Question

decodeDouble上的NSCoder会返回一个非可选值，但我想确定一个值在编码之前是否为零。

这是我的情景：

var optionalDouble: Double? = nil

func encode(with aCoder: NSCoder) {
    if let optionalDouble {
        aCoder.encode(optionalDouble, forKey: "myOptionalDouble")
    }
}

convenience required init?(coder aDecoder: NSCoder) {
    optionalDouble = aDecoder.decodeDouble(forKey: "myOptionalDouble") 
    // here optionalDouble is never nil anymore
}

因此，如果从未设置过值，则解码double将返回0，因此在编码之前，似乎无法识别值是否实际为0或nil / p>

我有办法在编码之前检查双精度是否为零

Answer 1

解决方案是在编码时使用NSNumber而不是Double，然后使用decodeObject返回double值（如果存在）。例如

class A: NSCoding {
    var optionalDouble: Double? = nil

    @objc func encodeWithCoder(aCoder: NSCoder) {
        if let optionalDouble = optionalDouble {
            aCoder.encodeObject(NSNumber(double: optionalDouble), forKey: "myOptionalDouble")
        }
    }

    @objc required init?(coder aDecoder: NSCoder) {
        if let decodedDoubleNumber = aDecoder.decodeObjectForKey("myOptionalDouble") as? NSNumber {
            self.optionalDouble = decodedDoubleNumber.doubleValue
        } else {
            self.optionalDouble = nil
        }
    }
}

根据@Hamish的建议，这里是Swift 3的版本。请注意我们需要将类继承到NSObject才能使NSEncoding工作（Got Unrecognized selector -replacementObjectForKeyedArchiver: crash when implementing NSCoding in Swift）

class A: NSObject, NSCoding {
    var optionalDouble: Double? = nil

    func encode(with aCoder: NSCoder) {
        if let optionalDouble = optionalDouble {
            aCoder.encode(optionalDouble, forKey: "myOptionalDouble")
        }
    }

    override init() {
        super.init()
    }

    required init?(coder aDecoder: NSCoder) {
        optionalDouble = aDecoder.decodeObject(forKey: "myOptionalDouble") as? Double
    }
}

NSCoder decodeDouble可选值

1 个答案: