++_referenceCount;
}

void Ref::release()
{
    CCASSERT(_referenceCount > 0,"reference count should greater than 0");
    --_referenceCount;

    if (_referenceCount == 0)
    {
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)
        auto poolManager = PoolManager::getInstance();
        if (!poolManager->getCurrentPool()->isClearing() && poolManager->isObjectInPools(this))
        {
            CCASSERT(false,"The reference shouldn't be 0 because it is still in autorelease pool.");
        }
#endif

#if CC_USE_MEM_LEAK_DETECTION
        untrackRef(this);
#endif
        delete this;
    }
}

Ref* Ref::autorelease()
{
    PoolManager::getInstance()->getCurrentPool()->addObject(this);
    return this;
}

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

#if CC_USE_MEM_LEAK_DETECTION

static std::list<Ref*> __refAllocationList;

void Ref::printLeaks()
{
    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());

        for (const auto& 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->getReferenceCount());
        }
    }
}

static void trackRef(Ref* ref)
{
    CCASSERT(ref,"Invalid parameter,ref should not be null!");
    __refAllocationList.push_back(ref);
}

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


NS_CC_END

上面的代码中我删除了一些注释，为了避免干扰。

我们从一看出Ref这个类中有一个

protected:
    //用于计数的变量
    unsigned int _referenceCount;

这个变量就是记录引用次数,并且在构造函数中初始化时1。

在代码retain中可以看到只有简单增加这个计数器。

重要的来了，那就是release这个函数，我们看到他是在计数器为0的时候，delete自己。这样的设计源于自己创建自己销毁这个原则。

我们还可以看到 autoRelease 这个接口，他调用了PoolManager。

从字面上猜到，这个接口的作用应该是自动释放。

那么他是怎么实现的呢？

我们又找到了PoolManager的源码。

CCAutoreleasePool.h

#ifndef __AUTORELEASEPOOL_H__
#define __AUTORELEASEPOOL_H__

#include <stack>
#include <vector>
#include <string>
#include "base/CCRef.h"

NS_CC_BEGIN

class CC_DLL AutoreleasePool
{
public:
    AutoreleasePool();
    
    AutoreleasePool(const std::string &name);
    
    ~AutoreleasePool();

    void addObject(Ref *object);

    void clear();
    
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)

    bool isClearing() const { return _isClearing; };
#endif
    
    bool contains(Ref* object) const;

    void dump();
    
private:
    std::vector<Ref*> _managedObjectArray;
    std::string _name;
    
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)
    bool _isClearing;
#endif
};

class CC_DLL PoolManager
{
public:
    CC_DEPRECATED_ATTRIBUTE static PoolManager* sharedPoolManager() { return getInstance(); }
    static PoolManager* getInstance();

    CC_DEPRECATED_ATTRIBUTE static void purgePoolManager() { destroyInstance(); }
    static void destroyInstance();
    
    AutoreleasePool *getCurrentPool() const;

    bool isObjectInPools(Ref* obj) const;

    friend class AutoreleasePool;
    
private:
    PoolManager();
    ~PoolManager();
    
    void push(AutoreleasePool *pool);
    void pop();
    
    static PoolManager* s_singleInstance;
    
    std::vector<AutoreleasePool*> _releasePoolStack;
};

NS_CC_END

#endif //__AUTORELEASEPOOL_H__

CCAutoreleasePool.cpp

#include "base/CCAutoreleasePool.h"
#include "base/ccMacros.h"

NS_CC_BEGIN

AutoreleasePool::AutoreleasePool()
: _name("")
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0),_isClearing(false)
#endif
{
    _managedObjectArray.reserve(150);
    PoolManager::getInstance()->push(this);
}

AutoreleasePool::AutoreleasePool(const std::string &name)
: _name(name)
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0),_isClearing(false)
#endif
{
    _managedObjectArray.reserve(150);
    PoolManager::getInstance()->push(this);
}

AutoreleasePool::~AutoreleasePool()
{
    CCLOGINFO("deallocing AutoreleasePool: %p",this);
    clear();
    
    PoolManager::getInstance()->pop();
}

void AutoreleasePool::addObject(Ref* object)
{
    _managedObjectArray.push_back(object);
}

void AutoreleasePool::clear()
{
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)
    _isClearing = true;
#endif
    for (const auto &obj : _managedObjectArray)
    {
        obj->release();
    }
    _managedObjectArray.clear();
#if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)
    _isClearing = false;
#endif
}

bool AutoreleasePool::contains(Ref* object) const
{
    for (const auto& obj : _managedObjectArray)
    {
        if (obj == object)
            return true;
    }
    return false;
}

void AutoreleasePool::dump()
{
    CCLOG("autorelease pool: %s,number of managed object %d\n",_name.c_str(),static_cast<int>(_managedObjectArray.size()));
    CCLOG("%20s%20s%20s","Object pointer","Object id","reference count");
    for (const auto &obj : _managedObjectArray)
    {
        CC_UNUSED_PARAM(obj);
        CCLOG("%20p%20u\n",obj,obj->getReferenceCount());
    }
}


//--------------------------------------------------------------------
//
// PoolManager
//
//--------------------------------------------------------------------

PoolManager* PoolManager::s_singleInstance = nullptr;

PoolManager* PoolManager::getInstance()
{
    if (s_singleInstance == nullptr)
    {
        s_singleInstance = new PoolManager();
        // Add the first auto release pool
        new AutoreleasePool("cocos2d autorelease pool");
    }
    return s_singleInstance;
}

void PoolManager::destroyInstance()
{
    delete s_singleInstance;
    s_singleInstance = nullptr;
}

PoolManager::PoolManager()
{
    _releasePoolStack.reserve(10);
}

PoolManager::~PoolManager()
{
    CCLOGINFO("deallocing PoolManager: %p",this);
    
    while (!_releasePoolStack.empty())
    {
        AutoreleasePool* pool = _releasePoolStack.back();
        
        delete pool;
    }
}


AutoreleasePool* PoolManager::getCurrentPool() const
{
    return _releasePoolStack.back();
}

bool PoolManager::isObjectInPools(Ref* obj) const
{
    for (const auto& pool : _releasePoolStack)
    {
        if (pool->contains(obj))
            return true;
    }
    return false;
}

void PoolManager::push(AutoreleasePool *pool)
{
    _releasePoolStack.push_back(pool);
}

void PoolManager::pop()
{
    CC_ASSERT(!_releasePoolStack.empty());
    _releasePoolStack.pop_back();
}

NS_CC_END

可以看到

AutoreleasePool* PoolManager::getCurrentPool()返回了一个AutoreleasePool的指针，<pre name="code" class="cpp">AutoreleasePool这个指针将在Ref autoRelease的时候添加到了一个叫<pre name="code" class="cpp">_managedObjectArray的vector里面去了。

那么自动管理是怎么做到呢？

转到CCDirector.cpp

<pre name="code" class="cpp">void DisplayLinkDirector::mainLoop()
{
    if (_purgeDirectorInNextLoop)
    {
        _purgeDirectorInNextLoop = false;
        purgeDirector();
    }
    else if (! _invalid)
    {
        drawScene();
     
        // release the objects
        PoolManager::getInstance()->getCurrentPool()->clear();
    }
}

在这个文件的上面那个函数中，我们可以看到每个loop里面在渲染场景之后PoolManager做了一个操作，这个操作就是AutoreleasePool类里面的clear，

这个clear就是将维护的_managedObjectArray里面的每个对象执行release操作，最后把_managedObjectArray清空。

<span style="font-family: Arial,Helvetica,sans-serif;">看到这里我们大约知道了他的原理了。</span>

1、我们在创建Ref的时候计数器值是1；

2、执行retain的时候计数器加1，执行release的时候计数器减1，当计数器为0时由对象自己删除自己。

3、当我们执行autorelease的时候，Ref把释放的操作的权利给了AutoreleasePool这个友元类，就是如果我们没有任何retain操作，这个Ref的对象将在这一帧结束后被AutoreleasePool的clear给释放掉。

4、当我们在创建后又retain，那么将保留对这个Ref对象的控制权，只有在你release后才会被释放，因为AutoreleasePool只执行一次便不再参与该对象的内存管理。

<span style="font-family: Arial,sans-serif;">
</span>

<span style="font-family: Arial,sans-serif;">差不多我了解的cocos2dx引用计数的原理就是这么多了，博主的水平也是非常有限，哪里写得不好或出错，欢迎指正交流。</span>

cocos2dx 3.x 源码阅读之一 引用计数原理

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