cocos2dx中onExitTransitionDidStart、onExit和onEnter、onEnterTransitionDidFinish处理机制

进入Node::addChildHelper定义发现如下代码

if( _running )

    {

        child->onEnter();

        // prevent onEnterTransitionDidFinish to be called twice when a node is added in onEnter

        if (_isTransitionFinished) {

            child->onEnterTransitionDidFinish();

        }

    }

也就是说，只有父节点已经在running的情况下，添加子节点时才会调用子节点的onEnter，由此可见，在scene中的添加的子节点，如果scene没有被渲染，那么之前在scene中添加的子节点(包括layer，sprite)时不会立刻调用子节点的onEnter。那么scene的onEnter()在什么时候调用呢？查看Director中的DrawScene()定义，发现如下代码：

 /* to avoid flickr, nextScene MUST be here: after tick and before draw.

     XXX: Which bug is this one. It seems that it can't be reproduced with v0.9 */

    if (_nextScene)

    {

        setNextScene();

    }

查看setNextScene定义发现

void Director::setNextScene()

{

    bool runningIsTransition = dynamic_cast<TransitionScene*>(_runningScene) != nullptr;

    bool newIsTransition = dynamic_cast<TransitionScene*>(_nextScene) != nullptr;

    // If it is not a transition, call onExit/cleanup

     if (! newIsTransition)

     {

         if (_runningScene)

         {

             _runningScene->onExitTransitionDidStart();

             _runningScene->onExit();

         }

         // issue #709. the root node (scene) should receive the cleanup message too

         // otherwise it might be leaked.

         if (_sendCleanupToScene && _runningScene)

         {

             _runningScene->cleanup();

         }

     }

    if (_runningScene)

    {

        _runningScene->release();

    }

    _runningScene = _nextScene;

    _nextScene->retain();

    _nextScene = nullptr;

    if ((! runningIsTransition) && _runningScene)

    {

        _runningScene->onEnter();

        _runningScene->onEnterTransitionDidFinish();

    }

}

在场景类型不是TransitionScene类型时，当当前场景不为null时，先退出当前场景，先调用当前场景的onExitTransitionDidStart、onExit并释放当前场景，接着讲下一个场景赋值给当前场景_runningScene = _nextScene;接着调用当前场景的onEnter()、onEnterTransitionDidFinish()，进行自身以及已添加子节点的onEnter调用，每个节点会在自身的onEnter中设置_running = true;这样后续在已有场景中添加子节点时，会直接调用子节点的onEnter。

在已渲染场景中，子节点动态从父类中移除时，会调用onExitTransitionDidStart以及onExit

void Node::detachChild(Node *child, ssize_t childIndex, bool doCleanup)

{

    // IMPORTANT:

    //  -1st do onExit

    //  -2nd cleanup

    if (_running)

    {

        child->onExitTransitionDidStart();

        child->onExit();

    }

#if CC_USE_PHYSICS

    child->removeFromPhysicsWorld();

#endif

    // If you don't do cleanup, the child's actions will not get removed and the

    // its scheduledSelectors_ dict will not get released!

    if (doCleanup)

    {

        child->cleanup();

    }

    // set parent nil at the end

    child->setParent(nullptr);

    _children.erase(childIndex);

}