It's also possible to use mutating or a nonmutating modifier at getter and setter in the computed property.

MutatingGetter.swift

struct Reference {
    private let _value: Double
    var count = 0

    init(value: Double) { _value = value }

    var value: Double { mutating get {
        count += 1 // Mutating property
        return _value
    }}
}

var reference = Reference(value: 0.125)
for _ in 0..<5 { print(reference.value) } // "0.125" is shown 5 times.

print(reference.count) // "5"

NonMutationgSetter.swift

struct Independent {
    private static var _pool = [Double]() // Type property
    let index: Int

    init(value: Double) {
        index = Independent._pool.count
        Independent._pool.append(value) // Add value to Array
    }

    var value: Double {
        get { return Independent._pool[index] }

        // Using only type property
        nonmutating set { Independent._pool[index] = newValue }
    }

    static func clear() { // Type method
        _pool = _pool.flatMap { _ in 0.0 }
    }
}

let independent = Independent(value: 0.125)
print(independent.value) // "0.125"

independent.value = 5.25 // Possible to substitute
print(independent.value) // "5.25"

Independent.clear()
print(independent.value) // "0.0"