One Script, Multiple Behaviors

See: Handlers

Because a single script slot cannot run multiple threads, you must borrow another entity's (or the map's) on_tick if you want a single script to trigger multiple simultaneous behaviors.

If you try to put all simultaneous behaviors into a single script, the actions will execute one after the other, but only after each action has completely finished. Three entities having a race would instead run the entire course one at a time, each waiting patiently for the runner before them to complete their route. And indeed, if one of the simultaneous behaviors has no stopping point, the actions listed afterwards will never execute at all.

The simultaneous behaviors must be managed by on_tick slots because on_interact and on_load script slots cannot execute actions at arbitrary times.

When the simultaneous behaviors are finished, however, the on_tick slots must be then set to something else or the new behavior will loop indefinitely (unless that's what you want). Halt an on_tick script by running null_script as its final action or by setting the target slot to a different on_tick script.

Example: Timid Goats

In the BMG2020, there are a pair of baby goats cavorting in the grass. However, if the player approaches them, they will run to another spot.

Three entities are involved in this behavior (apart from the player themselves):

• Billy (a baby goat)
• Kid (a baby goat)
• Verthandi (the goat handler)

Each goat has an on_tick script that moves it around in a small jaggedy circle: the vector shape high1 for Billy and high2 for Kid.

What's important here is the watcher, Verthandi. Her on_tick script, check_if_player_is_goat_high, watches for if the player has entered the goat trigger area called high, at which point her on_tick script jumps to one called move_goats_to_low.

1. move_goats_to_low contains the following actions (executed by Verthandi):
   • Set entity tick script for Billy to move_billy_to_low
   • Set entity tick script for Kid to move_kid_to_low
   • Set entity tick script for Verthandi to check_if_player_is_goat_low
2. move_billy_to_low (executed by Billy):
   • Walk Billy to the first point in geometry low1
   • Loop Billy along geometry low1
     • NOTE: this action will not end on its own.
3. move_kid_to_low (executed by Kid):
   • Walk Kid to the first point in geometry low2
   • Loop Kid along geometry low2
     • NOTE: this action will not end on its own.

This setup does what we want because #2 and #3 are run in on_tick slots, and because they are executed by entities other than the one running the watch script, #1.

The end behavior is a reversal of the above: Verthandi is now watching for the player to enter a goat trigger area called low, and due to the final action inside #2 and #3, the goats are now looping along their "low" vector paths until their on_tick scripts are changed again by #1.