function stateStr(state) {
  if (state & STATE_CANCELLED) {
    return 'cancel';
  } else if (state & STATE_ENDED) {
    return 'end';
  } else if (state & STATE_CHANGED) {
    return 'move';
  } else if (state & STATE_BEGAN) {
    return 'start';
  }
  return '';
}
@private
get a usable string, used as event postfix
@param {constant} state
@returns {String} state