An example of a simplified spiral loop algorithm for iterating in a square.

SpiralLoop.cs

public static class SpiralLoop
{
    public static void ExecuteCenterToOutter(int width, System.Action<int, int> callback)
    {
        // Run the loop from the center outwards
        for (int r = 0; r < width; r++)
        {
            // If we are on center, do not run the edge loops
            if (r == 0)
            {
                callback(0, 0);
                continue;
            }

            // For each edge of the square, run a loop from 0-size
            // Call one each of the edges using simple transformations
            for (int i = -r; i < r; i++)
            {
                callback(i, r);
                callback(r, -i);
                callback(-r, i);
                callback(-i, -r);
            }
        }
    }

    public static void ExecuteOutterToCenter(int width, System.Action<int, int> callback)
    {
        // Run the loop from the outside edges to the center
        for (int r = width-1; r >= 0; r--)
        {
            // If we are on center, do not run the edge loops
            if (r == 0)
            {
                callback(0, 0);
                continue;
            }

            // For each edge of the square, run a loop from 0-size
            // Call one each of the edges using simple transformations
            for (int i = -r; i < r; i++)
            {
                callback(i, r);
                callback(r, -i);
                callback(-r, i);
                callback(-i, -r);
            }
        }
    }
}