struct Person {
let firstName: String
let lastName: String
init!(firstName: String, lastName: String) {
guard !firstName.isEmpty && !lastName.isEmpty else {
return nil
}
self.firstName = firstName
self.lastName = lastName
}
}
swift 2.x
let nilPerson = Person(firstName: "", lastName: "Mathias")
nilPerson.firstName // `nilPerson ` 是 `Person!`类型，值为nil，程序崩溃！
let matt = Person(firstName: "Matt", lastName: "Mathias")
matt.firstName // `matt` 是 `Person!`类型
let anotherMatt = matt // `anotherMatt` 也是 `Person!`类型
swift 3
let nilPerson = Person(firstName: "", lastName: "Mathias")
nilPerson?.firstName// `nilPerson ` 是 `Person?`类型，可选链，安全！
let matt = Person(firstName: "Matt", lastName: "Mathias")
matt?.firstName // `matt` 是 `Person?`类型
let anotherMatt = matt // `anotherMatt` 也是 `Person?`类型
let runningWithScissors: Person! = Person(firstName: "Edward", lastName: "") // 显式声明为` Person!`类型
let safeAgain = runningWithScissors // `safeAgain ` 是 `Person?`类型
- 依靠编译器对类型进行推断
- 总结：引用
Optional 或 ImplicitlyUnwrappedOptional 类型，自己会变成 Optional 类型。
参考资料
WWDC 2016: Increased Safety in Swift 3.0
