Cocos2d-x V3.x内存管理分析

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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SceneLayerMenuItem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:

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/**  * Allocates and initializes a node.  * @return A initialized node which is marked as "autorelease".  */  * 分配空间并初始化Node  * 返回一个被初始化过且是autorelease的Node对象  */ static Node * create(); 

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。

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Node::create() {  ret = new Node();  if (ret && ret->init())  {  autorelease();  }  else  {  CC_SAFE_DELETE(ret);  }  return ret; } 

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:

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#define CC_SAFE_DELETE(p) do { delete (p); (p) = nullptr; } while(0) 

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:

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 * define a create function for a specific type,such as Layer  * @param \__TYPE__ class type to add create(),117)!important"> */ #define CREATE_FUNC(__TYPE__) \ static __TYPE__* create() \ { \  __TYPE__ *pRet = new __TYPE__(); \  if (pRet && pRet->init()) \  { \  pRet->autorelease(); \  return pRet; \  } \  else \  delete pRet; \  pRet = NULL; \  return NULL; \ } 

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:

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class CC_DLL Ref { public:   * Retains the ownership.  *  * This increases the Ref's reference count.  * @see release,autorelease  * @js NA  */   * 拿到所有权  * 这会增加引用计数  */  void retain();    * Releases the ownership immediately.  * This decrements the Ref's reference count.  * If the reference count reaches 0 after the descrement,this Ref is  * destructed.  * @see retain,117)!important"> * 立即释放所有权  * 这会减少引用计数  * 如果更新后的引用计数为0,该Ref对象会被销毁 release();    * Releases the ownership sometime soon automatically.  * This descrements the Ref's reference count at the end of current  * autorelease pool block.  * @returns The Ref itself.  * @see AutoreleasePool,retain,release  * @lua NA  * 自动释放所有权  */  Ref* autorelease();    * Returns the Ref's current reference count.  * @returns The Ref's reference count.  * 返回该Ref对象的引用计数 unsigned int getReferenceCount() const;  protected:   * Constructor  * The Ref's reference count is 1 after construction.  * 构造函数  * 初始引用计数为1 Ref();   */  virtual ~ * 采用引用计数(reference counting)  * _referenceCount就是计数值  */  // count of references  _referenceCount;   friend AutoreleasePool;   // Memory leak diagnostic data (only included when CC_USE_MEM_LEAK_DETECTION is defined and its value isn't zero)  // 以下函数用于开启内存泄露检测时打印出泄露信息 #if CC_USE_MEM_LEAK_DETECTION public:  static printLeaks(); #endif }; 

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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trackRef(ref); untrackRef(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  _referenceCount != 0)  untrackRef(#endif }  // retain只是单纯将计数递增 retain() {  // CCASSERT是cocos2d-x对C++的assert所封装的宏  CCASSERT(_referenceCount > 0, "reference count should greater than 0");  ++_referenceCount; }  release() {  // 首先计数递减  "reference count should greater than 0");  --_referenceCount;   // 计数为0,应当析构对象  _referenceCount == 0)  { #if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 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->getCurrentPool()->isClearing() && 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->autorelease(); // Wrong: If you wish to invoke autorelease several times,you should retain `obj` first.  // Wrong usage (2):  // auto obj = Node::create();  // obj->release(); // Wrong: obj is an autorelease Ref,it will be released when clearing current pool.  // Correct usage (1):  // |- new Node(); // `new` is the pair of the `autorelease` of next line  // |- autorelease(); // The pair of `new Node`.  // obj->retain();  // obj->autorelease(); // This `autorelease` is the pair of `retain` of prevIoUs line.  // Correct usage (2):  // obj->release(); // This `release` is the pair of `retain` of prevIoUs line.  CCASSERT(false,152)!important">"The reference shouldn't be 0 because it is still in autorelease pool.");  } // 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 #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 autorelease() {  getInstance()->addObject(this);  return this; }  const {  _referenceCount; }  #if CC_USE_MEM_LEAK_DETECTION  // 这里便是存放所追踪的对象指针的列表 std::list<Ref*> __refAllocationList;  printLeaks() {  // Dump Ref object memory leaks  __refAllocationList.empty())  {  log("[memory] All Ref objects successfully cleaned up (no leaks detected).\n");  }  "[memory] WARNING: %d Ref objects still active in memory.", (int)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& ref : __refAllocationList)  {  CC_ASSERT(ref);  const char* type = typeid(*ref).name();  "[memory] LEAK: Ref object '%s' still active with reference count %d.(type ? type : ""), ref->getReferenceCount());  }  } }  // 将对象指针放入列表中 ref) {  ref,152)!important">"Invalid parameter,ref should not be null!");   // Create memory allocation record.  push_back(ref); }  // 在列表中找到该对象指针并删除 ref) {  iter = find(begin(),210)!important">end(),210)!important">iter == end())  {  "[memory] CORRUPTION: Attempting to free (%s) with invalid ref tracking record.name());  return;  }   erase(iter); }  #endif // #if CC_USE_MEM_LEAK_DETECTION 

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:
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obj = create(); // 注意create函数调用autorelease方法,因此obj已经没有该指针的所有权了 obj->autorelease(); // obj没有所有权,因此无法再把所有权转交给AutoreleasePool,若要调用autorelease方法需要先调用retain拿到所有权 
- release缺乏对应的retain。

例如:

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release(); // obj没有所有权,因此无法再控制计数(所有权在AutoreleasePool),若要调用release方法需要先调用retain拿到所有权 
  • 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:
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// 前面我们分析过create函数,它会先用new operator得到对象,再调用autorelease方法 // 这里new和autorelease对应 create();  |- Node();  |- autorelease();  // 这里retain和autorelease对应,autorelease一个已经被autorelease过的对象(例如通过create函数构造的对象)必须先retain retain(); autorelease(); 

又如:

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create(); // 这里retain和release对应,release一个已经被autorelease过的对象(例如通过create函数构造的对象)必须先retain release(); 

AutoreleasePool类

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

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AutoreleasePool {  * @warn Don't create an auto release pool in heap,create it in stack.  * 警告:不要在heap上构造AutoreleasePool对象,要在stack上构造 AutoreleasePool();    * Create an autorelease pool with specific name. This name is useful for debugging. AutoreleasePool(const string &name);    */  ~ * Add a given object to this pool.  * The same object may be added several times to the same pool; When the  * pool is destructed,the object's Ref::release() method will be called  * for each time it was added.  * @param object The object to add to the pool.  * 把指定的对象指针放到AutoreleasePool对象中  * 注意:  * 同一对象的指针可能会被多次加入到同一AutoreleasePool对象中;  * 当该AutoreleasePool对象被析构时,该对象指针被加入多少次,就得调用多少次该对象的release()函数  * 这是因为AutoreleasePool用vector而非set来存放所管理的对象指针,因此不会去重 addObject(Ref *object);    * Clear the autorelease pool.  * Ref::release() will be called for each time the managed object is  * added to the pool.  * 清空AutoreleasePool  * 每个被管理的对象指针被加入多少次,就会调用多少次release()函数 clear();  #if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)   * Whether the pool is doing `clear` operation. bool isClearing() const { _isClearing; }; #endif    * Checks whether the pool contains the specified object.  * 检查AutoreleasePool对象是否管理某个对象指针 contains(object) const;    * Dump the objects that are put into autorelease pool. It is used for debugging.  * The result will look like:  * Object pointer address object id reference count dump();  private:   * The underlying array of object managed by the pool.  * Although Array retains the object once when an object is added,proper  * Ref::release() is called outside the array to make sure that the pool  * does not affect the managed object's reference count. So an object can  * be destructed properly by calling Ref::release() even if the object  * is in the pool.  * AutoreleasePool对象将它所管理的对象指针放到下面的vector中  * 尽管每次有对象指针加到该vector中时,该vector实际上retain拿到了所有权,  * 但是Ref::release()会被调用来保证AutoreleasePool不会改变它所管理的对象指针  * 的引用计数。  * 所以,当某个对象指针被放到AutoreleasePool类中管理时,仍然可以通过调用  * Ref::release()函数来析构它 vector<_managedObjectArray;  string _name;   * The flag for checking whether the pool is doing `clear` operation. _isClearing;  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:

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AutoreleasePool::AutoreleasePool() : _name("") #if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0) ,210)!important">_isClearing(false) #endif {  _managedObjectArray.reserve(150);  // 每个新创建的AutoreleasePool对象都交由PoolManager单例对象统一管理  push(this); }  name) : _name(name) AutoreleasePool::~AutoreleasePool() {  CCLOGINFO("deallocing AutoreleasePool: %p",22)!important">this);  // 清空该AutoreleasePool  clear();   // 要析构的AutoreleasePool对象不再由PoolManager管理  pop(); }  // 只是单纯调用vector::push_back加入所管理的对象 object) {  object); }  // clear函数就是AutoreleasePool调用release来管理对象的引用计数的地方 clear() { #if defined(COCOS2D_DEBUG) && (COCOS2D_DEBUG > 0)  _isClearing = true; // 调用每个在AutoreleasePool的对象指针的release方法  auto &obj : _managedObjectArray)  {  release();  }  // 清空存放管理对象的vector  clear(); false; // 线性搜索所管理的对象指针的vector,查看所指定的Ref指针是否存在 _managedObjectArray)  {  obj == object)  return true;  }  false; }  dump() {  CCLOG("autorelease pool: %s,number of managed object %d_name.c_str(),22)!important">static_cast<int>(size()));  "%20s%20s%20s",152)!important">"Object pointer",152)!important">"Object id",152)!important">"reference count");  CC_UNUSED_PARAM(obj);  "%20p%20uobj,210)!important">getReferenceCount());  } } 

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:

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PoolManager { CC_DEPRECATED_ATTRIBUTE PoolManager* sharedPoolManager() { getInstance(); }  getInstance();   purgePoolManager() { destroyInstance(); }  destroyInstance();    * Get current auto release pool,there is at least one auto release pool that created by engine.  * You can create your own auto release pool at demand,which will be put into auto releae pool stack. AutoreleasePool *getCurrentPool() const;   isObjectInPools(obj) AutoreleasePool;  private:  // singleton类把构造函数和析构函数设为private,避免被调用  PoolManager();  ~PoolManager();   push(pool);  pop();   s_singleInstance;   // 同样用vector来存放所管理AutoreleasePool对象指针的列表  AutoreleasePool*> _releasePoolStack; }; 

再来看类实现:

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s_singleInstance = nullptr;  getInstance() {  s_singleInstance == nullptr)  {  s_singleInstance = PoolManager();  // Add the first auto release pool  AutoreleasePool("cocos2d autorelease pool");  }  s_singleInstance; }  destroyInstance() {  delete s_singleInstance;  nullptr; }  PoolManager() {  _releasePoolStack.10); }  PoolManager::~"deallocing PoolManager: %p",22)!important">this);   // 逐个析构所管理的AutoreleasePool对象  while (!AutoreleasePool* pool = back();   pool;  } }  // 加入AutoreleasePool对象指针时用的是stl::vector的push_back函数 // 于是调用back函数就可以得到最新被加入的AutoreleasePool对象指针 back(); }  // 线性搜索每个被管理的AutoreleasePool, // 每个AutoreleasePool对象再用contains函数线性搜索一遍 pool : _releasePoolStack)  {  pool->obj))  pool) {  pool); }  pop() {  CC_ASSERT(!empty());  pop_back(); } 

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:

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DisplayLinkDirector::mainLoop() {  _purgeDirectorInNextLoop)  {  _purgeDirectorInNextLoop = false;  purgeDirector();  }  else if (! _invalid)  {  drawScene();   // release the objects  clear();  } } 

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。

原文链接:https://www.f2er.com/cocos2dx/339420.html

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