Source
static TransitionFunction custom(num amount) {
if (amount < -1) amount = -1;
if (amount > 1) amount = 1;
num easing(num t) {
if (amount == 0) return t;
if (amount < 0) return t * (t * -amount + 1 + amount);
return t * ((2 - t) * amount + (1 - amount));
}
return easing;
}