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;;; Chickadee Game Toolkit
;;; Copyright © 2016, 2018 David Thompson <dthompson2@worcester.edu>
;;;
;;; Licensed under the Apache License, Version 2.0 (the "License");
;;; you may not use this file except in compliance with the License.
;;; You may obtain a copy of the License at
;;;
;;; http://www.apache.org/licenses/LICENSE-2.0
;;;
;;; Unless required by applicable law or agreed to in writing, software
;;; distributed under the License is distributed on an "AS IS" BASIS,
;;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
;;; See the License for the specific language governing permissions and
;;; limitations under the License.
(define-module (chickadee game-loop)
#:export (run-game*
abort-game
current-timestep))
;;;
;;; Error handling
;;;
(define (call-with-error-handling handler thunk)
"Call THUNK and respond to any exceptions with HANDLER. Return #t if
an error was handled."
(if handler
(let ((stack #f))
(define (pre-unwind-handler . args)
(set! stack (make-stack #t 4)))
(define (throw-handler)
(with-throw-handler #t thunk pre-unwind-handler)
#f)
(define (exception-handler e)
(handler e stack)
#t)
(with-exception-handler exception-handler throw-handler #:unwind? #t))
(begin
(thunk)
#f)))
(define-syntax-rule (with-error-handling handler body ...)
(call-with-error-handling handler (lambda () body ...)))
;;;
;;; Game loop kernel
;;;
(define game-loop-prompt-tag (make-prompt-tag 'game-loop))
(define (abort-game)
(abort-to-prompt game-loop-prompt-tag #f))
(define current-timestep (make-parameter 0.0))
(define* (run-game* #:key init update render time error
(update-hz 60))
(let ((timestep (/ 1.0 update-hz)))
(parameterize ((current-timestep timestep))
(call-with-prompt game-loop-prompt-tag
(lambda ()
;; Catch SIGINT and kill the loop.
(sigaction SIGINT
(lambda (signum)
(abort-game)))
(init)
;; A simple analogy is that we are filling up a bucket
;; with water. When the bucket fills up to a marked
;; line, we dump it out. Our water is time, and each
;; time we dump the bucket we update the game. Updating
;; the game on a fixed timestep like this yields a
;; stable simulation.
(let loop ((previous-time (time))
(buffer 0.0))
(let* ((current-time (time))
(delta (- current-time previous-time)))
(let update-loop ((buffer (+ buffer delta)))
(if (>= buffer timestep)
;; Short-circuit the update loop if an error
;; occurred, and reset the current time to now in
;; order to discard the undefined amount of time
;; that was spent handling the error.
(if (with-error-handling error (update timestep))
(loop (time) 0.0)
(update-loop (- buffer timestep)))
(begin
;; We render upon every iteration of the loop, and
;; thus rendering is decoupled from updating.
;; It's possible to render multiple times before
;; an update is performed.
(if (with-error-handling error
(render (/ buffer timestep))
(usleep 1))
(loop (time) 0.0)
(loop current-time buffer))))))))
(lambda (cont callback)
#f)))))
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