Fire action on every keyframe

I want drive change of effect property on every keyframe: it would change its value from 0 to 1 and vice versa (basically on & off).

So far I have this expression for the effect "Dust & Scratches" sitting on its "Threshold" property:

var v = 1;
var n = timeRemap.numKeys;
if (n > 0){
    if(time < timeRemap.nearestKey(time).time){
        if(v == 0){
            v = 1;
        }else{
            v = 0
        }
    }
}
v

Problem is I would like to be able fire the change exactly on every new keyframe but I am not able to get there, this is the closest but of course it changes the value "erratically" once it reaches middle point between two frames.

I tried also this but to no success too as the variables kfA/kfB are initialized with every frame 😞

var kfA = 1;
var kfB = 2;
var v = 1;
var n = timeRemap.numKeys;
if (n > 0){
    if(time >= timeRemap.key(kfA).time && time < timeRemap.key(kfB).time){
        if(v == 0){
            v = 1;
        }else{
            v = 0
        }
    }
    if(time == timeRemap.key(kfB).time){
        kfA++;
        kfB++;
    }
}
v