import "math/event" for Event

class Observable{
  value{_value}
  value=(v){
    _event.emit([v])
    _value = v
  }
  value_no_update=(v){_value = v}

  event{_event}

  static new(value){
    var variable = new()
    variable.value_no_update = value
    return variable
  }

  construct new(){
    _event = Event.new()
  }

  on_change(fn){
    return _event.listen(fn)
  }

  on_change(initial, fn){
    if(initial){
      fn.call(value)
    }
    return on_change(fn)
  }

  not_on_change(fn){
    _event.unlisten(fn)
  }

  on_not_null(fn){
    if(value){
      fn.call(value)
    } else {
      _event.once(fn)
    }
  }
}