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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