Optional-assignment.swift

infix operator ?=: AssignmentPrecedence
infix operator ??=: AssignmentPrecedence

func ?= <T>(lhs: inout T, rhs: T?) {
    print("-- ?= start")
    defer { print("-- ?= end") }
    guard let actualRhs = rhs else { return }
    lhs = actualRhs
}

func ??= <T>(lhs: inout T, rhs: T?) {
    print("-- ??= start")
    defer { print("-- ??= end") }
    lhs = rhs ?? lhs
}

struct Book {
    var title: String {
        didSet { print("Book.title:didSet called with '\(title)'") }
    }
    var author: String {
        get {
            print("Book.author:getter called")
            return "default author"
         }
         set {
            print("Book.author:setter called with '\(newValue)'")
         }
    }
}

let optionalTitle1: String? = nil
let optionalTitle2: String? = "banana"

print("------------")

var book1 = Book(title: "apple")
book1.title ?= optionalTitle1
print("book1 title is '\(book1.title)'")
book1.title ?= optionalTitle2
print("book1 title is '\(book1.title)'")

print("------------")

var book2 = Book(title: "apple")
book2.author ?= optionalTitle1
book2.author ?= optionalTitle2

print("------------")

var book3 = Book(title: "apple")
book3.title ??= optionalTitle1
book3.title ??= optionalTitle2

print("------------")

var book4 = Book(title: "apple")
book4.author ??= optionalTitle1
book4.author ??= optionalTitle2

// Console output:
/*
------------
-- ?= start
-- ?= end
Book.title:didSet called with 'apple'
book1 title is 'apple'
-- ?= start
-- ?= end
Book.title:didSet called with 'banana'
book1 title is 'banana'
------------
Book.author:getter called
-- ?= start
-- ?= end
Book.author:setter called with 'default author'
Book.author:getter called
-- ?= start
-- ?= end
Book.author:setter called with 'banana'
------------
-- ??= start
-- ??= end
Book.title:didSet called with 'apple'
-- ??= start
-- ??= end
Book.title:didSet called with 'banana'
------------
Book.author:getter called
-- ??= start
-- ??= end
Book.author:setter called with 'default author'
Book.author:getter called
-- ??= start
-- ??= end
Book.author:setter called with 'banana'
*/