我有一个非常简单的关卡，其中有一个静态主体（和关联的精灵）。当用户触摸身体时，我移除精灵，销毁身体并用带有新精灵的动态身体替换它，然后我过渡到新场景。我的代码在第一次执行时运行良好，但是当重新加载这个场景时，当用户触摸身体时它会崩溃。
我将用于精灵的 .png 文件的文件名存储在精灵的 userData 字段中。然后当用户按下按钮（触摸身体）时，检索它是一件简单的事情。当我尝试访问精灵的 userData 字段时，在随后重新加载场景时会出现问题。而不是保存文件名，它是空的（null）。当我尝试使用文件名时，这反过来会使程序崩溃。
我不明白为什么它是第一次而不是第二次。我设置了一个断点并观察分配给精灵的 userData 字段的文件名，但它只能在我第一次创建场景时检索。这是 ccTouchesBegan 方法。崩溃发生在我尝试分配 newSpriteFileName 的地方，因为在第二次运行时 oldSpriteFileName 是空的。
我刚刚对这个发疯了，但我相信这是显而易见的。一如既往地感谢您的帮助！！
(void)ccTouchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
for( UITouch *touch in touches )
{
CGPoint touchLocation = [touch locationInView: [touch view]];
touchLocation = [[CCDirector sharedDirector] convertToGL: touchLocation];
touchLocation = [self convertToNodeSpace:touchLocation];
b2Vec2 locationWorld = b2Vec2(touchLocation.x/PTM_RATIO, touchLocation.y/PTM_RATIO);
NSLog(@"Location x = %f, y = %f", touchLocation.x, touchLocation.y);
b2AABB aabb;
b2Vec2 delta = b2Vec2(1.0/PTM_RATIO, 1.0/PTM_RATIO);
aabb.lowerBound = locationWorld - delta;
aabb.upperBound = locationWorld +delta;
SimpleQueryCallback callback(locationWorld);
m_world->QueryAABB(&callback, aabb);
//If they have not pressed a button yet, and this touch was actually inside
//one of the buttons then we will destroy the static button (and sprite)
//that they touched and replace it with a dynamic button
//(and new darker sprite) that falls, spinning off of the screen.
if(!replaceButtonPushedYet && callback.fixtureFound)
{
//Get a reference to the static body of the button they touched and wake it up
b2Body *body = callback.fixtureFound->GetBody();
body->SetAwake(true);
//Get the position of the body (button) they touched and use it to
//position the new button. This value is in Box2D coordinates,
//so when I position the new button I won't divide by PTM_RATIO.
b2Vec2 touchedButtonPosition = body->GetPosition();
//Get the sprite from the button the user pressed
CCSprite *bodySprite = (CCSprite*)body->GetUserData();
//Then extract the file name of the sprite. I assigned this to
//sprite's userData when I loaded the sprite in the method //"placeTheIndividualButton". Now I am going to extract it and
//then replace "(up)" with "(down)" in that string
//and that becomes the name of the sprite for the new (pressed)
//button I am getting ready to create. It is all about the file
//naming conventions!
NSString *oldSpriteFileName = (NSString*)bodySprite.userData;
NSString *newSpriteFileName = [oldSpriteFileName stringByReplacingOccurrencesOfString:@"up" withString:@"down"];
//First remove the sprite tied to the button the user pressed,
//then destroy the body of the button.
[self removeChild:bodySprite cleanup:YES];
body->GetWorld()->DestroyBody(body);
//Set the bool to true to keep this code from executing again.
//This ensures that once they press a button they can't
//press another one.
replaceButtonPushedYet = true;
//Build the new dynamic button that will fall and spin off the screen
b2BodyDef buttonBodyDef2;
b2PolygonShape buttonShape2;
b2FixtureDef buttonShapeDef2;
b2Vec2 vertices2[4];
//load the sprite for the second button (pressed down) and add it to the layer
level_down = [CCSprite spriteWithFile:newSpriteFileName];
level_down.userData=newSpriteFileName;
[self addChild:level_down];
//Define the polygon that forms the second button (pressed down)
buttonBodyDef2.type = b2_dynamicBody;
//Not dividing touchedButtonPosition.x or .y by PTM_RATIO because
//they are already in Box2D coordinates
buttonBodyDef2.position.Set(touchedButtonPosition.x, touchedButtonPosition.y);
buttonBodyDef2.angularVelocity = 1.5;
buttonBodyDef2.userData = level_down;
buttonBody2 = m_world->CreateBody(&buttonBodyDef2);
//Define the vertices for the replacement button
vertices2[0].Set(-94/PTM_RATIO, -32/PTM_RATIO);
vertices2[1].Set(94/PTM_RATIO, -32/PTM_RATIO);
vertices2[2].Set(94/PTM_RATIO, 32/PTM_RATIO);
vertices2[3].Set(-94/PTM_RATIO, 32/PTM_RATIO);
buttonShape2.Set(vertices2, 4);
//Define the shape for the replacement button
buttonShapeDef2.shape = &buttonShape2;
buttonShapeDef2.density = 50.01f;
buttonShapeDef2.friction = 0.75f;
buttonShapeDef2.restitution = 0.1f;
//The static buttons and the dynamic buttons are both in this groupIndex.
//Since it is a negative number they will never collide. If it was
//positive they would always collide.
buttonShapeDef2.filter.groupIndex = -1;
//Put the second button (pressed down) into the world.
buttonBody2->CreateFixture(&buttonShapeDef2);
//This starts a timer that fires every second to call the makeTransition
//method. The code inside that method will check to see if the button
//has fallen off the screen yet. If it has then it will transition to
//the new selected level.
//The colon after makeTransition sends a ccTime (dt). I don't need it,
//but may in the future so I left it in there.
buttonFalling = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector(makeTransition:) userInfo:nil repeats:YES];
}
}
}