SKPhysicsJointPin stretching

Question

I tried the solution posted here but it did not fix the issue. No one is responding to the my requests there since the question has already been marked as answered.

I have the player jumping across the screen to grab a rope. The grabbing is achieved by establishing an SKPhysicsJointPin between a rope segment and the player. The rope itself is made up of many segments connected to each other with SKPhysicsJointPins. Those behave as expected. However, the player seems to be joined for about a second but then as the player and rope swing together the joint between them stretches out and the player falls completely off screen.

Here is where the player (a monkey) gets added to the scene:

- (void)addMonkeyToWorld
{
    SKSpriteNode *monkeySpriteNode = [SKSpriteNode spriteNodeWithImageNamed:@"Monkey"];

    // Basic properties
    monkeySpriteNode.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:monkeySpriteNode.size];
    monkeySpriteNode.physicsBody.density = physicsParameters.monkeyDensity;
    monkeySpriteNode.physicsBody.restitution = physicsParameters.monkeyRestitution;
    monkeySpriteNode.physicsBody.linearDamping = physicsParameters.monkeyLinearDamping;
    monkeySpriteNode.physicsBody.angularDamping = physicsParameters.monkeyAngularDamping;
    monkeySpriteNode.physicsBody.velocity = physicsParameters.monkeyInitialVelocity;

    // Collision properties
    monkeySpriteNode.physicsBody.categoryBitMask = monkeyCategory;
    monkeySpriteNode.physicsBody.contactTestBitMask = ropeCategory;
    monkeySpriteNode.physicsBody.collisionBitMask = 0x0;
    monkeySpriteNode.physicsBody.usesPreciseCollisionDetection = YES;
} 

Here is where a contact event is sorted out:

- (void)didBeginContact:(SKPhysicsContact *)contact
{
    // Sort which bodies are which
    SKPhysicsBody *firstBody, *secondBody;
    if (contact.bodyA.categoryBitMask > contact.bodyB.categoryBitMask) {
        firstBody = contact.bodyA;
        secondBody = contact.bodyB;
    }
    else {
        firstBody = contact.bodyB;
        secondBody = contact.bodyA;
    }

    // Verify that the two bodies were the monkey and rope, then handle collision
    if ((firstBody.categoryBitMask & ropeCategory) != 0 && (secondBody.categoryBitMask & monkeyCategory) != 0)
    {
        [self monkey:secondBody didCollideWithRope:firstBody atPoint:contact.contactPoint];
    }
}

And here is where the joint is added between the player and the rope:

- (void)monkey:(SKPhysicsBody *)monkeyPhysicsBody didCollideWithRope:(SKPhysicsBody *)ropePhysicsBody atPoint:(CGPoint)contactPoint
{
    if (monkeyPhysicsBody.joints.count == 0) {
        // Create a new joint between the monkey and the rope segment
        CGPoint convertedMonkeyPosition  = CGPointMake(monkeyPhysicsBody.node.position.x + sceneWidth/2., monkeyPhysicsBody.node.position.y + sceneHeight/2.);
        CGPoint convertedRopePosition = CGPointMake(ropePhysicsBody.node.position.x + sceneWidth/2., ropePhysicsBody.node.position.y + sceneHeight/2.);
        CGFloat leftMostX = convertedMonkeyPosition.x < convertedRopePosition.x ? convertedMonkeyPosition.x : convertedRopePosition.x;
        CGFloat bottomMostY = convertedMonkeyPosition.y < convertedRopePosition.y ? convertedMonkeyPosition.y : convertedRopePosition.y;
        CGPoint midPointMonkeyAndRope = CGPointMake(leftMostX + fabsf(ropePhysicsBody.node.position.x - monkeyPhysicsBody.node.position.x) / 2.,
                                                bottomMostY + fabsf(ropePhysicsBody.node.position.y - monkeyPhysicsBody.node.position.y) / 2.);
        SKPhysicsJointPin *jointPin = [SKPhysicsJointPin jointWithBodyA:monkeyPhysicsBody bodyB:ropePhysicsBody anchor:midPointMonkeyAndRope]; // FIXME: Monkey-rope joint going to weird position
        jointPin.upperAngleLimit = M_PI/4;
        jointPin.shouldEnableLimits = YES;
        [self.scene.physicsWorld addJoint:jointPin];
    }
}

Any ideas what would cause an SKPhysicsJointPin to stretch out?

Answer 1

This issue was related to another one I posted here. I'll repeat the answer here for anyone coming along.

I added the following convenience method to my GameScene.m.

-(CGPoint)convertSceneToFrameCoordinates:(CGPoint)scenePoint
{
    CGFloat xDiff = myWorld.position.x - self.position.x;
    CGFloat yDiff = myWorld.position.y - self.position.y;
    return CGPointMake(scenePoint.x + self.frame.size.width/2 + xDiff, scenePoint.y + self.frame.size.height/2 + yDiff);
}

I use this method to add joints. It handles all of the coordinate system transformations that need to be dealt with that lead to the issue raised in this question. For example, the way I add joints

    CGPoint convertedRopePosition = [self convertSceneToFrameCoordinates:ropePhysicsBody.node.position];
    SKPhysicsJointPin *jointPin = [SKPhysicsJointPin jointWithBodyA:monkeyPhysicsBody bodyB:ropePhysicsBody anchor:convertedRopePosition];
    jointPin.upperAngleLimit = M_PI/4;
    jointPin.shouldEnableLimits = YES;
    [self.scene.physicsWorld addJoint:jointPin];