Answer: Declare a var with a get block that returns the value and a set block that updates the underlying stored state. The setter receives the incoming value, named newValue by default.

Code Example:

struct Temperature {
    var celsius: Double

    var fahrenheit: Double {
        get {
            celsius * 9 / 5 + 32
        }
        set(newFahrenheit) {               // explicit parameter name
            celsius = (newFahrenheit - 32) * 5 / 9
        }
    }

    var kelvin: Double {
        get { celsius + 273.15 }
        set { celsius = newValue - 273.15 } // shorthand: implicit `newValue`
    }
}

var t = Temperature(celsius: 0)
t.fahrenheit = 212
print(t.celsius) // 100.0

Key Points:

• Shorthand setter — omit the parameter name and use newValue.
• Shorthand getter — if the getter is a single expression, the return is implicit (Swift 5.1+).
• A computed property cannot be setter-only; set always requires a get.
• On a struct, the setter is implicitly mutating, so it can only be called on a var instance.