SpriteKit 节点旋转问题

Question

我正在尝试为 iOS 开发一个涉及布娃娃状实体的基本游戏。基本上（如下所示），我有一个头、一个身体和一个右臂（这三个都是基本的精灵节点），它们通过针关节连接，模拟人体中的连接（大致）。

头部和身体完美地工作，当施加力时，身体完美地围绕头部旋转并最终在头部下方垂直休息（见图）。

手臂的底部通过销关节固定在身体上，应该围绕其底部旋转（有点像肩膀），并且设置为初始旋转 45 度，因此它看起来像物理之前的手臂引擎接管。

我的问题是：为什么手臂不会因为重力而处于垂直位置（如身体）？难道重力不应该导致手臂围绕其底部旋转，直到手臂的尖端直接位于手臂顶部（肩膀）的下方？此外，当对身体施加力时（如下面的示例代码所示），身体会完全按照应有的方式围绕颈部关节旋转，但手臂不会从当前方向移动（这是不可取的）。

如果不是这样，我该如何实现这个效果？

感谢您的宝贵时间，如果需要，我很乐意提供任何其他信息

物理模拟静止图片：

演示问题的相关代码：

//make the head node
SKSpriteNode *head = [SKSpriteNode spriteNodeWithImageNamed:@"head"];
head.size = CGSizeMake(20 * [CFLConstants universalWidthScaleFactor], 20 * [CFLConstants universalWidthScaleFactor]);
head.position = position;
head.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:head.size.width/2];
head.physicsBody.categoryBitMask = CFLPhysicsCategoriesHead;
head.physicsBody.collisionBitMask = 0;
head.physicsBody.dynamic = NO;
[self.ragdollLayer addChild:head];

//make the body node
SKSpriteNode *body = [SKSpriteNode spriteNodeWithImageNamed:@"body"];
body.size = CGSizeMake(head.size.width, head.size.width * 3);
body.position = CGPointMake(head.position.x, head.position.y - head.size.height/2 - body.size.height/2);
body.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:body.size];
body.physicsBody.categoryBitMask = CFLPhysicsCategoriesBody;
body.physicsBody.collisionBitMask = 0;
[self.ragdollLayer addChild:body];

//attach the head and the body (via a neck joint)
SKPhysicsJointPin *neckJoint = [SKPhysicsJointPin jointWithBodyA:head.physicsBody bodyB:body.physicsBody anchor:CGPointMake(head.position.x, head.position.y - head.size.height/2)];
[self.physicsWorld addJoint:neckJoint];

//make the right arm
SKSpriteNode *rightArm = [SKSpriteNode spriteNodeWithImageNamed:@"arm"];
rightArm.size = CGSizeMake(head.size.width/5, head.size.width/5 * 10);
rightArm.anchorPoint = CGPointZero;
CGPoint rightArmPosition = CGPointMake(body.position.x + body.size.width * 1/5, body.position.y + body.size.height * 1/5);
rightArm.position = rightArmPosition;
rightArm.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:rightArm.size];
rightArm.physicsBody.categoryBitMask = CFLPhysicsCategoriesRightArm;
rightArm.physicsBody.collisionBitMask = 0;
rightArm.zRotation = -M_PI_4;

//force which makes the arm problem even more noticeable
[body.physicsBody applyImpulse:CGVectorMake(100, 0)];
[self.ragdollLayer addChild:rightArm];

//make the joint which holds the right arm to the body, but should allow the arm to rotate about this point (and doesn't)
SKPhysicsJointPin *rightShoulderJoint = [SKPhysicsJointPin jointWithBodyA:body.physicsBody bodyB:rightArm.physicsBody anchor:rightArmPosition];
[self.physicsWorld addJoint:rightShoulderJoint];

Answer 1

根据我的经验，这是因为更改精灵上的锚点不会更改物理主体的锚点。虽然我发誓有时你不必这样做，所以也许这是操作顺序的事情。但无论如何，偏移物理主体的中心以考虑精灵锚点。比如：

spriteRightArm.anchorPoint = CGPointMake(0, 1);

spriteRightArm.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:spriteRightArm.size center:CGPointMake(spriteRightArm.size.width/2, -spriteRightArm.size.height/2)];