IOS Sprite Kit 2对象碰撞并保持不变

Question

我希望同一类别的物体在相互碰撞时不会反弹或移动。我尝试过使用collisionBitMask，contactTestBitMask，质量，密度等等。有谁知道怎么做？

self.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:self.frame.size];
self.physicsBody.affectedByGravity = NO;
self.physicsBody.allowsRotation = NO;
self.physicsBody.categoryBitMask = CollisionTypeRock;
self.physicsBody.collisionBitMask = CollisionTypeRock;
self.physicsBody.contactTestBitMask = CollisionTypeRock;

感谢。