In the previous chapters our code examples used the OnTimer() callback with a rough fixed timestep to update the state and the OnDrawFrame() callback to render graphics. This is not suitable for a real-time game where we should update the state based on the real time elapsed since the last frame. However, it is still desirable to use a small fixed timestep in the call to OnTimer(). We can solve this problem by interleaving calls to OnTimer() and OnDrawFrame() in a tricky fashion and put this logic into a game main loop.
There is a very interesting article called Fix Your Timestep! available at http://gafferongames.com/game-physics/fix-your-timestep, which explains in great detail different approaches to the implementation of a game main loop and why fixed timesteps are important.