SpriteKit节点轮换问题

Question

我正在尝试开发iOS的基本游戏，涉及一个类似布娃娃的实体。基本上（正如你在下面所说），我有一个头部，一个身体和一个右臂（所有三个都是基本的精灵节点）通过针接头连接，模拟人体内的连接（大致）。

头部和身体完美地工作，当施加一个力时，身体完全围绕头部旋转，最终垂直地在头部下方休息（见图）。

手臂的底座固定有一个针脚连接到身体上，应该围绕它的底座旋转（有点像肩膀），它的初始旋转设置为45度，所以看起来像一个物理引擎接管之前的手臂。

我的问题是：为什么手臂由于重力而不能垂直放置在垂直位置（如身体）？重力不应导致手臂围绕其基部旋转，直到手臂的尖端直接位于手臂顶部（肩部）的下方？此外，当一个力施加到身体上时（如下面的示例代码所示），身体围绕颈部关节旋转，正如它应该的那样，但是手臂不会从其当前方向移动（这是不可取的）。

如果不是这样，我将如何实现这种效果？

感谢您的时间，如果需要，我很乐意提供任何其他信息

静止时的物理模拟图片：

证明问题的相关代码：

//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，不会改变物理体的锚点。虽然我发誓有时你没有必要，所以也许这是一个操作的顺序。但无论如何，偏移物理体的中心来解释精灵锚点。类似的东西：

spriteRightArm.anchorPoint = CGPointMake(0, 1);

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