错误症状

最近做实训cocos2dx项目的TA，一个同学在场景切换时遇到如下问题，场景B->C,C->B,B->C,然后场景C->B，返回场景B时程序就崩溃了

主要涉及代码

bool StartScene::init() {
      if (!Scene::init())
        return  false;
    MenuItemFont *skipMenuItem =           MenuItemFont::create("skip",this,menu_selector(StartScene::SkipMenuItemCallback));
    skipMenuItem->setAnchorPoint(Vec2(0,0));
    skipMenuItem->setPosition(Vec2(900,25));
    Menu *skipMenu = Menu::create(skipMenuItem,NULL);
    skipMenu->setAnchorPoint(Vec2::ANCHOR_BOTTOM_LEFT);
    skipMenu->setPosition(0,0);
    this->addChild(skipMenu);
    return true;

回调函数

void StartScene::SkipMenuItemCallback(Ref* sender)
 {
auto levelChosenScene = LevelChooseScene::createScene();
Director::getInstance()->pushScene(levelChosenScene);
}

bool LevelChooseScene::init() {
    if (!Scene::init())
        return  false;
    CCMenuItemFont *levelChosenMenuItem = CCMenuItemFont::create("LevelChosen",menu_selector(LevelChooseScene::LevelChosenMenuItemCallback));
    levelChosenMenuItem->setAnchorPoint(Vec2(0,0));
    levelChosenMenuItem->setPosition(Vec2(500,300));
    CCMenu *levelChosenMenu = CCMenu::create(levelChosenMenuItem,NULL);
    levelChosenMenu->setAnchorPoint(Vec2::ANCHOR_BOTTOM_LEFT);
    levelChosenMenu->setPosition(0,0);
    this->addChild(levelChosenMenu);

    return true;
}

点击回调函数：

void LevelChooseScene::LevelChosenMenuItemCallback(Ref* sender) { Director::getInstance()->popScene(); }

错误定位

通过简单调试，发现startScene的引用计数异常，在一创建之后就为2，而在第二次pop之后变为0，而另外一个TA发现把startScene中的menuitem用CC_CALLBACK_1创建而不是menu_selector创建的话就没问题

MenuItemFont *skipMenuItem = MenuItemFont::create("skip",CC_CALLBACK_1(StartScene::SkipMenuItemCallback,this));

发现由原来的函数创建时:

bool MenuItemFont::initWithString(const std::string& value,Ref* target,SEL_MenuHandler selector)
{
    CCASSERT( !value.empty(),"Value length must be greater than 0");

    _target = target;
    CC_SAFE_RETAIN(target);
    return initWithString(value,std::bind(selector,target,std::placeholders::_1) );
}

会维护了target指针，此例中就是StartScene指针
然后就是定位_target何时release(),

因为scene继承Ref,在Ref的release()函数中增加语句

if (!strcmp(typeid(*this).name(),"class StartScene"))
    {
        CCLOG("hehe");
    }

然后把断点设在CCLOG处，观察堆栈帧

发现在menuitem::onExit()函数中

会realse()一次，导致了错误的发生
因为每次切换场景都对scene额外多realse()了一次，引擎的bug在于不应该在menuitem中维护target指针

这个问题已经在最新版的cocos2dx引擎中解决了，新版本中不在提供menu_item 维护target指针的create函数重载