3. GUI programming

Graphics User Interfaces (GUI) reflect the state of the system and let the user interact with it. There are two things:

  • Frames: pictures that are displayed on screen

  • Interaction: given the currently displayed frame and the state of the system (i.e. the currently displayed frame is part of the state in fact), how to react to events (mouse click, key press, etc.)

3.1. Basic game loop

A basic game loop acts like this:

gameLoop :: IO ()
gameLoop = do

   -- upate the state
   whileM (currentTime - lastSimulationTime >= timeStep) do
      -- process input events and other system events that have happened
      -- during [lastSimulationTime, lastSimulationTime+timeStep]

      updateSimulation timeStep  -- update physics in constant meaningful steps

      lastSimulationTime += timeStep

   -- perform frame rendering
   -- Parameter is used for interpolation (1 frame lag) or extrapolation
   -- (potential glitches) of the remaining time
   tmpState <- simulateUpdate (currentTime - lastSimulationTime)
   render tmpState

   unlessM quitTheGame

3.2. Modifying the state

With MonadFlow M we track access to values from the Monad M. Basically we build a tree of commands but if the values we read from M are the same as those in cache, we know that the tree is exactly the same (pure function with the same inputs).

We build a pure tree representing the new state or we indicate if no change occurred: then we don’t have to redraw at all.

M must be able to give a coherent view of the system at a given point in time.