ios - SpriteKit 节点旋转问题

Question

我正在尝试为 iOS 开发一款基本游戏，涉及一个类似布娃娃的实体。基本上(如下所示)，我有一个头部、一个 body 和一个右臂(所有三个都是基本的 Sprite 节点)通过销关节连接，模拟人体中的连接(大致)。

头部和 body 完美地配合，当施加力时， body 完美地围绕头部旋转，并最终垂直地停在头部下方(见图)。

ARM 的底座通过销接头固定在 body 上，并且应该绕其底座旋转(有点像肩膀)，并且它的初始旋转设置为 45 度，因此在物理之前它看起来像 ARM 引擎接管。

我的问题是:为什么 ARM 不会由于重力而保持垂直位置(就像 body 一样)？难道重力不会导致 ARM 围绕其底座旋转，直到 ARM 尖端直接位于 ARM 顶部(肩部)下方吗？此外，当向 body 施加力时(如下面的示例代码所示)， body 会完全按照应有的方式围绕颈部关节旋转，但 ARM 不会从其当前方向移动(这是不可取的)。

如果不是这种情况，我该如何实现这种效果？

感谢您的宝贵时间，如果需要，我很乐意提供任何其他信息

静态物理模拟图片:

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 Sprite 上的 anchor 不会更改物理体的 anchor 。虽然我发誓有时你不必这么做，所以也许这是一个操作顺序的问题。但无论如何，偏移物理体的中心以考虑 Sprite anchor 。像这样的东西:

spriteRightArm.anchorPoint = CGPointMake(0, 1);

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