easing method

Tween easing([
  1. num easingFunction(
    1. num, [
    2. num?
    ])?
])

Implementation

Tween easing([num Function(num, [num?])? easingFunction]) {
  easingFunction ??= Easing.Linear[ETTypes.None];
  this._easing = easingFunction!;
  return this;
}