ios - 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 are connected via SKPhysicsPinJoints to the box, so they can act as wheels.

Here's my code. I've tried to make it as concise as possible:

- (SKNode*) createWheelWithRadius:(float)wheelRadius {
    CGRect wheelRect = CGRectMake(-wheelRadius, -wheelRadius, wheelRadius*2, wheelRadius*2);

    SKShapeNode* wheelNode = [[SKShapeNode alloc] init];
    wheelNode.path = [UIBezierPath bezierPathWithOvalInRect:wheelRect].CGPath;

    wheelNode.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:wheelRadius];

    return wheelNode;
}


- (void) createCar {

    // Create the car
    SKSpriteNode* carNode = [SKSpriteNode spriteNodeWithColor:[SKColor yellowColor] size:CGSizeMake(150, 50)];
    carNode.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:carNode.size];
    carNode.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame));
    [self addChild:carNode];

    // Create the left wheel
    SKNode* leftWheelNode = [self createWheelWithRadius:30];
    leftWheelNode.position = CGPointMake(carNode.position.x-80, carNode.position.y);
    [self addChild:leftWheelNode];

    // Create the right wheel
    SKNode* rightWheelNode = [self createWheelWithRadius:30];
    rightWheelNode.position = CGPointMake(carNode.position.x+80, carNode.position.y);
    [self addChild:rightWheelNode];

    // Attach the wheels to the body
    CGPoint leftWheelPosition = leftWheelNode.position;
    CGPoint rightWheelPosition = rightWheelNode.position;

    SKPhysicsJointPin* leftPinJoint = [SKPhysicsJointPin jointWithBodyA:carNode.physicsBody bodyB:leftWheelNode.physicsBody anchor:leftWheelPosition];
    SKPhysicsJointPin* rightPinJoint = [SKPhysicsJointPin jointWithBodyA:carNode.physicsBody bodyB:rightWheelNode.physicsBody anchor:rightWheelPosition];

    [self.physicsWorld addJoint:leftPinJoint];
    [self.physicsWorld addJoint:rightPinJoint];
}

What I'm expecting is that the pin joints are anchored at their centre points; however, when I test this, the anchors for the joints appear to be far off.

Am I missing something really obvious?

Answer 1

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