忍者吃西瓜里面的this指针的问题，谢谢

Cocos2d-x
#1

在init里面：
bool HelloWorld::init()
{
if ( !Layer::init() )
{
return false;
}

SimpleAudioEngine::getInstance()->preloadEffect("pickup.mp3");
SimpleAudioEngine::getInstance()->preloadEffect("hit.mp3");
SimpleAudioEngine::getInstance()->preloadEffect("move.mp3");
SimpleAudioEngine::getInstance()->playBackgroundMusic("TileMap.mp3");
auto winSize=Director::getInstance()->getWinSize();
std::string file = "TileMap.tmx";
auto str = String::createWithContentsOfFile(FileUtils::getInstance()->fullPathForFilename(file.c_str()).c_str());
_tileMap = TMXTiledMap::createWithXML(str->getCString(),"");
_background = _tileMap->getLayer("Background");
_foreground = _tileMap->getLayer("Foreground");
_meta = _tileMap->getLayer("Meta");
_meta->setVisible(false);

TMXObjectGroup *objects = _tileMap->getObjectGroup("Objects");
CCASSERT(NULL != objects, "'Objects' object group not found");
auto spawnPoint = objects->getObject("SpawnPoint");
CCASSERT(!spawnPoint.empty(), "SpawnPoint object not found");
int x = spawnPoint"x"].asInt();
int y = spawnPoint"y"].asInt();

_player = Sprite::create("Player.png");
_player->setPosition(x, y);
addChild(_player);

setViewPointCenter(_player->getPosition());

addChild(_tileMap, -1);

for (auto& eSpawnPoint : objects->getObjects()) {
    ValueMap& dict = eSpawnPoint.asValueMap();
    if (dict"Enemy"].asInt() == 1) {
        x = dict"x"].asInt();
        y = dict"y"].asInt();
        this->addEnemyAtPos(Point(x,y));
    }
}

auto listener = EventListenerTouchOneByOne::create();
//lambda expression: advanced feature in C++ 11
listener->onTouchBegan = &](Touch *touch, Event *unused_event)->bool { return true; };
listener->onTouchEnded = CC_CALLBACK_2(HelloWorld::onTouchEnded, this);
this->_eventDispatcher->addEventListenerWithSceneGraphPriority(listener, this);

_numCollected = 0;
_mode = 0;

this->schedule(schedule_selector(HelloWorld::testCollisions));

return true;

}

下面是ontouchend函数
void HelloWorld::onTouchEnded(Touch *touch, Event *unused_event)
{
if (_mode == 0) {
// old contents of onTouchEnded
auto touchLocation = touch->getLocation();
touchLocation = this->convertToNodeSpace(touchLocation);

    auto playerPos = _player->getPosition();
    auto diff = touchLocation - playerPos;
    if (abs(diff.x) > abs(diff.y)) {
        if (diff.x > 0) {
            playerPos.x += _tileMap->getTileSize().width;
        }
        else {
            playerPos.x -= _tileMap->getTileSize().width;
        }
    }
    else {
        if (diff.y > 0) {
            playerPos.y += _tileMap->getTileSize().height;
        }
        else {
            playerPos.y -= _tileMap->getTileSize().height;
        }
    }

    if (playerPos.x <= (_tileMap->getMapSize().width * _tileMap->getMapSize().width) &&
        playerPos.y <= (_tileMap->getMapSize().height * _tileMap->getMapSize().height) &&
        playerPos.y >= 0 &&
        playerPos.x >= 0)
    {
        this->setPlayerPosition(playerPos);
    }

this->setViewPointCenter(_player->getPosition());
} else {
// code to throw ninja stars will go here
// Find where the touch is
auto touchLocation = touch->getLocation();
touchLocation = this->convertToNodeSpace(touchLocation);

    // Create a projectile and put it at the player's location
    auto projectile = Sprite::create("Projectile.png");
    projectile->setPosition(_player->getPosition());
    this->addChild(projectile);

    // Determine where we wish to shoot the projectile to
    int realX;

    // Are we shooting to the left or right?
    auto diff = touchLocation - _player->getPosition();
    if (diff.x > 0)
    {
        realX = (_tileMap->getMapSize().width * _tileMap->getTileSize().width) +
        (projectile->getContentSize().width / 2);
    }
    else {
        realX = -(_tileMap->getMapSize().width * _tileMap->getTileSize().width) -
        (projectile->getContentSize().width / 2);
    }
    float ratio = (float)diff.y / (float)diff.x;
    int realY = ((realX - projectile->getPosition().x) * ratio) + projectile->getPosition().y;
    auto realDest = Point(realX, realY);

    // Determine the length of how far we're shooting
    int offRealX = realX - projectile->getPosition().x;
    int offRealY = realY - projectile->getPosition().y;
    float length = sqrtf((offRealX*offRealX) + (offRealY*offRealY));
    float velocity = 480 / 1; // 480pixels/1sec
    float realMoveDuration = length / velocity;

    // Move projectile to actual endpoint
    auto actionMoveDone = CallFuncN::create(CC_CALLBACK_1(HelloWorld::projectileMoveFinished, this));
    projectile->runAction(Sequence::create(MoveTo::create(realMoveDuration, realDest), actionMoveDone, NULL));

    _projectiles.pushBack(projectile);
}

}

这里面的this指针的调用对象是什么，为什么地图图层会偏移呢，__tileMap在那里调用setviewpoint了？