什么是可选类型
swift编译器要求开发者明确变量/属性的状态，即确定是否不为nil。如果不确定，则使用变量/属性时需要针对两种场景进行处理。
表示变量/属性有两种状态：1是nil，2.是个确定值。可选类型的定义如下（实际上是枚举类型）：
public enum Optional<Wrapped> : ExpressibleByNilLiteral {
/// The absence of a value.
///
/// In code, the absence of a value is typically written using the `nil`
/// literal rather than the explicit `.none` enumeration case.
case none
/// The presence of a value, stored as `Wrapped`.
case some(Wrapped)
/// Creates an instance that stores the given value.
public init(_ some: Wrapped)
}
语法规则，下面两种写法等价
let shortForm: Int? = Int("42")
let longForm: Optional<Int> = Int("42")
什么是解包解包
Optional类型通常不能直接使用，需要解包后获取nil或者确定的值，才能为程序所用？
如何解包解包
1.gurad let
let name: String? = "Antoine van der Lee"
guard let unwrappedName = name else {
return
}
print(unwrappedName.count)
2.if let
2.if let
let name: String? = "Antoine van der Lee"
if let unwrappedName = name {
print(unwrappedName.count)
}
3.使用“??” 运算符
let name: String? = "Antoine van der Lee"
print(name?.count ?? 0)
- 使用"!"运算符强制解包，如果值为nil则抛出fatalError异常，除非能确保有值，通常不推荐使用
var name: String? = "Antoine van der Lee"
print(name!.count)
- 链式解包
struct BlogPost {
let title: String?
}
let post: BlogPost? = BlogPost(title: "Learning everything about optionals")
print(post?.title?.count ?? 0)
