Swift可选型（optional）各种用法示例
原文：optionals

the optionals

let optional: String? = "unicorn"
var optional2: Int?
let optionals: [String?] = ["unicorn", nil, "dragon"]
var optionals2: [Int]?

if let

if let value = optional {

multiple variables

if let value = optional, let value2 = optional2 {

boolean clause

if let value = optional, value.hasSuffix("saurus") {

guard

guard let value = optional else { 
  return
}

while let

while let value = optional {

nil coalescing

let value = optional ?? "nil"

force unwrapping

let value = optional!

switch block

switch optional {
case .some(let value):
  print(value)
case .none:
  print("nil")
}

map()

let value = optional.map(String.init(describing:))

flatMap()

let value = optional.flatMap(URL.init(string:))

compactMap()

let values = optionals.compactMap { $0 }

type casting

let value = optional as! String

optional chaining

let value = optional?.uppercased()

for loop

for element in optionals {
  if let value = element {

for case let

for case let optional? in optionals {

for case .some

for case .some(let value) in optionals {

forEach

optionals2?.forEach { value in

assignment

assigns if the optional has a value
optional2? = 2014

pattern matching

switch optional {
case "unicorn"?:
  print("Unicorn!")
default:
  print("Not Unicorn")
}

enums

enum Animal {
  case pet(type: String?)
}

switching with associated optionals

let enumValue = Animal.pet(type: optional)
switch enumValue {
case .pet(.some(let value)):
  print("I am a \(value).")
case .pet(.none):
  print("I am unknown.")
}

switching on optional enums

let enumValue2: Animal? = nil
switch enumValue2 {
case .pet?:
  print("Pets")
default:
  print("No pets.")
}

参考

Apple Developer Documentation: Optional
The Complete Guide to Optionals in Swift
Swift optionals explained simply
Extending optionals in Swift