问题描述

我最近才开始搞乱 cocos2d 的 Box2D 集成，虽然大部分过程都很简单直接，但在使用 CCPhysicsSprite(将 b2body 与 sprite 集成的 CCSprite 子类)时，我一直遇到 EXC_BAD_ACCESS 错误.我使用的代码是:

I just recently started messing with cocos2d's Box2D integration, while most of the process has been simple and straight forward, I keep running into a EXC_BAD_ACCESS error when using a CCPhysicsSprite (CCSprite subclass that integrates a b2body with the sprite). The code I'm using is:

- (void)spawnBallAtPoint:(CGPoint)point
{
    count++;

    CCPhysicsSprite *sprite = [CCPhysicsSprite spriteWithFile:@"ball.png" rect:CGRectMake(0, 0, 52, 52)];
    sprite.position = point;
    [self addChild:sprite];

    b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;
    bodyDef.position.Set(point.x / PTM_RATIO, point.y / PTM_RATIO);
    bodyDef.userData = sprite;
    b2Body *body = world->CreateBody(&bodyDef);

    b2CircleShape circleShape;
    circleShape.m_radius = (26.0 / PTM_RATIO);

    b2FixtureDef fixtureDef;
    fixtureDef.shape = &circleShape;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.2f;
    fixtureDef.restitution = 0.8f;
    body->CreateFixture(&fixtureDef);

    sprite.b2Body = body;
}

此代码触发 EXC_BAD_ACCESS，我知道它是 CCPhysicsSprite，因为将 CCPhysicsSprite 更改为 CCSprite 会引发零错误:

This code triggers an EXC_BAD_ACCESS, I know it's the CCPhysicsSprite because changing CCPhysicsSprite to CCSprite throws zero errors:

- (void)spawnBallAtPoint:(CGPoint)point
{
    count++;

    CCSprite *sprite = [CCSprite spriteWithFile:@"ball.png" rect:CGRectMake(0, 0, 52, 52)];
    sprite.position = point;
    [self addChild:sprite];

    b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;
    bodyDef.position.Set(point.x / PTM_RATIO, point.y / PTM_RATIO);
    bodyDef.userData = sprite;
    b2Body *body = world->CreateBody(&bodyDef);

    b2CircleShape circleShape;
    circleShape.m_radius = (26.0 / PTM_RATIO);

    b2FixtureDef fixtureDef;
    fixtureDef.shape = &circleShape;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.2f;
    fixtureDef.restitution = 0.8f;
    body->CreateFixture(&fixtureDef);
}

我环顾四周，找不到真正的答案(因为所有示例代码都以这种方式使用 CCPhysicsSprite 没有错误).我确定我犯了一个愚蠢的错误，但我想在学习新东西时这是可以预料的:P

I've been looking around and can't find a real answer (as all the sample code uses CCPhysicsSprite in this way without error). I'm sure I'm making a stupid mistake but I guess that's to be expected when learning something new :P

提前致谢！

推荐答案

使用 CCPhysicsSprite 必须在设置精灵位置之前设置主体.我假设这条线是发生崩溃的地方:

With CCPhysicsSprite you must set the body BEFORE you set the position of the sprite. I assume that this line is where the crash occurs:

sprite.position = point;

如果您查看 CCPhysicsSprite，它会覆盖 position/setPosition 以始终使用包含的 b2body 对象中的值.所以它真的访问了它的 b2body 属性，在那个时候是 nil → Crash.通常，在创建 CCPhysicsSprite 后，您希望尽快设置其 .b2body - 可能会出现更多类似问题，因为您需要先设置 .b2body 属性.

If you look at CCPhysicsSprite it overrides the position/setPosition to always use the values from the contained b2body object. So it really accesses its b2body property, which is nil at that point → Crash. Generally after creating a CCPhysicsSprite you want to set its .b2body as soon as possible - there might be more occasions where similar problems might occur just because you need to set the .b2body property first.

所以，将 sprite.position = point; 移到 sprite.b2Body = body; 下方.

So, move sprite.position = point; below sprite.b2Body = body;.

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