Cocos2d Box2D之碰撞检测

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在Box2D中碰撞事件由b2ContactListener类函数实现，b2ContactListener是Box2D提供的抽象类，它的抽象函数：

 /// Called when two fixtures begin to touch.两个物体开始接触时会响应，但只调用一次。
 virtual void BeginContact(b2Contact* contact) { B2_NOT_USED(contact); }

 /// Called when two fixtures cease to touch.分离时响应。但只调用一次。
 virtual void EndContact(b2Contact* contact) { B2_NOT_USED(contact); }

 /// This is called after a contact is updated. This allows you to inspect a
 /// contact before it goes to the solver. If you are careful, you can modify the
 /// contact manifold (e.g. disable contact).
 /// A copy of the old manifold is provided so that you can detect changes.
 /// Note: this is called only for awake bodies.
 /// Note: this is called even when the number of contact points is zero.
 /// Note: this is not called for sensors.
 /// Note: if you set the number of contact points to zero, you will not
 /// get an EndContact callback. However, you may get a BeginContact callback
 /// the next step.持续接触时响应，它会被多次调用。
 virtual void PreSolve(b2Contact* contact, const b2Manifold* oldManifold)
 {
         B2_NOT_USED(contact);
         B2_NOT_USED(oldManifold);
 }

 /// This lets you inspect a contact after the solver is finished. This is useful
 /// for inspecting impulses.
 /// Note: the contact manifold does not include time of impact impulses, which can be
 /// arbitrarily large if the sub-step is small. Hence the impulse is provided explicitly
 /// in a separate data structure.
 /// Note: this is only called for contacts that are touching, solid, and awake.

 /// 持续接触时响应，调用完preSolve后调用。
 virtual void PostSolve(b2Contact* contact, const b2ContactImpulse* impulse)
 {
         B2_NOT_USED(contact);
         B2_NOT_USED(impulse);
 }

第一步：我们检测碰撞的类要继承自public b2ContactListener

第二步：创建物理世界的监听对象(在init()中添加)

第三步：在类中添加检测碰撞的函数(上面的四个中选择一个)

第四步：进行碰撞检测

如上：我要检测小鸟是否受到了碰撞，那么我通过contact->GetFixtureA()和contact->GetFixtureB()去取得当前的物理世界中产生碰撞的物体是否等于小鸟的刚体变量(birdBody)。如果相等说明产生了碰撞，如果不相等则说明没有碰撞。

摘自：http://www.cocos2d-x.org/wiki/Box2D

To find out when a fixture collides with another fixture in Box2D, we need to register a contact listener. A contact listener is a C++ object that we give Box2D, and it will call methods on that object to let us know when two objects begin to touch and stop touching. we can’t just store references to the contact points that are sent to the listener, because they are reused by Box2D. So we have to store copies of them instead.

void MyContactListener::BeginContact(b2Contact* contact)
{
    // We need to copy out the data because the b2Contact passed in
    // is reused.
    MyContact myContact = { contact->GetFixtureA(), contact->GetFixtureB() };
    _contacts.push_back(myContact);
}

The following iterates through all of the buffered contact points, and checks to see if any of them are a match between the ball and the bottom of the screen.

void MyContactListener::EndContact(b2Contact* contact)
{
    MyContact myContact = { contact->GetFixtureA(), contact->GetFixtureB() };
    std::vector::iterator pos;
    pos = std::find(_contacts.begin(), _contacts.end(), myContact);
    if (pos != _contacts.end())
    {
        _contacts.erase(pos);
    }
}

In the Helloworld.cpp,“tick” method wil use _contactListener to go through the contact points of bodies that are colliding. If a sprite is intersecting with a block, we add the block to a list of objects to destroy.

b2Body *bodyA = contact.fixtureA->GetBody();
b2Body *bodyB = contact.fixtureB->GetBody();
if (bodyA->GetUserData() != NULL && bodyB->GetUserData() != NULL) {
CCSprite *spriteA = (CCSprite *) bodyA->GetUserData();
CCSprite *spriteB = (CCSprite *) bodyB->GetUserData();

// Sprite A = ball, Sprite B = Block
if (spriteA->getTag() == 1 && spriteB->getTag() == 2) {
    if (std::find(toDestroy.begin(), toDestroy.end(), bodyB)
             == toDestroy.end()) {
              toDestroy.push_back(bodyB);
      }
}
// Sprite B = block, Sprite A = ball
else if (spriteA->getTag() == 2 && spriteB->getTag() == 1) {
            if (std::find(toDestroy.begin(), toDestroy.end(), bodyA)
            == toDestroy.end()) {
            toDestroy.push_back(bodyA);
       }
}　　

Now you have even more techniques to add to Box2D in your game. I look forward to seeing some cool physics games from you guys!

运行代码：

.h files

#ifndef _GAME_PLAY_H_
#define _GAME_PLAY_H_

#define PTM_RATIO      32

#include "cocos2d.h"
#include "Box2D\Box2D.h"

USING_NS_CC;
using namespace CocosDenshion;

class GamePlay : public cocos2d::Layer, public b2ContactListener
{
private:
        cocos2d::Sprite* backgroundA;
        cocos2d::Sprite* backgroundB;
        cocos2d::Sprite* ready;
        cocos2d::Sprite* tutorial;
        cocos2d::Sprite* bird;
        cocos2d::Sprite* land;
        cocos2d::Sprite* num;
        cocos2d::Sprite* upPipe;
        cocos2d::Sprite* downPipe;
        cocos2d::Sprite* pipeContainer;
        cocos2d::Sprite* model;
        cocos2d::Sprite* gameEnd;
        cocos2d::LabelTTF* score;
        cocos2d::LabelTTF* best;
        cocos2d::MenuItemImage* play;
        cocos2d::MenuItemImage* exit;
        b2World* world;
        b2Body* birdBody;
        b2Body* landBody;
        b2Body* downBody;
        b2Body* upBody;
        int bestScore;
        int times = 0;

private:
        void replaceBackground(int);
        void tipInformation();
        void addBird();
        void addLand();
        void addPipe(float dt);
        void gameBegin(float dt);
        void gameOver();
        void timeAnimate();
        void upperBoundary();
        //int birdSelect(float);

public:
        static cocos2d::Scene* createScene();
        virtual bool init();
        void initPhysicsWorld();
        virtual void update(float);

        /// Called when two fixtures begin to touch.
        virtual void BeginContact(b2Contact* contact);

        /** Callback function for multiple touches began.
        *
        * @param touches Touches information.
        * @param unused_event Event information.
        * @js NA
        */
        virtual void onTouchesBegan(const std::vector<Touch*>& touches, Event *unused_event);
        void goPlay(cocos2d::Ref* pSender);
        void goExit();
        CREATE_FUNC(GamePlay);
};

#endif // _GAME_PLAY_H_

.cpp files
#include "GamePlay.h"
#include "GameUnit.h"
#include "GameData.h"

unit u3;

cocos2d::Scene* GamePlay::createScene()
{
        auto scene = Scene::create();
        auto layer = GamePlay::create();
        scene->addChild(layer);
        return scene;
}

bool GamePlay::init()
{
        if (!Layer::init())
        {
               return false;
        }
        this->initPhysicsWorld();
        this->replaceBackground(1);
        this->tipInformation();
        this->upperBoundary();
        this->addLand();
        this->addBird();
        this->timeAnimate();

        //设置多点触屏事件的监听器
        auto listener = EventListenerTouchAllAtOnce::create();
        listener->onTouchesBegan = CC_CALLBACK_2(GamePlay::onTouchesBegan, this);
        //注册监听器
        _eventDispatcher->addEventListenerWithSceneGraphPriority(listener, this);

        //设置物理世界监听
        world->SetContactListener(this);

        scheduleOnce(schedule_selector(GamePlay::gameBegin), 3);
        //scheduleUpdate();

        return true;
}

void GamePlay::initPhysicsWorld()
{
        //设置重力加速度为9.8;方向向下
        b2Vec2 gravity;
        gravity.Set(0.0f, -9.8f);
        //创建一个新的物理世界
        world = new b2World(gravity);

        //设置是否允许物体休眠
        world->SetAllowSleeping(true);
        //连续物理测试，防止发生非子弹特性的物体发生穿透现象
        world->SetContinuousPhysics(true);
}

void GamePlay::replaceBackground(int flag)
{
        switch (flag)
        {
        case 1:
               backgroundA = Sprite::create("background/light.png");
               backgroundA->setPosition(Vec2(u3.winOrigin().x + u3.winSize().width / 2,
                       u3.winOrigin().y + u3.winSize().height / 2));
               backgroundA->setScale(u3.scaleX(backgroundA, u3.winSize()),
                       u3.scaleY(backgroundA, u3.winSize()));
               this->addChild(backgroundA, 0);
               break;
        default:
               break;
        }
}

void GamePlay::tipInformation()
{
        ......
        //exit
        exit = MenuItemImage::create(
               "button/exit_f.png",
               "button/exit_b.png",
               CC_CALLBACK_0(GamePlay::goExit, this)
               );
        Menu* menu = Menu::create(exit, NULL);
        menu->setPosition(Vec2(u3.winOrigin().x + exit->getContentSize().width / 2,
               u3.winOrigin().y + exit->getContentSize().height / 2));
        this->addChild(menu, 7);
}

void GamePlay::addBird()
{
        bird = Sprite::create("bird/littleBird.png");
        bird->setPosition(Vec2(u3.winOrigin().x + u3.winSize().width / 3,
               u3.winOrigin().y + u3.winSize().height - 5 * (ready->getContentSize().height)));
        bird->setScale(1.5);
        auto repeat = RepeatForever::create(Animate::create(u3.gameAnimate(1)));
        bird->runAction(repeat);
        this->addChild(bird, 1);

        //创建刚体
        b2BodyDef birdBodyDef;
        birdBodyDef.type = b2_dynamicBody;
        birdBodyDef.position.Set(bird->getPosition().x / PTM_RATIO,
               bird->getPosition().y / PTM_RATIO);
        //将刚体，精灵与世界关联起来
        birdBody = world->CreateBody(&birdBodyDef);
        birdBody->SetUserData(bird);

        //定义一个盒子
        b2PolygonShape birdBox;
        birdBox.SetAsBox(bird->getContentSize().width / 3 / PTM_RATIO,
               bird->getContentSize().height / 3 / PTM_RATIO);
        //夹具
        b2FixtureDef fixtureDef;
        //设置夹具的形状
        fixtureDef.shape = &birdBox;
        birdBody->CreateFixture(&fixtureDef);
}

void GamePlay::update(float dt)
{
        world->Step(dt, 8, 3);
        for (b2Body* bb = world->GetBodyList(); bb; bb = bb->GetNext())
        {
               if (bb->GetUserData() != nullptr)
               {
                       Sprite* sprite = (Sprite*)bb->GetUserData();
                       //设置精灵的当前的位置，通过取得精灵的位置乘上相应的像素，这里的PTM_RATIO为32个像素为1m，就可以判断出精灵的位置处于物理世界的何处
                       sprite->setPosition(Vec2(bb->GetPosition().x*PTM_RATIO,
                               bb->GetPosition().y*PTM_RATIO));
                       //设置精灵的角度偏转
                       sprite->setRotation(0);
               }
        }
}

//添加地面
void GamePlay::addLand()
{
        //地板也是物理世界的一个刚体，是一个静态的刚体
        land = Sprite::create("background/land.png");
        land->setPosition(Vec2(u3.winOrigin().x + u3.winSize().width / 2,
               u3.winOrigin().y + land->getContentSize().height / 2));
        land->setScaleX(u3.winSize().width / 1.5*land->getContentSize().width);
        land->setScaleY(u3.winSize().height / (land->getContentSize().height * 3));
        this->addChild(land, 4);

        //创建刚体
        b2BodyDef landBodyDef;
        landBodyDef.type = b2_staticBody;
        landBodyDef.position.Set(u3.winSize().width / 2 / PTM_RATIO,
               land->getPosition().y / PTM_RATIO);

        landBody = world->CreateBody(&landBodyDef);
        landBody->SetUserData(land);

        b2PolygonShape landShape;
        landShape.SetAsBox(u3.winSize().width / 2 / PTM_RATIO,
               backgroundA->getContentSize().height / 2 / PTM_RATIO);
        //1.4*land->getContentSize().height / PTM_RATIO
        b2FixtureDef landFixtureDef;
        landFixtureDef.shape = &landShape;
        landBody->CreateFixture(&landFixtureDef);
}

void GamePlay::upperBoundary()
{
        .......
}

void GamePlay::addPipe(float dt)
{
        times++;

        float randPipe = -rand() % 3;
        //down bar
        downPipe = Sprite::create("pipe/down.png");
        downPipe->setScaleY(u3.winSize().height / (downPipe->getContentSize().height*1.5));
        this->addChild(downPipe, 3);
        b2BodyDef downBodyDef;
        downBodyDef.position = b2Vec2(u3.winSize().width / PTM_RATIO + 2,
               downPipe->getContentSize().height / 2 / PTM_RATIO + randPipe);
        // + land->getContentSize().height / PTM_RATIO
        downBodyDef.type = b2_kinematicBody;
        //修改速度可以提升游戏的难度
        downBodyDef.linearVelocity = b2Vec2(-2, 0);
        downBody = world->CreateBody(&downBodyDef);
        downBody->SetUserData(downPipe);
        b2PolygonShape downShape;
        downShape.SetAsBox(downPipe->getContentSize().width / 2 / PTM_RATIO,
               1.6*downPipe->getContentSize().height / PTM_RATIO);
        b2FixtureDef downPipeFixture;
        downPipeFixture.shape = &downShape;
        downBody->CreateFixture(&downPipeFixture);

        ......
}

void GamePlay::gameOver()
{
        //显示Game Over的Logo
        .......
        //显示奖章牌
        ......

        //奖章
        ......
        //显示成绩
        ......
        //设置字体的颜色为黑色，并将成绩显示在panel面板上
        ......

        unscheduleUpdate();
        unschedule(schedule_selector(GamePlay::addPipe));
}

void GamePlay::gameBegin(float dt)
{
        scheduleUpdate();
        //修改时间可以提升游戏的难易程度
        schedule(schedule_selector(GamePlay::addPipe), 2);
}

void GamePlay::BeginContact(b2Contact* contact)
{
        if (contact->GetFixtureA()->GetBody() == birdBody || contact->GetFixtureB()->GetBody() == birdBody)
        {
               this->gameOver();
        }
}

运行截图：