用box2d设置游戏边框使用b2EdgeShape,小球无法下落

Cocos2d-x
#1

刚刚开始学习，想做一个小球可以在屏幕中央来回弹跳的游戏，但是遇到了如下问题。

// on "init" you need to initialize your instance

bool HelloWorld::init()

{


//////////////////////////////

// 1. super init first

if ( !Layer::init() )

{

    return false;

}



size = Director::getInstance()->getVisibleSize();



world = new b2World(b2Vec2(0, -10.0f));



addBall();



addEdge();





scheduleUpdate();



return true;



}


void HelloWorld::update(float dt){


world->Step(dt, 8, 3);



for(b2Body *b = world->GetBodyList(); b; b=b->GetNext()){

    if(b->GetUserData() == sBall){

        sBall->setPosition(b->GetPosition().x*RATIO, b->GetPosition().y*RATIO);

        printf("x=%f, y=%fn", b->GetPosition().x, b->GetPosition().y);

    }

}



return;



}


void HelloWorld::addEdge(){


b2BodyDef def;

def.position.SetZero();

edgeBody = world->CreateBody(&def);



float widthInMeters = size.width / RATIO;

float heightInMeters = size.height / RATIO;

printf("widthInMeters=%f, heightInMeters=%fn", widthInMeters, heightInMeters);

b2Vec2 lowerLeftCorner = b2Vec2(0, 0);

b2Vec2 lowerRightCorner = b2Vec2(widthInMeters, 0);

b2Vec2 upperLeftCorner = b2Vec2(0, heightInMeters);

b2Vec2 upperRightCorner = b2Vec2(widthInMeters, heightInMeters);



b2EdgeShape edge;

//edge.Set(b2Vec2(0, 0), b2Vec2(size.width/RATIO, 0));

edge.Set(lowerLeftCorner, lowerRightCorner);

edgeBody->CreateFixture(&edge, 0);

edge.Set(lowerRightCorner, upperRightCorner);

edgeBody->CreateFixture(&edge, 0);

edge.Set(upperRightCorner, upperLeftCorner);

edgeBody->CreateFixture(&edge, 0);

edge.Set(upperLeftCorner, lowerLeftCorner);

edgeBody->CreateFixture(&edge, 0);



return;



}


void HelloWorld::addBall(){


sBall = Sprite::create("180px-Basketball.png");

addChild(sBall);



b2BodyDef b2Body;

b2Body.position = b2Vec2(5, 5);

b2Body.type = b2_dynamicBody;

b2Body.userData = sBall;

ball = world->CreateBody(&b2Body);



b2CircleShape circle;

circle.m_radius = 26.0;



b2FixtureDef b2Fixture;

b2Fixture.shape = &circle;

b2Fixture.density = 0.6f;

b2Fixture.friction = 0.0f;

b2Fixture.restitution = 0.8f;

ball->CreateFixture(&b2Fixture);



return;



}


<p> </p>
当注释掉init()中的addEdge()后小球可以正常下落，但是按照上面代码运行后小球会被卡在屏幕中央，网上找了一下答案，在stack overflow中有关于增加边界的回答，和我做的对照一下没有什么差别，实在想不出来为什么了。</p>
<p> </p>
#2

circle.m_radius = 26.0;

问题在于这一行，给小球设置的半径太大了，所以卡在了屏幕中央。