Agendas (The Chickadee Game Toolkit)

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5.5.1 Agendas

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To schedule a task to be performed later, an “agenda” is used. -There is a default, global agenda that is ready to be used, or -additional agendas may be created for different purposes. The -following example prints the text “hello” when the agenda has -advanced to time unit 10. -

(at 10 (display "hello\n"))
-

- -

Most of the time it is more convenient to schedule tasks relative to -the current time. This is where after comes in handy: -

(after 10 (display "hello\n"))
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Time units in the agenda are in no way connected to real time. It’s -up to the programmer to decide what agenda time means. A simple and -effective approach is to map each call of the update procedure -(see Kernel) to 1 unit of agenda time, like so: -

(define (update dt)
-  (update-agenda 1))
-

- -

It is important to call update-agenda periodically, otherwise -no tasks will ever be run! -

In addition to using the global agenda, it is useful to have multiple -agendas for different purposes. For example, the game world can use a -different agenda than the user interface, so that pausing the game is -a simple matter of not updating the world’s agenda while continuing to -update the user interface’s agenda. The current agenda is dynamically -scoped and can be changed using the with-agenda special form: -

(define game-world-agenda (make-agenda))
-
-(with-agenda game-world-agenda
-  (at 60 (spawn-goblin))
-  (at 120 (spawn-goblin))
-  (at 240 (spawn-goblin-king)))
-

- -

Procedure: make-agenda: Return a new task scheduler. -

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Procedure: agenda? obj: Return #t if obj is an agenda. -

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Procedure: current-agenda
Procedure: current-agenda agenda: When called with no arguments, return the current agenda. When called -with one argument, set the current agenda to agenda. -

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Syntax: with-agenda agenda body …: Evaluate body with the current agenda set to agenda. -

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Procedure: agenda-time: Return the current agenda time. -

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Procedure: update-agenda dt: Advance the current agenda by dt. -

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Procedure: schedule-at time thunk: Schedule thunk, a procedure of zero arguments, to be run at -time. -

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Procedure: schedule-after delay thunk: Schedule thunk, a procedure of zero arguments, to be run after -delay. -

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Procedure: schedule-every interval thunk [n]: Schedule thunk, a procedure of zero arguments, to be run every -interval amount of time. Repeat this n times, or -indefinitely if not specified. -

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Syntax: at time body …: Schedule body to be evaluated at time. -

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Syntax: after delay body …: Schedule body to be evaluated after delay. -

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Syntax: every interval body …
Syntax: every (interval n) body …: Schedule body to be evaluated every interval amount of -time. Repeat this n times, or indefinitely if not specified. -

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It is also possible to schedule things that are not dependent on how -much time passes. The agenda will periodically poll to see if any -registered conditions are met. -

Procedure: call-when pred thunk: Call thunk sometime in the future when pred is satisfied -(returns a value other than #f.) -

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