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。