cocos2dx内存管理

今天想探索一下cocos2dx的内存管理，我们就先从CCObject开始吧

class CC_DLL CCObject : public CCCopying
{
public:
    // object id, CCScriptSupport need public m_uID
    unsigned int        m_uID;
    // Lua reference id
    int                 m_nLuaID;
protected:
    // count of references
    unsigned int        m_uReference;
    // count of autorelease
    unsigned int        m_uAutoReleaseCount;
public:
    CCObject(void);
    virtual ~CCObject(void);
    
    void release(void);
    void retain(void);
    CCObject* autorelease(void);
    CCObject* copy(void);
    bool isSingleReference(void);
    unsigned int retainCount(void);
    virtual bool isEqual(const CCObject* pObject);

    virtual void update(float dt) {CC_UNUSED_PARAM(dt);};
    
    friend class CCAutoreleasePool;
    
    unsigned int getAutoReleaseCount();
};

以上是CCObject的定义，可以看出CCObject主要有两个保护成员：m_uReference和m_uAutoReleaseCount，这个两个成员到底有什么用呢？让我们一探究竟，首先我们来看一下CCObject的构造函数：

CCObject::CCObject(void)
: m_nLuaID(0)
, m_uReference(1) // when the object is created, the reference count of it is 1
, m_uAutoReleaseCount(0)
{
    static unsigned int uObjectCount = 0;

    m_uID = ++uObjectCount;
}

从构造函数我么可以看出m_uReference初始化为1，m_uAutoReleaseCount初始化为0，另外CCObject维护了一个静态无符号整型变量：uObjectCount来记录CCObject实例的总数目。然后我们来看一下release()和retain()函数：

 

void CCObject::release(void)
{
    if (m_uReference == 0) {//modify by yangm
        CCLOG("CCObject::release reference count should greater than 0");
        return;
    }
    CCAssert(m_uReference > 0, "reference count should greater than 0");
    --m_uReference;
    
    if (m_uReference == 0)
    {
        delete this;
    }
}

由release()函数可以看到每执行一次release()，m_uReference自减一，只有当m_uReference为零时CCObject对象才会被释放；

void CCObject::retain(void)
{
    if (m_uReference == 0) {//modify by yangm
        CCLOG("CCObject::retain reference count should greater than 0,m_uReference=%d",m_uReference);
        return;
    }
    CCAssert(m_uReference > 0, "reference count should greater than 0");

    ++m_uReference;
}

retain()函数和release()函数功能正好相反，每调用一次retain()，m_uReference会自增一；接下来我们再来看看autorelease()函数：

CCObject* CCObject::autorelease(void)
{
    CCPoolManager::sharedPoolManager()->addObject(this);
    return this;
}
void CCPoolManager::addObject(CCObject* pObject)
{
    getCurReleasePool()->addObject(pObject);  //获取栈顶得CCAutoreleasePool实例，并将CCObject加入到CCAutoreleasePool中
}

此处涉及到一个CCPoolManager类，该类内部有一个CCAutoreleasePool类型的栈，而CCPoolManager又是什么呢？其实CCPoolManager就是一个CCArray一个可增长的数组，我们来看看他得addObject(CCObject* pObject)函数：

 

void CCAutoreleasePool::addObject(CCObject* pObject)
{
    m_pManagedObjectArray->addObject(pObject);  //向CCArray中添加CCObject实例

    CCAssert(pObject->m_uReference > 1, "reference count should be greater than 1");
    ++(pObject->m_uAutoReleaseCount);   //CCObject的m_uAutoReleaseCount自增一
    pObject->release(); // no ref count, in this case autorelease pool added.  
}

在调试的时候我们会发现CCObject在release()时他的m_uReference为2，也就避免了被释放，其实该对象实例在添加到自动释放池（CCAutoreleasePool）的时候retain了一下；我们来验证一下，来看看下面的几个函数：

void CCArray::addObject(CCObject* object)
{
    ccArrayAppendObjectWithResize(data, object);
}
/** Appends an object. Capacity of arr is increased if needed. */
void ccArrayAppendObjectWithResize(ccArray *arr, CCObject* object)
{
    ccArrayEnsureExtraCapacity(arr, 1);
    ccArrayAppendObject(arr, object);
}

/** Appends an object. Behavior undefined if array doesn't have enough capacity. */
void ccArrayAppendObject(ccArray *arr, CCObject* object)
{
    CCAssert(object != NULL, "Invalid parameter!");
    object->retain();                           //此处果然retain了一下
    arr->arr[arr->num] = object;
    arr->num++;
}

接下来我们来看一下cocos2dx如何销毁那些无效的对象实例，以下是整个程序的主循环：

 

void CCDisplayLinkDirector::mainLoop(void)
{
    if (m_bPurgeDirecotorInNextLoop)
    {
        m_bPurgeDirecotorInNextLoop = false;
        purgeDirector();
    }
    else if (! m_bInvalid)
     {
         drawScene();
     
         // release the objects
         CCPoolManager::sharedPoolManager()->pop();        
     }
}

由上面函数可知coco2dx在每一帧的帧尾会调用自动释放内存池的管理者—CCPoolManager的pop()函数，我们来看看该函数主要做了哪些工作？

 

void CCPoolManager::pop()
{
    if (! m_pCurReleasePool)
    {
        return;
    }

     int nCount = m_pReleasePoolStack->count();

    m_pCurReleasePool->clear();
 
      if(nCount > 1)
      {
        m_pReleasePoolStack->removeObjectAtIndex(nCount-1);

//         if(nCount > 1)
//         {
//             m_pCurReleasePool = m_pReleasePoolStack->objectAtIndex(nCount - 2);
//             return;
//         }
        m_pCurReleasePool = (CCAutoreleasePool*)m_pReleasePoolStack->objectAtIndex(nCount - 2);
    }

    /*m_pCurReleasePool = NULL;*/
}

主要做的就两点：1、清除当前的自动释放池，2、更新当前自动释放池指针；我们来看看他是如何clear？

 

void CCAutoreleasePool::clear()
{
    if(m_pManagedObjectArray->count() > 0)
    {
        //CCAutoreleasePool* pReleasePool;
#ifdef _DEBUG
        int nIndex = m_pManagedObjectArray->count() - 1;
#endif

        CCObject* pObj = NULL;
        CCARRAY_FOREACH_REVERSE(m_pManagedObjectArray, pObj)
        {
            if(!pObj)
                break;

            --(pObj->m_uAutoReleaseCount);
            
            if( (int)(pObj->m_uAutoReleaseCount)<0 ){
                pObj->m_uAutoReleaseCount = 0;
            }
            //(*it)->release();
            //delete (*it);
#ifdef _DEBUG
            nIndex--;
#endif
        }

        m_pManagedObjectArray->removeAllObjects();
    }
}

以上函数主要做的工作是：1、将内部的所有元素的自动释放引用m_uAutoReleaseCount减一。2、清除内部的所有元素。看到这里我很诧异：为什么要清除所有的元素呢？难道不是清除那些无效的对象实例吗？紧接着看下面的函数看他如何removeAllObjects。

 

void CCArray::removeAllObjects()
{
    ccArrayRemoveAllObjects(data);
}

void ccArrayRemoveAllObjects(ccArray *arr)
{
    while( arr->num > 0 )
    {
        (arr->arr[—arr->num])->release();  //注意此处的—arr->num
    }
}

以上函数其实就是将数组内部的所有元素release一下，并将元素个数减为0。到这里整个对象创建—添加到自动释放池—帧尾的释放的过程就完了，我一开始很是纳闷，假如我有定义了一个类如下：

class A  : public CCNode
{
private:
    CCSprite* m_pSprite1;
    CCSprite* m_pSprite2;
public:
    CREATE_FUNC(A);
    virtual bool init();
};
bool A::init()
{
    m_pSprite1 = CCSprite::create(“1.png”);   //创建精灵1
    m_pSprite2 = CCSprite::create(“2.png”);  //创建精灵2
    this->addChild(m_pSprite);  //只将精灵1添加到父节点
}

实际上到下一帧的时候，m_pSprite2所指向的内存已经无效，m_Sprite1仍然有效。上面的过程是如何做到的呢？只有这种情况才能解释：m_pSprite1和m_pSprite2在create的时候加入到了自动释放池被监视，而m_pSprite1再加入到父节点时retain了一下，才不会在帧尾release的时候被释放掉。我们来验证一下CCNode::addChild(…)(参数就不写了):

 

void CCNode::addChild(CCNode *child, int zOrder, int tag)
{    
    CCAssert( child != NULL, "Argument must be non-nil");
    CCAssert( child->m_pParent == NULL, "child already added. It can't be added again");

    if( ! m_pChildren )
    {
        this->childrenAlloc();
    }

    this->insertChild(child, zOrder);

    child->m_nTag = tag;

    child->setParent(this);
    child->setOrderOfArrival(s_globalOrderOfArrival++);

    if( m_bRunning )
    {
        child->onEnter();
        child->onEnterTransitionDidFinish();
    }
}

发现没有retain,继续看insertChild函数：

void CCNode::insertChild(CCNode* child, int z)
{
    m_bReorderChildDirty = true;
    ccArrayAppendObjectWithResize(m_pChildren->data, child);
    child->_setZOrder(z);
}

发现也没有retain，继续看ccArrayAppendObjectWithResize函数

 

void ccArrayAppendObjectWithResize(ccArray *arr, CCObject* object)
{
    ccArrayEnsureExtraCapacity(arr, 1);
    ccArrayAppendObject(arr, object);
}

任然没有retain，继续看ccArrayAppendObject函数

 

void ccArrayAppendObject(ccArray *arr, CCObject* object)
{
    CCAssert(object != NULL, "Invalid parameter!");
    object->retain();
    arr->arr[arr->num] = object;
    arr->num++;
}

终于发现了retain。

总结：当一个CCObject实例被创建：

1、若被autorelease()，那么在当前帧的帧尾会被release()一次（注意仅仅一次，以后就会被移除监视），若之前加入到了父节点中那么帧尾的release()时会避免释放，反之则会被无情的释放掉。如果没有加入到父节点又想想拥有该对象，那么需要自己retain();

2、若没有autorelease(),那么这个实例就需要自己来管理。