NSCoder上的decodeDouble返回一个非可选值，但我想确定一个值在编码之前是否为nil。这是我的场景:varoptionalDouble:Double?=nilfuncencode(withaCoder:NSCoder){ifletoptionalDouble{aCoder.encode(optionalDouble,forKey:"myOptionalDouble")}}conveniencerequiredinit?(coderaDecoder:NSCoder){optionalDouble=aDecoder.decodeDouble(forKey:"myOptiona