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);
    /** 
    called after the action has finished. It will set the 'target' to nil.
    IMPORTANT: You should never call "[action stop]" manually. Instead,use: "target->stopAction(action);"
    */
    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 loop
void 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;
}