Move two sprites together with offset?

Question

I'm making a simple game in C# for practice and i'm having some problems when dragging a sprite and making another sprite follow it.

What i'm trying to achieve:

• When dragging a sprite, i want another sprite to move with it.

• While dragging, i want the two sprites to keep the same distances they where before from one another.

• If possible, but not related to the question, i would like to adjust this distance with another offset.

Example:

• Lets suppose i have two sprites, one at position (10, 10) and another at (10, 20).
• If i drag the first sprite to (10, 20) the second sprite should be at (10, 30).

Example code of what i'm trying to do:

public class Drag
{
    public Sprite first;
    public Sprite second;
    
    public bool dragging = true;
    public Point mouse_position;
    
    public void Update()
    {
        if (dragging == true)
        {
            first.position = mouse_position;
            
            var offset = ???
            var another_offset = ???
            second.position = ???
        }
    }
}

Edit: I tried the code that @DMGregory posted and @Tyyppi_77 linked but it is not working as expected. Here is the code that i'm using. Currently, the second sprite is flying all over the place.

public class Sprite
{
    public Sprite parent;
    public List<Sprite> childrens;
    public Rectangle bounds;
    
    public Point World { get { return new Point(bounds.X, bounds.Y); } }
    public Point Local { get { return new Point(World.X - parent.World.X, World.Y - parent.World.Y); } }
}

public class Drag
{
    public Sprite first;
    public Sprite second;

    public Point mouse;
    public bool dragging;
    
    public void Update()
    {
        if (dragging == true)
        {   
            first.bounds.X = mouse.X - (first.bounds.Width / 2);
            first.bounds.Y = mouse.Y - (first.bounds.Height / 2);
            
            second.bounds.X = second.Local.X;
            second.bounds.Y = second.Local.Y;
        }
    }
}

Thanks in advance!

Answer 1

The sprite class you have written is not fit for purpose. As I've repeated to you a few times now, you should treat your local position as the source of truth, and derive the world position from it and the parent.

Your code does the opposite - it takes the world bounds as the source of truth, and derives local position from it and the parent.

That means whenever your parent moves without moving the child's bounds, the child will stay in place in the world and its local position will change to compensate. This is not at all what you want.

We can fix that by making the local position the source of truth:

public class Sprite
{
    public Sprite parent;
    public List<Sprite> children; // No "s" - "children" is already plural
    public Rectangle localBounds;
    
    public Point World { 
        get {
            Point origin = default;
            if (parent != null) origin = parent.World;

            return new Point(origin.X + localBounds.X, origin.Y + localBounds.Y);
        }

        set {
            Point origin = default;
            if (parent != null) origin = parent.World;

            localBounds.x = value.X - origin.X;
            localBounds.y = value.Y - origin.Y;
        }
    }

    public Point Local { 
        get {
            return new Point(localBounds.X, localBounds.y); 
        }

        set {
            localBounds.X = value.X;
            localBounds.Y = value.Y;
        }
    }
}

With that in hand, you can drag two sprites and maintain their offset something like this.

public class Drag
{
    public Sprite first;
    public Sprite second;

    public Point mouse;
    public bool dragging;

    // Alter this offset if you want to adjust the spacing during a drag.
    private Point _offset;
    
    // Call this when you want to begin dragging a pair of objects.
    public void StartDrag() {
         _offset = new Point(second.World.X - first.World.X,
                             second.World.Y - first.World.Y);

         dragging = true;
    }

    public void Update()
    {
        if (dragging == true)
        {   
            var position = new Point(mouse.X - (first.localBounds.Width / 2),
                                     mouse.Y - (first.localBounds.Height / 2));

            first.World = position;
            second.World = new Point(position.X + _offset.X, 
                                     position.Y + _offset.Y);

            // In case the first could be a child or grandchild of second,
            // then we need to apply a fix-up after second moves.
            first.World = position;

            // (Though in that case, you'd be better off just dragging
            //  second alone, without touching first, and let it come
            //  along for the ride automatically through parenting)
        }
    }
}