RAF.hs

-----------------------------------------------------------------------------
{-# LANGUAGE OverloadedStrings #-}
-----------------------------------------------------------------------------
-- |
-- Module      :  Miso.Subscription.RAF
-- Copyright   :  (C) 2016-2026 David M. Johnson
-- License     :  BSD3-style (see the file LICENSE)
-- Maintainer  :  David M. Johnson <code@dmj.io>
-- Stability   :  experimental
-- Portability :  non-portable
--
-- = Overview
--
-- "Miso.Subscription.RAF" provides 'rAFSub', a subscription that hooks
-- into the browser's
-- <https://developer.mozilla.org/en-US/docs/Web/API/Window/requestAnimationFrame requestAnimationFrame>
-- loop. On each frame the browser calls back with a
-- <https://developer.mozilla.org/en-US/docs/Web/API/DOMHighResTimeStamp DOMHighResTimeStamp>
-- (milliseconds since page load, sub-millisecond precision), which is
-- forwarded to the component as an action.
--
-- This is the recommended driver for canvas-based animations and games
-- because the browser throttles the callback to the display refresh rate
-- (typically 60 fps) and pauses it automatically when the tab is hidden.
--
-- = Quick start
--
-- @
-- import "Miso"
-- import "Miso.Subscription.RAF"
-- import "Miso.Canvas"
--
-- data Action = Tick Double   -- DOMHighResTimeStamp in ms
--
-- myComponent = ('Miso.component' model update view)
--   { 'Miso.Types.subs'   = [ 'rAFSub' Tick ]
--   , 'Miso.Types.events' = 'Miso.Event.Types.defaultEvents'
--   }
--
-- update :: Action -> 'Miso.Effect.Effect' p props Model Action
-- update (Tick t) = do
--   'Miso.State.modify' (\\m -> m { time = t })
-- @
--
-- = Lifecycle
--
-- Internally 'rAFSub' uses 'Miso.Subscription.Util.createSub':
--
-- * __Acquire__ — schedules the first @requestAnimationFrame@ callback,
--   which re-schedules itself on every invocation.
-- * __Release__ — calls 'Miso.DSL.freeFunction' to cancel the callback
--   and release the JS reference when the component unmounts.
--
-- = See also
--
-- * "Miso.Canvas" — canvas drawing API driven by 'rAFSub' ticks
-- * "Miso.Subscription.Util" — 'Miso.Subscription.Util.createSub'
-- * "Miso.Subscription" — re-export hub
----------------------------------------------------------------------------
module Miso.Subscription.RAF
  ( rAFSub
  , rAFSubElapsed
  ) where
----------------------------------------------------------------------------
import           Data.IORef
----------------------------------------------------------------------------
import           Miso.DSL
import           Miso.Effect (Sub)
import           Miso.Subscription.Util (createSub)
----------------------------------------------------------------------------
-- | A 'Sub' for 60FPS animations when using 'requestAnimationFrame'.
--
-- The 'Double' returned is a [DOMHighResTimeStamp](https://developer.mozilla.org/en-US/docs/Web/API/DOMHighResTimeStamp) expressed in milliseconds.
--
rAFSub
  :: (Double -> action)
  -- ^ Callback fired each frame with a @DOMHighResTimeStamp@ in milliseconds
  -> Sub model action
rAFSub toAction sink = createSub acquire release sink
  where
    acquire = do
      cbRef <- newIORef (error "rAFSub: uninitialized, impossible")
      idRef <- newIORef (0 :: Int)
      callback <-
        syncCallback1 $ \jsval -> do
          sink . toAction =<< fromJSValUnchecked jsval
          writeIORef idRef =<< requestAnimationFrame =<< readIORef cbRef

      writeIORef cbRef callback
      writeIORef idRef =<< requestAnimationFrame callback
      pure (callback, idRef)

    -- N.B. the queued frame must be cancelled before the callback is
    -- freed: the browser holds a reference to it, and invoking a freed
    -- callback on the next frame crashes the runtime.
    release (callback, idRef) = do
      cancelAnimationFrame =<< readIORef idRef
      freeFunction (Function callback)
----------------------------------------------------------------------------
-- | Like 'rAFSub' but fires @action@ at most once per @interval@ milliseconds.
--
-- Elapsed time is accumulated in 'IORef's inside the subscription so the
-- model is not touched between ticks — Miso only re-renders when a tick
-- actually fires, rather than on every animation frame.
--
-- @
-- app = defaultApp model update view
--   { subs = [ rAFSubElapsed 175 Tick ] }
-- @
--
rAFSubElapsed
  :: Double
  -- ^ Minimum interval between ticks in milliseconds (e.g. @175@ for ~6 fps)
  -> action
  -- ^ Action to dispatch each time the interval elapses
  -> Sub model action
rAFSubElapsed interval action sink = createSub acquire release sink
  where
    acquire = do
      cbRef <- newIORef (error "rAFSubElapsed: uninitialized, impossible")
      idRef <- newIORef (0 :: Int)
      let go lastT elap = do
            cb <- syncCallback1 $ \jsval -> do
              t <- fromJSValUnchecked jsval
              let dt      = if lastT == 0 then 0 else min interval (t - lastT)
                  newElap = elap + dt
              if newElap >= interval
                then sink action *> go t (newElap - interval)
                else go t newElap
            writeIORef cbRef cb
            writeIORef idRef =<< requestAnimationFrame cb
      go 0 0
      pure (cbRef, idRef)
    -- N.B. cancel the queued frame before freeing the callback (see 'rAFSub')
    release (cbRef, idRef) = do
      cancelAnimationFrame =<< readIORef idRef
      freeFunction . Function =<< readIORef cbRef
----------------------------------------------------------------------------