On-demand nil coalescing infix operator

on-demand.swift

import Foundation

infix operator ??=: NilCoalescingPrecedence
func ??= <T>(left: inout T?, right: T) -> T? {
    left = left ?? right
    return left
}

class MyClass {
    private var _onDemand: Date?
    var onDemand: Date? {
        get { return _onDemand ??= Date() }
        set { _onDemand = newValue }
    }
}

let myClass = MyClass()

let formatter = DateFormatter()
formatter.dateFormat = "y-MM-dd H:m:ss.SSSS"

if let onDemand = myClass.onDemand {
    print(formatter.string(from: onDemand))   // 2019-01-03 8:51:52.9600
}

myClass.onDemand = nil

if let onDemand = myClass.onDemand {
    print(formatter.string(from: onDemand))   // 2019-01-03 8:51:52.9620
}

myClass.onDemand = Date().addingTimeInterval(84000)

if let onDemand = myClass.onDemand {
    print(formatter.string(from: onDemand))   // 2019-01-04 8:51:52.9620
}