Sprite Kit pin joints appear to have an incorrect anchor

Question

I\'m testing out pin joints with Sprite Kit, and I\'m finding something unusual happening.

My desired setup is this: one wide, flat box, and two circles; the circles

Answer 1

Try this code, I used yours and got weird issues, so started from scratch.

- (SKShapeNode*) makeWheel
{
    SKShapeNode *wheel = [[SKShapeNode alloc] init];
    CGMutablePathRef myPath = CGPathCreateMutable();
    CGPathAddArc(myPath, NULL, 0,0, 16, 0, M_PI*2, YES);
    wheel.path = myPath;
    return wheel;
}


- (void) createCar 
{

    self.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:CGRectMake(0, 0, self.size.width, self.size.height)];

    // 1. car body
    SKSpriteNode *carBody = [SKSpriteNode spriteNodeWithColor:[UIColor whiteColor] size:CGSizeMake(120, 8)];
    carBody.position = CGPointMake(200, 200);
    carBody.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:carBody.size];
    [self addChild:carBody];

    // 2. wheels
    SKShapeNode *leftWheel = [self makeWheel];
    leftWheel.position = CGPointMake(carBody.position.x - carBody.size.width / 2, carBody.position.y);
    leftWheel.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:16];
    [self addChild:leftWheel];

    SKShapeNode *rightWheel = [self makeWheel];
    rightWheel.position = CGPointMake(carBody.position.x + carBody.size.width / 2, carBody.position.y);
    rightWheel.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:16];
    [self addChild:rightWheel];

    // 3. Join wheels to car
    [self.physicsWorld addJoint:[SKPhysicsJointPin jointWithBodyA:carBody.physicsBody bodyB:leftWheel.physicsBody anchor:leftWheel.position]];
    [self.physicsWorld addJoint:[SKPhysicsJointPin jointWithBodyA:carBody.physicsBody bodyB:rightWheel.physicsBody anchor:rightWheel.position]];

    // 4. drive car
    [carBody.physicsBody applyForce:CGVectorMake(10, 0)];
}

Answer 2

I also had this issue and the cause is setting the physics body before setting the sprites position.

carNode.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:carNode.size];
carNode.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame));

Change the above to

carNode.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame));
carNode.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:carNode.size];

It should work. Thanks Smick.

SpriteKit: How to create Basic Physics Joints