Function that accepts any type of Measurement in swift

user_3002785 0
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Swift implements the Measurement type which can be used to hold values such as speeds, altitudes, etc. in their desired metric.

For example:

// Going to explicitly type here so the question is clear

var speed: Measurement<UnitSpeed> = Measurement(value: 10, unit: UnitSpeed.metersPerSecond)
var altitude: Measurement<UnitLength> = Measurement(value: 500, unit: UnitLength.meters)

altitude.convert(to: UnitLength.feet)

print("500 meters is roughly \(Int(altitude.value)) feet")
// Prints: 500 meters is roughly 1640 feet

However, even though UnitLength and UnitSpeed are both subclasses of Unit, measurements using these types can not be passed in polymorphically into a function in swift.

For example, the following function does not work if you pass in an object of type Measurement<UnitLength> or Measurement<UnitSpeed>.

func formatMeasurementAsString(measurement: Measurement<Unit>) -> String {
    return "\(Int(measurement.value)) \(measurement.unit.symbol)"
}

Passing in either speed or altitude defined above into this function yields the following error: Cannot convert value of type 'Measurement<UnitSpeed>' to expected argument type 'Measurement<Unit>'

EDIT:

So you can explicitly type both speed and altitude in this example as type Measurement<Unit>. However, if you do this, they lose the incredibly useful .convert(to:) function unless you typecast them back into Measurement<UnitLength> before converting, which is not very helpful.

swift polymorphism
1 Answers

You can make the function generic

func formatMeasurementAsString<T: Unit>(measurement: Measurement<T>) -> String {
    return "\(Int(measurement.value)) \(measurement.unit.symbol)"
}

Example

let length = Measurement(value: 180.0, unit: UnitLength.meters)
let speed = Measurement(value: 60, unit: UnitSpeed.kilometersPerHour)

print(formatMeasurementAsString(measurement: length))
print(formatMeasurementAsString(measurement: speed))

180 m
60 km/h

user_92238390
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