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@@ -173,6 +173,51 @@ second, waiting for 60 updates means that the NPC will wait one second in between each step. +*** Actions + Actions encapsulate a procedure that operates over a certain + period of time. Action objects have two properties: an arbitrary + procedure and a duration in game ticks. Action procedures accept + one argument: a time delta in the range [0, 1]. Use actions in + combination with coroutines for things that are a function of + time, such as moving a sprite across the screen. + + #+BEGIN_SRC scheme + (schedule-action + ;; Move horizontally across the screen, starting at x=0 and moving to + ;; x=800, in 60 ticks. + (lerp (lambda (x) + (set-sprite-position! sprite (vector x (/ window-height 2)))) + 0 800 60)) + #+END_SRC + + =schedule-action= is used to schedule a coroutine that will + perform the given action in the current agenda. =lerp= is a type + of action, short for linear interpolation. =lerp= takes an + arbitrary procedure to apply at each tick, a start value, an end + value, and like all other actions, a duration. The code above + interpolates from 0 to 800 over 60 ticks. The result of this + action is a sprite moving across the screen from left to right. + + Actions can be combined to run in a sequence or in parallel. + + #+BEGIN_SRC scheme + (schedule-action + (action-parallel + (lerp (lambda (x) + (set-sprite-position! sprite (vector x (/ window-height 2)))) + 0 800 60) + ;; Rotate sprite 1080 degrees in 120 ticks. + (lerp (lambda (angle) + (set-sprite-rotation! sprite angle)) + 0 1080 120))) + #+END_SRC + + =action-parallel= will combine many actions into one action that + does everything at the same time. In the example above, the sprite + will still move across the screen from left to right, but while + it's doing so (and for 60 ticks after), it will be rotating from 0 + to 1080 degrees. + ** REPL Driven Development The read-eval-print-loop present in Guile allows you to develop your game while it is running! This allows you to see in real time |