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cocos2d-x动作原理
首先CCAction是所有动作的基类，如下图继承关系：
那么来看看CCAction的定义：
class CC_DLL CCAction : public CCObject {public: CCAction(void); virtual ~CCAction(void); const char* description(); virtual CCObject* copyWithZone(CCZone *pZone); //! return true if the action has finished virtual bool isDone(void); virtual void startWithTarget(CCNode *pTarget); virtual void stop(void); //! called every frame with it‘s delta time. DON‘T override unless you know what you are doing. virtual void step(float dt); virtual void update(float time); inline CCNode* getTarget(void) { return m_pTarget; } inline void setTarget(CCNode *pTarget) { m_pTarget = pTarget; } inline CCNode* getOriginalTarget(void) { return m_pOriginalTarget; } inline void setOriginalTarget(CCNode *pOriginalTarget) { m_pOriginalTarget = pOriginalTarget; } inline int getTag(void) { return m_nTag; } inline void setTag(int nTag) { m_nTag = nTag; }public: static CCAction* create();protected: CCNode *m_pOriginalTarget; CCNode *m_pTarget; int m_nTag;};在类定义最后有三个成员变量，而继承自CCAction的CCFiniteTimeAction主要新增加了一个用于保存该动作总完成时间的成员变量float m_fDuration;
对于其两个子类CCActionInstant和CCActionInterval，前者没有新增任何函数和变量，而后者增加了两个成员变量：float m_elapsed（记录从动作开始起逝去的时间）;和bool m_bFirstTick（一个控制变量）;
那么动作是如何执行的呢？
当一个节点调用runAction方法时，动作管理类CCActionManager（单例类）会将新的动作和节点添加到其管理的动作表中。
CCAction * CCNode::runAction(CCAction* action){ CCAssert( action != NULL, "Argument must be non-nil"); m_pActionManager->addAction(action, this, !m_bRunning); return action;}
在addAction中，将动作添加到动作队列后，就会对该动作调用其成员函数startWithTarget(CCNode* pTarget)来绑定该动作的执行节点，和初始化动作类的成员变量。
这些工作都完成后，每一帧刷新屏幕时，系统就会在CCActionManager中遍历动作表中的每一个动作，并调用动作的step(float)方法。而step方法主要负责计算m_elapsed的值，并调用update(float)方法。
void CCActionInterval::step(float dt){ if (m_bFirstTick) { m_bFirstTick = false; m_elapsed = 0; } else { m_elapsed += dt; } this->update(MAX (0, // needed for rewind. elapsed could be negative MIN(1, m_elapsed / MAX(m_fDuration, FLT_EPSILON) // division by 0 ) ) );} 传入update方法的float型参数表示逝去的时间与动作完成需要的时间的比值，介于0-1之间，即动作完成的百分比。然后在update方法中，通过完成比例对节点的属性进行操作来达到动作的效果。
例如：对MoveBy调用update时，通过传入的比例调用setPosition直接修改节点的属性。
最后在每一帧结束后，CCActionManager的update会检查动作队列中每个动作的isDone函数是否返回true，如果返回true，则动作结束，将其从队列中删除。
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从上面知道：动作都是由CCActionManager来管理。那我们再来看看CCActionManager的工作原理。
在CCDirector初始化时，执行了如下代码：
// scheduler m_pScheduler = new CCScheduler(); // action manager m_pActionManager = new CCActionManager(); m_pScheduler->scheduleUpdateForTarget(m_pActionManager, kCCPrioritySystem, false);
可见动作管理类在创建时就注册了Update定时器，那么CCScheduler在游戏的每一帧mainLoop更新中都会触发CCActionManager注册的update(float )方法。调度器原理请参照此链接：http://www.cnblogs.com/songcf/p/3162414.html
// main loopvoid CCActionManager::update(float dt){ //枚举动作表中的每一个节点 for (tHashElement *elt = m_pTargets; elt != NULL; ) { m_pCurrentTarget = elt; m_bCurrentTargetSalvaged = false; if (! m_pCurrentTarget->paused) { //枚举节点的每一个动作 actions数组可能会在循环中被修改 for (m_pCurrentTarget->actionIndex = 0; m_pCurrentTarget->actionIndex < m_pCurrentTarget->actions->num; m_pCurrentTarget->actionIndex++) { m_pCurrentTarget->currentAction = (CCAction*)m_pCurrentTarget->actions->arr[m_pCurrentTarget->actionIndex]; if (m_pCurrentTarget->currentAction == NULL) { continue; } m_pCurrentTarget->currentActionSalvaged = false; m_pCurrentTarget->currentAction->step(dt); if (m_pCurrentTarget->currentActionSalvaged) { // The currentAction told the node to remove it. To prevent the action from // accidentally deallocating itself before finishing its step, we retained // it. Now that step is done, it‘s safe to release it. m_pCurrentTarget->currentAction->release(); } else if (m_pCurrentTarget->currentAction->isDone()) { m_pCurrentTarget->currentAction->stop(); CCAction *pAction = m_pCurrentTarget->currentAction; // Make currentAction nil to prevent removeAction from salvaging it. m_pCurrentTarget->currentAction = NULL; removeAction(pAction); } m_pCurrentTarget->currentAction = NULL; } } // elt, at this moment, is still valid // so it is safe to ask this here (issue #490) elt = (tHashElement*)(elt->hh.next); // only delete currentTarget if no actions were scheduled during the cycle (issue #481) if (m_bCurrentTargetSalvaged && m_pCurrentTarget->actions->num == 0) { deleteHashElement(m_pCurrentTarget); } } // issue #635 m_pCurrentTarget = NULL;}cocos2d-x动作原理