Cocos2d-iPhone与Box2D：CCPhysicsSprite EXC_BAD_ACCESS

问题描述

我刚刚开始混淆cocos2d的Box2D集成，虽然大多数过程是简单和直接，我使用CCPhysicsSprite（CCSprite子类，集成b2body与sprite时）一直运行到一个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;
}

这段代码触发一个EXC_BAD_ACCESS，我知道它是CCPhysicsSprite，因为改变CCPhysicsSprite为CCSprite抛出零错误：

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);
}

我一直在环顾四周，找不到真正的答案所有的示例代码使用CCPhysicsSprite以这种方式没有错误）。我相信我犯了一个愚蠢的错误，但我想这是预期当学习新的东西：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

感谢高级！

推荐答案

使用 CCPhysicsSprite ，你必须设置body之前设置sprite的位置。
我假设这一行是发生崩溃的地方：

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;

如果你看一下 CCPhysicsSprite position / setPosition始终使用所包含的 b2body 对象中的值。所以它真正访问它的b2body属性，这是 nil 在那一点→崩溃。
一般在创建CCPhysicsSprite之后，您希望尽快设置其.b2body - 可能会有更多的情况下，类似的问题可能发生只是因为您需要先设置.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，移动 sprite.position = point; 下面 sprite.b2Body = body; 。

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