swift - 如何在 Swift 中存储通用测量值？

Question

我想要一个可以表示单个 Measurement 的变量但可能有不同的单位类型。例如，它可以存储长度或质量。看起来很简单，但我想不通。

这是我尝试过的:

struct Data {
    var weight: Measurement<UnitMass>
    var length: Measurement<UnitLength>
    var target = "Weight"

    var valueOfTarget: Measurement<Unit> {
        if target == "Weight" {
            return weight
        } else {
            return length
        }
    }
}

var data = Data(weight: Measurement<UnitMass>(value: 10, unit: UnitMass.kilograms),
                length: Measurement<UnitLength>(value: 10, unit: UnitLength.centimeters),
                target: "Weight")

print(data.valueOfTarget)

我也试过使用 <Dimension>正如另一个答案中所建议的那样，但也有类似的错误。

这会导致编译器错误:

error: cannot convert return expression of type 'Measurement<UnitMass>' to return type 'Measurement<Unit>'

我是否遗漏了一些明显的东西，或者这是不可能的？

Answer 1

您可以创建新的通用返回值。这对我来说似乎可以编译。

struct Data {
    var weight: Measurement<UnitMass>
    var length: Measurement<UnitLength>
    var target = "Weight"

    var valueOfTarget: Measurement<Unit> {
        if target == "Weight" {
            return Measurement<Unit>(value: weight.value, unit: weight.unit)
        } else {
            return Measurement<Unit>(value: length.value, unit: length.unit)
        }
    }
}

var data = Data(weight: Measurement<UnitMass>(value: 10, unit: UnitMass.kilograms),
                length: Measurement<UnitLength>(value: 10, unit: UnitLength.centimeters),
                target: "Weight")

print(data.valueOfTarget)