probe.onChange

Fire when a value becomes different from what it was.

Reads the expression once per frame and fires on the transition, so it reaches anything the engine will evaluate — a component field, a light's intensity, a transform's position, a table lookup — without that value needing to publish an event of its own.

probe.onChange("entity.find('Lamp').component.get('Light').intensity", {
    notice = "the lamp got bright",
    when   = "value >= 5",
    run    = "return value",
})

The context carries value and previous, so a probe can report the step as well as the destination.

Pair it with when to name the value you are waiting for rather than every change on the way there: the example fires the first time intensity reaches five, not on each increment toward it.

It samples once per frame. A value that changes and changes back inside one frame reads as no change at all, and one held for a frame or two is an edge it may catch going up and miss coming down. When what you are chasing is that short-lived, probe the CODE that sets it with onLine instead: a line fires every time it runs, whatever the frame rate.

The value at arm time is the baseline, not a hit — a probe that fired on arming would report the state you already knew. An expression that cannot be evaluated yet (an entity not spawned, a component not added) is not a change; it becomes one when it starts answering.