Tutorial: Using GKPolygonObstacle to draw line of sight

Using GKPolygonObstacle a line of sight polygon (path) can be calculated that could then be used to draw over the graphics to show the "line of sight" for a given game object (ie enemy soldier).

If you don't know what this means have a look at the (interactive) examples in this article:
http://www.redblobgames.com/articles/visibility/

Or watch this video: https://www.youtube.com/watch?v=-4jRNOmAvlk

Estimation: 2-3 days