OnTriggerExit2D called multiple times when two triggers touch

Question

I have two GameObjects tagged "Point", each with a Collider 2D set to Is Trigger.

I have another GameObject, "pencil", which also has a Collider2D set to Is Trigger

I want to move the pencil the first point to the start drawing a line, and then stop drawing when it hits the second point.

My problem is that when the pencil hits a point, it calls OnTriggerExit2D multiple times. How can I only detect first contact with the point?

Pencil script:

private void OnTriggerExit2D(Collider2D collision)
{
    if (!collision.isTrigger)
    {
        if (collision.gameObject.tag == "Point")
        {
            writing = !writing;
        }
    }
}

Answer 1

It looks like your Pecil needs some memory, so it can disregard repeated contacts with the same collider multiple times in a row.

This is related to the idea of "debouncing" a switch - the input to our systems, especially when they come from players or physics - can be messy. The mouse cursor might hover just at the border of a zone for several frames, alternately entering and exiting. So we often need to make our trigger responses "sticky", by remembering we've been activated, and requiring a larger action to deactivate/reset, something called hysteresis.

Collider2D _lastPointTouched;

void OnTriggerExit2D(Collider2D other)
{
    // Disregard collisions with anything but our points.
    if(other.gameObject.CompareTag("Point") == false)
        return;

    // Disregard repeated visits to the same point.
    if(other == _lastPointTouched)
        return;

    // Toggle our writing state, and remember this point for future events.
    writing = !writing;
    _lastPointTouched = other;
}

If you want, you can clear this memory if a condition is met (like the mouse button/touch is released, or the pencil gets far enough away, or if a certain amount of time passes) to allow starting a new line at this point when the player does so deliberately, without accidentally triggering it from a brushing contact.