code: function getHookArgumentsLength (fn) {
    if (isUndef(fn)) {
      return false
    }
    var invokerFns = fn.fns;
    if (isDef(invokerFns)) {
      // invoker
      return getHookArgumentsLength(
        Array.isArray(invokerFns)
          ? invokerFns[0]
          : invokerFns
      )
    } else {
      return (fn._length || fn.length) > 1
    }
  }
docs: Normalize a transition hook's argument length. The hook may be:
  - a merged hook (invoker) with the original in .fns
  - a wrapped component method (check ._length)
  - a plain function (.length)