Examples/elm-js/FrameRate/FrameRate.elm · elk

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2module FrameRate where

4import Data.Maybe (mapMaybe)

5import Foreign.JavaScript

6import Signal.Keyboard.Raw

7import Signal.Input

10foreign import jsevent "trigger" (castIntToJSNumber 0)

11 jsTime :: Signal JSNumber

13foreign export jsevent "finished"

14 done :: Signal JSBool

16foreign export jsevent "desiredFPS"

17 desiredFPS :: Signal JSNumber

19desiredFPS = constant (castIntToJSNumber 30)

21time = lift castJSNumberToFloat jsTime

24-- Calculate FPS based on incoming times.

26getFPS ts =

27 let len = length ts in

28 if len > 1 then (len-1) / (head ts - last ts) else 0

29truncate n = castJSNumberToInt (castFloatToJSNumber $ 100 * n) / 100

30fps = lift (truncate . getFPS) (foldp (\t ts -> take 10 (t:ts)) [0] time)

33-- Use time and keyboard input to determine the

34-- position of the black square.

36addVecs (x1,y1) (x2,y2) = (x1+x2, y1+y2)

37scaleBy s (x,y) = (x*s, y*s)

39addKey k (x,y) =

40 if k < 37 || k > 40 then (x,y) else

41 if k `mod` 2 == 1 then (x + k - 38, y)

42 else (x, y + k - 39)

44velocity = lift (foldl addKey (0,0)) keysDown

46timestep = lift snd $ foldp (\t1 (t0,d) -> (t1, t1-t0)) (0,0) time

48delta = dropIf (\(x,y) -> x == 0 && y == 0) (0,0) $

49 lift (scaleBy 100) (lift2 scaleBy timestep velocity)

51position = foldp addVecs (0,0) delta

54-- Display moving square and FPS on screen.

56screen pos actual desired =

57 flow down [ collage 400 400 [ outlined black (rect 40 40 pos) ]

58 , plainText "Move the square around with the arrow keys."

59 , text $ toText "Actual frames per second: " ++ show actual

60 , text $ toText "Desired frames per second: " ++ show desired

61 ]

64game = lift3 screen position fps desiredFPS

65done = lift (\_ -> castBoolToJSBool True) game

67main = game