Scripting

From FreeGameDevWiki

Jump to: navigation, search

Contents

Coroutines

Coroutines, also known as green threads or micro threads, are pieces of code that behave similar to a real thread, but instead of letting the OS do the scheduling, they let the process to the scheduling. This leads to a much reduced overhead and also to much more control over the threads. The reason why they are important in gaming is that they allow the creation of actor based scripting and cutscene scripting without falling back to state machine handling. Simple example (pseudo language): 

 def cutscene():
   bob.walk_to(alice)
   bob.say("Hi")
   alice.say("Welcome, follow me")
   bob.follow(alice)
   alice.walk_to(exit)

What happens here is that Bob goes to Alice, talks a bit and then follows her. Unlike normal code, the running game is not interrupted, but instead the code just inserts tasks into the engine and then waits for their completion. This allows to program sequences. The implementation would look something like this: 

 class Actor:
   def walk_to(target):
      set_target(target)
      suspend()

The set_target() call, schedules the next tasks, while the suspend() call interrupts the script and returns control to the engine. Once the task is completed, the engine will resume the script and continue at the exact point where suspend() left. On the engine side things look something like this: 

 scripts = [Script("cutscene1.script"), Script("background-animation.script")]
 
 while(1):
   for script in scripts:
     script.run()
   process_input()
   update_world()
   draw()

There is a list with running scripts that gets called each frame. Each frame runs till it encounters suspend() and then gives control back to the engine. The whole stack of the script is preserved and allows the script to continue where it left of. Some script engine allow more tight control and for example give you the ability to limit how much CPU a script might take, so that a script gets automatically interrupted when its over its threshold.

Scripts with callbacks

Game State Saving

One issue that arises with scripting is that they are often hard to serialize. Proper solution for this is unknown.

Scripting Languages

Retrieved from "http://freegamedev.net/wiki/Scripting"
Personal tools