Now that we can schedule tasks, let’s take things to the next level. It sure would be great if we could make procedures that described a series of actions that happened over time, especially if we could do so without contorting our code into a nest of callback procedures. This is where scripts come in. With scripts we can write code in a linear way, in a manner that appears to be synchronous, but with the ability to suspend periodically in order to let other scripts have a turn and prevent blocking the game loop. Building on top of the scheduling that agendas provide, here is a script that models a child trying to get their mother’s attention:
(script (while #t (display "mom!") (newline) (sleep 60))) ; where 60 = 1 second of real time
This code runs in an endless loop, but the sleep
procedure
suspends the script and schedules it to be run later by the agenda.
So, after each iteration of the loop, control is returned back to the
game loop and the program is not stuck spinning in a loop that will
never exit. Pretty neat, eh?
Scripts can suspend to any capable handler, not just the agenda.
The yield
procedure will suspend the current script and pass
its “continuation” to a handler procedure. This handler procedure
could do anything. Perhaps the handler stashes the continuation
somewhere where it will be resumed when the user presses a specific
key on the keyboard, or maybe it will be resumed when the player picks
up an item off of the dungeon floor; the sky is the limit.
Sometimes it is necessary to abruptly terminate a script after it has been started. For example, when an enemy is defeated their AI routine needs to be shut down. When a script is spawned, a handle to that script is returned that can be used to cancel it when desired.
(define script (script (while #t (display "hey\n") (sleep 60)))) ;; sometime later (cancel-script script)
Apply thunk as a script and return a handle to it.
Evaluate body as a script and return a handle to it.
Return #t
if obj is a script handle.
Return #t
if obj has been cancelled.
Return #t
if obj has not yet terminated or been
cancelled.
Return #t
if obj has terminated.
Prevent further execution of the script co.
Suspend the current script and pass its continuation to the procedure handler.
Wait duration before resuming the current script.
Evaluate body in an endless loop.