Question

I’m making a vector graphics game in XNA. I’ve designed a Line class to rotate around a central point to help draw specific shapes. In order to maintain a single point of truth, is there a way to pass a reference to the center of the shape to all of the lines I create, so that updating the center’s position will also update the lines’ positions? I thought something like this would work:

class Line
{
    private Vector2 start;
    private double length;
    private double angle;

    public Line(ref Vector2 start, double length, double angle){
        this.start = start;
        this.length = length;
        this.angle = angle;
    }
}




class Obj
{
    private Vector2 center;
    private Line[] lines;
    public Obj(){
        center = new Vector2(50,50);
        lines = new Lines[5];
        for (int i = 0; i < 5; i++){
            lines[i] = new Line(ref center,30, (i/5 * 2 * Math.PI));

        }

    }
}

but the lines do not update when I move the center. What am I doing wrong?

Answer 1

Although the struct is correctly passed by reference to Line, when you assign it internally:

public Line(ref Vector2 start, double length, double angle){
    this.start = start;
}

You are actually taking a copy of the struct.

If you ever find yourself needing reference type semantics of struct beyond passing it to a single method then you likely need to reconsider using class.

You can either re-implement the type in a class or wrap the Vector2 in a class and use that:

class Vector2Class
{
    public Vector2 Centre;

    public Vector2Class(Vector2 inner)
    {
        Centre = inner;
    }
}

class Line
{
    private Vector2Class _centre;

    public Line(Vector2Class centre)
    {
        _centre = centre;    
    }
}

Be aware that you are still working against a copy, but if you share the class you’ll all be working on the same copy.

Personally, I would avoid the wrapper and make my own class for representing “centre”. This is supported by the largely accepted idea that struct types should be immutable, but you seem to need to mutate the values to keep the representation true.

class CentreVector<T>
{
    public <T> X { get; set; }
    public <T> Y { get; set; }
}

This only lets you share the data, it doesn’t actually notify the lines that the centre has changed. For that you would need some sort of event.