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I'm creating a simple game development framework with Ruby.

There is a node system. A node is a game entity, and it has position. It can have children nodes (and one parent node). Children are always drawn relatively to their parent.

Nodes have a @position field. Anyone can modify it. When such position is modified, the node must update its children accordingly to properly draw them relatively to it.

@position contains a Point instance (a class with x and y properties, plus some other useful methods).

I need to know when a node's @position's state changes, so I can tell the node to update its children.

This is easy if the programmer does something like this:

@node.position = Point.new(300,300)

Because it is equivalent to calling this:

# Code in the Node class
def position=(newValue)
  @position = newValue
  update_my_children    # <--- I know that the position changed
end

But, I'm lost when this happens:

@node.position.x = 300

The only one that knows that the position changed is the Point instance stored in the @position property of the node. But I need the node to be notified!

It was at this point that I considered the Observer pattern. Basically, Point is now observable.

When a node's position property is given a new Point instance (through the assignment operator), it will stop observing the previous Point it had (if any), and start observing the new one. When a Point instance gets a state change, all observers (the node owning it) will be notified, so now my node can update its children when the position changes.

A problem is when this happens:

@someNode.position = @anotherNode.position

This means that two nodes are observing the same point. If I change one of the node's position, the other would change as well. To fix this, when a position is assigned, I plan to create a new Point instance, copy the passed argument's x and y, and store my newly created point instead of storing the passed one.

Another problem I fear is this:

somePoint = @node.position
somePoint.x = 500

This would, technically, modify @node's position. I'm not sure if anyone would be expecting that behavior. I'm under the impression that people see Point as some kind of primitive rather than an actual object.

Is this approach even reasonable? Reasons I'm feeling skeptical:

  • I've heard that the Observer pattern should be used with, well, many observers. Technically, in this scenario there is only one observer at a time.
  • When assigning a node's position as another's (@someNode.position = @anotherNode.position), where I create a whole new instance rather than storing the passed point, it feels hackish, or even inefficient.
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Having your setter make a defensive copy seems reasonable; having your accessor return a defensive copy would prevent @node.position.x = 500, which would keep you from needing the Observable pattern, but would make assigning new positions more cumbersome. –  Michael Brewer-Davis Sep 12 '13 at 2:59
    
It would be nice if the absolute values were only updated when they were accessed, maybe keyed off of a frame counter. That way you wouldn't be processing the entire subtree every time its root moved, only the bits you care about for that frame. –  jzx Nov 11 '13 at 15:24

3 Answers 3

You could solve this issue by making Point an immutable object. Immutable objects are objects with read-only properties which are set at creation and can not change during the lifetime of the object. This might seem limiting at first, but it greatly simplifies code because you have less effect-at-a-distance.

When you need to change a position, you would create a new Point and assign it. You could have an utility method in Point for this, like position.moveBy(3, -2) which doesn't change the Point object itself but returns a new Point object with a position relative to the current one.

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Bingo. Thanks for writing the answer I was going to write. –  Wayne Conrad Jan 10 at 12:53

I think you're missing the observer (subscribe) side of the Observer pattern. Take a look at the Observable module.

You could create an ObservablePosition class that manages a node's Position (getters, setters, etc.) and notifies observers as needed. Child nodes would subscribe to changes on their parent's OP. This would have the desired affect of automatically cascading a node change to all children in the tree below it. This design could also be generalized to allow child nodes to listen for changes on multiple node properties.

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Why not have a separate class or method responsible for updating the node and its children with the new values. You can have it return the new value to update your reference with and then it is a just simple calculation. If you go that route, you don't have to worry about knowing who is subscribed to who and all of that stateful mess.

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