detect the first time a collision happened

[andrew-peng-144]

like box2d's onEnter() and onExit(). This is really good for projectiles collision but not wanting to remove the projectile, like piercing.

[andrew-peng-144]

idea: track when pairs of IDs collide with each other. Cases:

• If collided in the current frame but not previous, that means it just started, onEnter
• if yes previous yes current, it's in the middle of colliding
• if yes previous no current, it just stopped colliding, onExit

[andrew-peng-144]

use 2 lists of hashed IDs: "prev" and "curr", with fast add, remove, and search. then follow up with the logic in above comment

[andrew-peng-144]

Did the onenter, in the form of a boolean in the handler function.

The onexit works, but still need to have a way to add a onexit function for each pair of types.