Box2D - b2body GetUserData 始终返回 null

我正在尝试根据 box2d 中的 b2body 调整精灵的位置和旋转。

创建主体后，我将 userData 属性设置为保存精灵和位置等的主体对象的属性。问题是在勾选方法 b->GetUserData 中永远不会检索我放入其中的对象。你能看出以下有什么问题吗？

这是我的添加播放器方法：

-(void) addPlayerShip
{
    CCSpriteSheet *sheet = [CCSpriteSheet spriteSheetWithFile:@"PlayerShip.png" capacity:1];

    //Get sprite sheet
    CCSprite *sprite = [CCSprite spriteWithSpriteSheet:sheet rect:CGRectMake(0,0,64,64)];
    [sheet addChild:sprite];

    CGSize screenSize = [CCDirector sharedDirector].winSize;

    sprite.position = ccp( screenSize.width/2, screenSize.height/2);

    [self addChild:sheet];

    // Define the dynamic body.
    b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;
    bodyDef.position.Set(sprite.position.x/PTM_RATIO, sprite.position.y /PTM_RATIO);
    bodyDef.angularVelocity=0;

    b2Body *body = world->CreateBody(&bodyDef);

    b2PolygonShape dynamicTriangle;

    dynamicTriangle.m_vertexCount=3;
    dynamicTriangle.m_vertices[0].Set(0,-1);
    dynamicTriangle.m_vertices[1].Set(1,1);
    dynamicTriangle.m_vertices[2].Set(-1,1);

    // Define the dynamic body fixture.
    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicTriangle;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.0f;
    fixtureDef.restitution=0.0f;

    b2Fixture *fixture = body->CreateFixture(&fixtureDef);

    b2Vec2 *vector = new b2Vec2;

    BodyObject *bodyObject = [[BodyObject alloc] initWithBody:body 
                                                   andFixture:fixture 
                                                  andVelocity:vector 
                                                    andSprite:sprite];


    bodyDef.userData = bodyObject;
}

以及在 b->GetUserData 上始终返回 null 的 tick 方法

-(void) tick: (ccTime) dt
{
    int32 velocityIterations = 8;
    int32 positionIterations = 1;

    world->Step(dt, velocityIterations, positionIterations);

    //Iterate over the bodies in the physics world
    for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
    {
        if (b->GetUserData() != NULL) {
            //Synchronize the Sprites position and rotation with the corresponding body
            GameObject *myObject = (GameObject*)b->GetUserData();
            myObject.Sprite.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
            myObject.Sprite.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
        }   
    }
}

最后是 bodyObject

#import "BodyObject.h"


@implementation BodyObject

@synthesize Body;
@synthesize Fixture;

-(id) initWithBody:(b2Body*) body 
        andFixture:(b2Fixture*) fixture 
       andVelocity:(b2Vec2*) velocity 
         andSprite:(CCSprite*) sprite
{
    self = [super initWithSprite:(CCSprite*)sprite 
                     andVelocity:(b2Vec2*)velocity];

    self.Body=body;
    self.Fixture=fixture;

    return self;
}

@end

我是 Objective C 的新手，所以如果我做错了什么，请告诉我！

编辑： 我在这里尝试了另一段代码，我似乎能够更进一步。

if I set bodyDef.UserData = sprite; 

就在该行之后

bodyDef.position.Set(sprite.position.x/PTM_RATIO, sprite.position.y /PTM_RATIO); 

然后 GetUserData 返回精灵。但是，如果我在 addPlayerShip 方法末尾将用户数据设置为精灵，那么它会再次返回 null。精灵/身体对象是否被丢弃在某处？

您正在将用户数据分配给 bodyDef AFTER 你调用CreateBody()，所以它永远不会被添加到实际的主体中。

b2BodyDef 是一个结构体，它保存创建主体的设置。一旦调用了 CreateBody()，对该结构的更改就不起作用。

您通过移动 bodyDef.UserData = sprite; 自己找到了解决方案在调用 CreateBody() 之前的某个地方。这是放置它的正确位置。如果您认为出于某种原因（其他变量？）在函数末尾需要它，那么您需要重构您的代码，以便情况并非如此。

我会更改这一行（以及 BodyObject 类）：

BodyObject *bodyObject = [[BodyObject alloc] initWithBody:body 
                                               andFixture:fixture 
                                              andVelocity:vector 
                                                andSprite:sprite];

这样您就可以实例化它，而不需要 box2d 元素（即在定义它们之前），然后在创建它们后使用属性来分配这些值。这样，您可以将此对象作为主体的用户数据传递，创建主体，然后使用分配 b2Body*[bodyObject setBody:body];

不要忘记确保您的 Body 和 Fixture 属性not只读。

最终代码可能如下所示：

-(void) addPlayerShip
{
    CCSpriteSheet *sheet = [CCSpriteSheet spriteSheetWithFile:@"PlayerShip.png" capacity:1];

    //Get sprite sheet
    CCSprite *sprite = [CCSprite spriteWithSpriteSheet:sheet rect:CGRectMake(0,0,64,64)];
    [sheet addChild:sprite];

    CGSize screenSize = [CCDirector sharedDirector].winSize;

    sprite.position = ccp( screenSize.width/2, screenSize.height/2);

    [self addChild:sheet];

    b2Vec2 *vector = new b2Vec2;

    BodyObject *bodyObject = [[BodyObject alloc] initWithSprite:sprite 
                                                   andVelocity:vector];

    //MOVES THE BODY DEFINITION AFTER THE BODYOBJECT DEFINITION
    // Define the dynamic body.
    b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;
    bodyDef.position.Set(sprite.position.x/PTM_RATIO, sprite.position.y /PTM_RATIO);
    bodyDef.angularVelocity=0;
    bodyDef.userData = bodyObject;

    b2Body *body = world->CreateBody(&bodyDef);

    b2PolygonShape dynamicTriangle;

    dynamicTriangle.m_vertexCount=3;
    dynamicTriangle.m_vertices[0].Set(0,-1);
    dynamicTriangle.m_vertices[1].Set(1,1);
    dynamicTriangle.m_vertices[2].Set(-1,1);

    // Define the dynamic body fixture.
    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicTriangle;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.0f;
    fixtureDef.restitution=0.0f;

    b2Fixture *fixture = body->CreateFixture(&fixtureDef);

    [bodyObject setBody:body];
    [bodyObject setFixture:fixture];
}

bodyObject 类将如下所示：

#import "BodyObject.h"


@implementation BodyObject

@synthesize Body;
@synthesize Fixture;

-(id) initWithSprite:(CCSprite*) sprite 
       andVelocity:(b2Vec2*) velocity 
{
    self = [super initWithSprite:(CCSprite*)sprite 
                     andVelocity:(b2Vec2*)velocity];

    return self;
}

@end