步骤如下：
1.使用上一篇的工程；
2.下载本游戏所需的资源，将资源放置Resources目录下：

删除旧的资源player.png和projectile.png；
3.在HelloWorldScene.cpp文件，init函数，修改创建玩家精灵：

        _player = Sprite::create(player2.png);

    Sprite* projectile = Sprite::create(projectile2.png);

5.计算炮塔旋转的角度。看下面图：

数学的知识就是，tan(angle) = 对边 / 邻边，利用反正切angle = arctan(对边 / 邻边)，这时计算出的是弧度，用CC_RADIANS_TO_DEGREES宏转换成角度。另外在数学中，逆时针为正，在Cocos2D-x中，顺时针为正，就如下图所示：

需要将最后计算出的角度乘以-1。在onTouchesEnded函数里，添加如下代码在projectile精灵runAction之前：

    float angleRadians = atanf((float)offRealY / (float)offRealX);
    float angleDegrees = CC_RADIANS_TO_DEGREES(angleRadians);
    float cocosAngle = -1 * angleDegrees;
    _player->setRotation(cocosAngle);

8.旋转再射击。炮塔的旋转是瞬间完成的，这不符合现实，需要让它有个动作移动炮塔的方向。在HelloWorldScene.h文件中，添加如下声明：

    Sprite* _nextProjectile;

    _player = NULL;
    _nextProjectile = NULL;

void HelloWorld::onTouchesEnded(const std::vector &touches, cocos2d::Event *event)
{
    if (_nextProjectile != NULL) {
        return;
    }
    
    
    Touch* touch = touches.front();
    Point location = this->convertTouchToNodeSpace(touch);
    
    
    Size winSize = Director::getInstance()->getWinSize();
    
    _nextProjectile = Sprite::create(projectile2.png);
    _nextProjectile->retain();
    _nextProjectile->setScale(2);
    _nextProjectile->setPosition(Point(20, winSize.height / 2));
    
    Point offset = ccpSub(location, _nextProjectile->getPosition());
    
    if (offset.x <= 0) {
        return;
    }
    
   
    
    int realX = winSize.width + _nextProjectile->getContentSize().width / 2;
    float ratio = (float)offset.y / (float)offset.x;
    int realY = realX * ratio + _nextProjectile->getPosition().y;
    Point realDest = Point(realX, realY);
    
    int offRealX = realX - _nextProjectile->getPosition().x;
    int offRealY = realY - _nextProjectile->getPosition().y;
    float length = sqrtf(offRealX * offRealX + offRealY * offRealY);
    float velocity = 480 / 1;
    
    float realMoveDuration = length / velocity;
    
    float angleRadians = atanf((float)offRealY / (float)offRealX);
    float angleDegrees = CC_RADIANS_TO_DEGREES(angleRadians);
    float cocosAngle = -1 * angleDegrees;
    
    float rotateDegreesPerSecond = 180 / 0.5;
    float degreesDiff = _player->getRotation() - cocosAngle;
    float rotateDuration = fabs(degreesDiff / rotateDegreesPerSecond);

    RotateTo* rotate = RotateTo::create(rotateDuration, cocosAngle);
    CallFunc* callFc = CallFunc::create(std::bind(&HelloWorld::finishShoot, this));
    
    _player->runAction(Sequence::create(rotate,callFc, NULL));
    
    
    _nextProjectile->runAction(Sequence::create(MoveTo::create(realMoveDuration, realDest),
                                                CallFuncN::create(std::bind(&HelloWorld::spriteMoveFinished, this,_nextProjectile)), NULL));
    _nextProjectile->setTag(2);
    
    

}

void HelloWorld::finishShoot()
{
    this->addChild(_nextProjectile);
    _projectiles->addObject(_nextProjectile);
    _nextProjectile->release();
    _nextProjectile = NULL;
}