NinePatch.cs

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using rpg.Source.Assets;

namespace rpg {
    public class NinePatch {

        private const int DefaultSpacing = 1;
        private const int Bleed = 1;
        
        private readonly Sprite sprite;
        private readonly Rectangle topLeft, top, topRight;
        private readonly Rectangle midLeft, mid, midRight;
        private readonly Rectangle botLeft, bot, botRight;

        public NinePatch(string spriteRef, int top, int bottom, int left, int right, int spacing = DefaultSpacing) {
            sprite = Assets.GetSprite(spriteRef);
            Rectangle spriteRect = sprite.Rectangle;

            int midWidth  = spriteRect.Width  - (left + right + (spacing + Bleed) * 2);
            int midHeight = spriteRect.Height - (top + bottom + (spacing + Bleed) * 2);

            int left1 = spriteRect.X;
            int left2 = spriteRect.X + left + spacing + Bleed;
            int left3 = spriteRect.X + left + midWidth + (spacing + Bleed) * 2;

            int top1 = spriteRect.Y;
            int top2 = spriteRect.Y + top + spacing + Bleed;
            int top3 = spriteRect.Y + top + midHeight + (spacing + Bleed) * 2;

            this.topLeft  = new Rectangle(left1, top1, left,     top);
            this.top      = new Rectangle(left2, top1, midWidth, top);
            this.topRight = new Rectangle(left3, top1, right,    top);
            
            this.midLeft  = new Rectangle(left1, top2, left,     midHeight);
            this.mid      = new Rectangle(left2, top2, midWidth, midHeight);
            this.midRight = new Rectangle(left3, top2, right,    midHeight);
            
            this.botLeft  = new Rectangle(left1, top3, left,     bottom);
            this.bot      = new Rectangle(left2, top3, midWidth, bottom);
            this.botRight = new Rectangle(left3, top3, right,    bottom);
        }

        public void Draw(int x, int y, int w, int h) {
            float scaleX = (w - (topLeft.Width  + topRight.Width)) / (float) top.Width;
            float scaleY = (h - (topLeft.Height + botLeft.Height)) / (float) mid.Height;
            
            DrawPatch(topLeft,  new Vector2(x, y),                                             Vector2.One);
            DrawPatch(top,      new Vector2(x + topLeft.Width, y),                             new Vector2(scaleX, 1));
            DrawPatch(topRight, new Vector2(x + w - topRight.Width, y),                        Vector2.One);
            
            DrawPatch(midLeft,  new Vector2(x, y + topLeft.Height),                            new Vector2(1, scaleY));
            DrawPatch(mid,      new Vector2(x + midLeft.Width, y + topLeft.Height),            new Vector2(scaleX, scaleY));
            DrawPatch(midRight, new Vector2(x + w - midRight.Width, y + topLeft.Height),       new Vector2(1, scaleY));
            
            DrawPatch(botLeft,  new Vector2(x, y + (h - botLeft.Height)),                      Vector2.One);
            DrawPatch(bot,      new Vector2(x + botLeft.Width, y + (h - botLeft.Height)),      new Vector2(scaleX, 1));
            DrawPatch(botRight, new Vector2(x + w - botRight.Width, y + (h - botLeft.Height)), Vector2.One);
        }

        private void DrawPatch(Rectangle rect, Vector2 pos, Vector2 scale) {
            Texture2D texture = sprite.Texture;
            Main.sb.Draw(texture, pos, rect,  Color.White, 0f, Vector2.Zero, scale, SpriteEffects.None, 0f);
        }

    }
}