动态对象之间的Box2d碰撞

我想用Box2d做一个简单的射击游戏。

敌人和子弹都是充满活力的身体。我在屏幕左侧创建敌人,然后向他们施加线性力,使他们向右移动。玩家需要击中敌人三次才能杀死敌人。

我的问题是,当敌人头两次被子弹击中时,我如何才能让敌人保持移动?现在,当敌人第一次被击中时,它会被击倒,甚至从屏幕上被击倒。我希望敌人只有在被杀死(第三次被击中)时才会表现出这样的行为。我该怎么做?

谢谢你的建议!

此处的一些代码:

    // Apply force
    rock.isFlying = 1;
    b2Vec2 force = b2Vec2(-2.0f*distance*cos,-2.0f*distance*sin);
    rockBody->ApplyForce(force,rockBody->GetWorldCenter());
    rockBody->SetLinearDamping(0.2f);

- (void)birdFly:(ccTime)dt{
    for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
    {
        ZLBird *myBird = (__bridge ZLBird*)b->GetUserData();
        if (b->GetUserData() != NULL && myBird.tag == TAG_BIRD)
        {
            // Apply force
            b2Vec2 gravityOffset;
            gravityOffset.x = 0;
            gravityOffset.y = myBird.multiplier*(myBird.startY - myBird.position.y);
            if (!myBird.isDead)
                b->ApplyForce(gravityOffset , b->GetWorldCenter());
        }
    }
}

void ContactListener::BeginContact(b2Contact* contact)
{
    b2Body* bodyA = contact->GetFixtureA()->GetBody();
    b2Body* bodyB = contact->GetFixtureB()->GetBody();
    CCSprite* spriteA = (__bridge CCSprite*)bodyA->GetUserData();
    CCSprite* spriteB = (__bridge CCSprite*)bodyB->GetUserData();

    if (spriteA != NULL && spriteB != NULL)
    {
        // if hit by rock
        if (spriteA.tag == -1 && spriteB.tag == TAG_BIRD){
            ZLBird *myBird = (__bridge ZLBird*)bodyB->GetUserData();
            ZLRock *myRock = (__bridge ZLRock*)bodyA->GetUserData();
            [myBird getHurt];
        }
        else if(spriteB.tag == -1 && spriteA.tag == TAG_BIRD) {
            ZLBird *myBird = (__bridge ZLBird*)bodyA->GetUserData();
            ZLRock *myRock = (__bridge ZLRock*)bodyB->GetUserData();
            [myBird getHurt];
        }
}
}