Sprite Kit - Move scene and camera with sprite when sprite is going off screen

Question

I am trying to make a scene that extends up off the screen for a certain distance, and I want the camera to stay centered on the player node. When the player reaches the top of the screen, I want the bottom nodes to disappear below the screen bounds and the new off screen part of the scene to become visible. Like Mario!

I create the scene size like this:

CGSize screenSize = CGSizeMake(skView.bounds.size.width, skView.bounds.size.height + (skView.bounds.size.height/3));//+ (skView.bounds.size.height/3)
scene = [GameScene sceneWithSize:screenSize];

And then, add all of my nodes to a WorldNode as in This question.

I have modified it to be:

- (void)didSimulatePhysics
{
    CGFloat margin = self.size.height/3;// previously defined

    // make sure the cat's position is defined in scene coordinates
    CGPoint catPosition = [self convertPoint:cat.position fromNode:cat.parent];

    if (catPosition.y > (self.size.height - margin))
    {
        CGPoint worldPosition = worldNode.position;
        worldPosition.y -= catPosition.y - (self.size.height - margin);
        worldNode.position = worldPosition;
    }
    else if (catPosition.y < (self.size.height - margin))
    {
        CGFloat maxWorldHeight = self.size.height;
        CGPoint worldPosition = worldNode.position;
        // this keeps the cat on the margin line
        if (worldPosition.y != worldStartPosition.y)
        {
            worldPosition.y += (self.size.height - margin) - catPosition.y;
        }
        if (worldPosition.y > maxWorldHeight) // assume maxWorldHeight is defined
        {
            worldPosition.y = maxWorldHeight;
        }
        worldNode.position = worldPosition;
    }

}

Ok, I have it almost working. I can't seem to stop the world at the top boarder of the scene, and the character does not fall when he reaches below the margin, as in the world keeps following him down past the worlds initial start point. How can I get the screen to only move up and down if the character is above the margin, and then if the character is below the margin, the screen does not move?

Answer 1

You want to move your world node down by an amount equal to how far the cat is above where you want it to be. Your didSimulatePhysics method would end up looking something like this:

- (void)didSimulatePhysics
{
    CGFloat margin; // set this to be whatever you want it to be

    // make sure the cat's position is defined in scene coordinates
    CGPoint catPosition = [self convertPoint:cat.position fromNode:cat.parent];

    if (catPosition.y > (self.size.height - margin))
    {
        // the cat is above the margin, we need to move the world node down until the
        // cat is at the margin

        // pull world position into local variable so you can modify the y component
        CGPoint worldPosition = worldNode.position;
        // move the world down so the cat is on the margin
        worldPosition.y -= catPosition.y - (self.size.height - margin);
        worldNode.position = worldPosition;
    }
    else
    {
        // the cat is below the margin, we need to move the world node up, but only
        // until the world node is at its starting position

        CGPoint worldPosition = worldNode.position;
        // move the cat up to the margin line
        worldPosition.y += (self.size.height - margin) - catPosition.y;
        // if this caused the world to be too high, move it back down
        if (worldPosition.y > maxWorldHeight)
        {
            worldPosition.y = maxWorldHeight;
        }
        worldNode.position = worldPosition;
    }
}

One thing to take note of is that maxWorldHeight is not how tall the world is, but rather how far above the bottom of the screen it's allowed to be. You should set it to the y-component of the word node's position when you first place it in the scene.

Answer 2

- (void)didSimulatePhysics
{
  CGPoint target = [self pointToCenterViewOn:cat.position];
  CGPoint newPosition = _worldNode.position;
  newPosition.x += (target.x - worldNode.position.x) * 0.1f;
  newPosition.y += (target.y - worldNode.position.y) * 0.1f;
  worldNode.position = newPosition;
}

- (CGPoint)pointToCenterViewOn:(CGPoint)centerOn
{ 
  CGFloat x = Clamp(centerOn.x, self.size.width / 2, worldNode.size.width - self.size.width / 2);
  CGFloat y = Clamp(centerOn.y, self.size.height / 2, worldNode.size.height - self.size.height/2);
  return CGPointMake(-x, -y);
}