Optional 是Swift标准库中的类型，
- 代表一个解包值或者一个缺席值nil
- 是枚举类型
- 遵守ExpressibleByNilLiteral协议
只要使用了可选值就会用到Optional类型,没有出现Optional关键字是因为?座位简写替代之。Int? == Optional<Int>
let shortForm: Int? = Int("42")
let longForm: Optional<Int> = Int("42")
Optional是有两个cases的枚举，Optional.none匹配nil,Optional.some(Wrapped)存储解包值。
/// let number: Int? = Optional.some(42)
/// let noNumber: Int? = Optional.none
/// print(noNumber == nil)
/// // Prints "true"
很多上下文中，必须解包Optional实例才能使用。Swift推出了几种安全的解包方式实现简洁清晰的代码。
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)
/// Evaluates the given closure when this `Optional` instance is not `nil`,
/// passing the unwrapped value as a parameter.
///
/// Use the `map` method with a closure that returns a nonoptional value.
/// This example performs an arithmetic operation on an
/// optional integer.
///
/// let possibleNumber: Int? = Int("42")
/// let possibleSquare = possibleNumber.map { $0 * $0 }
/// print(possibleSquare)
/// // Prints "Optional(1764)"
///
/// let noNumber: Int? = nil
/// let noSquare = noNumber.map { $0 * $0 }
/// print(noSquare)
/// // Prints "nil"
///
/// - Parameter transform: A closure that takes the unwrapped value
/// of the instance.
/// - Returns: The result of the given closure. If this instance is `nil`,
/// returns `nil`.
public func map<U>(_ transform: (Wrapped) throws -> U) rethrows -> U?
/// Evaluates the given closure when this `Optional` instance is not `nil`,
/// passing the unwrapped value as a parameter.
///
/// Use the `flatMap` method with a closure that returns an optional value.
/// This example performs an arithmetic operation with an optional result on
/// an optional integer.
///
/// let possibleNumber: Int? = Int("42")
/// let nonOverflowingSquare = possibleNumber.flatMap { x -> Int? in
/// let (result, overflowed) = x.multipliedReportingOverflow(by: x)
/// return overflowed ? nil : result
/// }
/// print(nonOverflowingSquare)
/// // Prints "Optional(1764)"
///
/// - Parameter transform: A closure that takes the unwrapped value
/// of the instance.
/// - Returns: The result of the given closure. If this instance is `nil`,
/// returns `nil`.
public func flatMap<U>(_ transform: (Wrapped) throws -> U?) rethrows -> U?
/// Creates an instance initialized with `nil`.
///
/// Do not call this initializer directly. It is used by the compiler when you
/// initialize an `Optional` instance with a `nil` literal. For example:
///
/// var i: Index? = nil
///
/// In this example, the assignment to the `i` variable calls this
/// initializer behind the scenes.
public init(nilLiteral: ())
/// The wrapped value of this instance, unwrapped without checking whether
/// the instance is `nil`.
///
/// The `unsafelyUnwrapped` property provides the same value as the forced
/// unwrap operator (postfix `!`). However, in optimized builds (`-O`), no
/// check is performed to ensure that the current instance actually has a
/// value. Accessing this property in the case of a `nil` value is a serious
/// programming error and could lead to undefined behavior or a runtime
/// error.
///
/// In debug builds (`-Onone`), the `unsafelyUnwrapped` property has the same
/// behavior as using the postfix `!` operator and triggers a runtime error
/// if the instance is `nil`.
///
/// The `unsafelyUnwrapped` property is recommended over calling the
/// `unsafeBitCast(_:)` function because the property is more restrictive
/// and because accessing the property still performs checking in debug
/// builds.
///
/// - Warning: This property trades safety for performance. Use
/// `unsafelyUnwrapped` only when you are confident that this instance
/// will never be equal to `nil` and only after you've tried using the
/// postfix `!` operator.
public var unsafelyUnwrapped: Wrapped { get }
/// Returns a Boolean value indicating whether an argument matches `nil`.
///
/// You can use the pattern-matching operator (`~=`) to test whether an
/// optional instance is `nil` even when the wrapped value's type does not
/// conform to the `Equatable` protocol. The pattern-matching operator is used
/// internally in `case` statements for pattern matching.
///
/// The following example declares the `stream` variable as an optional
/// instance of a hypothetical `DataStream` type, and then uses a `switch`
/// statement to determine whether the stream is `nil` or has a configured
/// value. When evaluating the `nil` case of the `switch` statement, this
/// operator is called behind the scenes.
///
/// var stream: DataStream? = nil
/// switch stream {
/// case nil:
/// print("No data stream is configured.")
/// case let x?:
/// print("The data stream has \(x.availableBytes) bytes available.")
/// }
/// // Prints "No data stream is configured."
///
/// - Note: To test whether an instance is `nil` in an `if` statement, use the
/// equal-to operator (`==`) instead of the pattern-matching operator. The
/// pattern-matching operator is primarily intended to enable `case`
/// statement pattern matching.
///
/// - Parameters:
/// - lhs: A `nil` literal.
/// - rhs: A value to match against `nil`.
public static func ~= (lhs: _OptionalNilComparisonType, rhs: Wrapped?) -> Bool
/// Returns a Boolean value indicating whether the left-hand-side argument is
/// `nil`.
///
/// You can use this equal-to operator (`==`) to test whether an optional
/// instance is `nil` even when the wrapped value's type does not conform to
/// the `Equatable` protocol.
///
/// The following example declares the `stream` variable as an optional
/// instance of a hypothetical `DataStream` type. Although `DataStream` is not
/// an `Equatable` type, this operator allows checking whether `stream` is
/// `nil`.
///
/// var stream: DataStream? = nil
/// if stream == nil {
/// print("No data stream is configured.")
/// }
/// // Prints "No data stream is configured."
///
/// - Parameters:
/// - lhs: A value to compare to `nil`.
/// - rhs: A `nil` literal.
public static func == (lhs: Wrapped?, rhs: _OptionalNilComparisonType) -> Bool
/// Returns a Boolean value indicating whether the left-hand-side argument is
/// not `nil`.
///
/// You can use this not-equal-to operator (`!=`) to test whether an optional
/// instance is not `nil` even when the wrapped value's type does not conform
/// to the `Equatable` protocol.
///
/// The following example declares the `stream` variable as an optional
/// instance of a hypothetical `DataStream` type. Although `DataStream` is not
/// an `Equatable` type, this operator allows checking whether `stream` wraps
/// a value and is therefore not `nil`.
///
/// var stream: DataStream? = fetchDataStream()
/// if stream != nil {
/// print("The data stream has been configured.")
/// }
/// // Prints "The data stream has been configured."
///
/// - Parameters:
/// - lhs: A value to compare to `nil`.
/// - rhs: A `nil` literal.
public static func != (lhs: Wrapped?, rhs: _OptionalNilComparisonType) -> Bool
/// Returns a Boolean value indicating whether the right-hand-side argument is
/// `nil`.
///
/// You can use this equal-to operator (`==`) to test whether an optional
/// instance is `nil` even when the wrapped value's type does not conform to
/// the `Equatable` protocol.
///
/// The following example declares the `stream` variable as an optional
/// instance of a hypothetical `DataStream` type. Although `DataStream` is not
/// an `Equatable` type, this operator allows checking whether `stream` is
/// `nil`.
///
/// var stream: DataStream? = nil
/// if nil == stream {
/// print("No data stream is configured.")
/// }
/// // Prints "No data stream is configured."
///
/// - Parameters:
/// - lhs: A `nil` literal.
/// - rhs: A value to compare to `nil`.
public static func == (lhs: _OptionalNilComparisonType, rhs: Wrapped?) -> Bool
/// Returns a Boolean value indicating whether the right-hand-side argument is
/// not `nil`.
///
/// You can use this not-equal-to operator (`!=`) to test whether an optional
/// instance is not `nil` even when the wrapped value's type does not conform
/// to the `Equatable` protocol.
///
/// The following example declares the `stream` variable as an optional
/// instance of a hypothetical `DataStream` type. Although `DataStream` is not
/// an `Equatable` type, this operator allows checking whether `stream` wraps
/// a value and is therefore not `nil`.
///
/// var stream: DataStream? = fetchDataStream()
/// if nil != stream {
/// print("The data stream has been configured.")
/// }
/// // Prints "The data stream has been configured."
///
/// - Parameters:
/// - lhs: A `nil` literal.
/// - rhs: A value to compare to `nil`.
public static func != (lhs: _OptionalNilComparisonType, rhs: Wrapped?) -> Bool
}
extension Optional : CustomDebugStringConvertible {
/// A textual representation of this instance, suitable for debugging.
public var debugDescription: String { get }
}
extension Optional : CustomReflectable {
/// The custom mirror for this instance.
///
/// If this type has value semantics, the mirror should be unaffected by
/// subsequent mutations of the instance.
public var customMirror: Mirror { get }
}
extension Optional {
/// Returns a Boolean value indicating whether two values are not equal.
///
/// Inequality is the inverse of equality. For any values `a` and `b`, `a != b`
/// implies that `a == b` is `false`.
///
/// This is the default implementation of the not-equal-to operator (`!=`)
/// for any type that conforms to `Equatable`.
///
/// - Parameters:
/// - lhs: A value to compare.
/// - rhs: Another value to compare.
public static func != (lhs: Optional<Wrapped>, rhs: Optional<Wrapped>) -> Bool
}
extension Optional : Encodable where Wrapped : Encodable {
/// Encodes this optional value into the given encoder.
///
/// This function throws an error if any values are invalid for the given
/// encoder's format.
///
/// - Parameter encoder: The encoder to write data to.
public func encode(to encoder: Encoder) throws
}
extension Optional : Decodable where Wrapped : Decodable {
/// Creates a new instance by decoding from the given decoder.
///
/// This initializer throws an error if reading from the decoder fails, or
/// if the data read is corrupted or otherwise invalid.
///
/// - Parameter decoder: The decoder to read data from.
public init(from decoder: Decoder) throws
}
extension Optional : Hashable where Wrapped : Hashable {
/// Hashes the essential components of this value by feeding them into the
/// given hasher.
///
/// - Parameter hasher: The hasher to use when combining the components
/// of this instance.
public func hash(into hasher: inout Hasher)
}
extension Optional : Equatable where Wrapped : Equatable {
/// Returns a Boolean value indicating whether two optional instances are
/// equal.
///
/// Use this equal-to operator (`==`) to compare any two optional instances of
/// a type that conforms to the `Equatable` protocol. The comparison returns
/// `true` if both arguments are `nil` or if the two arguments wrap values
/// that are equal. Conversely, the comparison returns `false` if only one of
/// the arguments is `nil` or if the two arguments wrap values that are not
/// equal.
///
/// let group1 = [1, 2, 3, 4, 5]
/// let group2 = [1, 3, 5, 7, 9]
/// if group1.first == group2.first {
/// print("The two groups start the same.")
/// }
/// // Prints "The two groups start the same."
///
/// You can also use this operator to compare a nonoptional value to an
/// optional that wraps the same type. The nonoptional value is wrapped as an
/// optional before the comparison is made. In the following example, the
/// `numberToMatch` constant is wrapped as an optional before comparing to the
/// optional `numberFromString`:
///
/// let numberToFind: Int = 23
/// let numberFromString: Int? = Int("23") // Optional(23)
/// if numberToFind == numberFromString {
/// print("It's a match!")
/// }
/// // Prints "It's a match!"
///
/// An instance that is expressed as a literal can also be used with this
/// operator. In the next example, an integer literal is compared with the
/// optional integer `numberFromString`. The literal `23` is inferred as an
/// `Int` instance and then wrapped as an optional before the comparison is
/// performed.
///
/// if 23 == numberFromString {
/// print("It's a match!")
/// }
/// // Prints "It's a match!"
///
/// - Parameters:
/// - lhs: An optional value to compare.
/// - rhs: Another optional value to compare.
public static func == (lhs: Wrapped?, rhs: Wrapped?) -> Bool
/// Returns a Boolean value indicating whether two optional instances are not
/// equal.
///
/// Use this not-equal-to operator (`!=`) to compare any two optional instances
/// of a type that conforms to the `Equatable` protocol. The comparison
/// returns `true` if only one of the arguments is `nil` or if the two
/// arguments wrap values that are not equal. The comparison returns `false`
/// if both arguments are `nil` or if the two arguments wrap values that are
/// equal.
///
/// let group1 = [2, 4, 6, 8, 10]
/// let group2 = [1, 3, 5, 7, 9]
/// if group1.first != group2.first {
/// print("The two groups start differently.")
/// }
/// // Prints "The two groups start differently."
///
/// You can also use this operator to compare a nonoptional value to an
/// optional that wraps the same type. The nonoptional value is wrapped as an
/// optional before the comparison is made. In this example, the
/// `numberToMatch` constant is wrapped as an optional before comparing to the
/// optional `numberFromString`:
///
/// let numberToFind: Int = 23
/// let numberFromString: Int? = Int("not-a-number") // nil
/// if numberToFind != numberFromString {
/// print("No match.")
/// }
/// // Prints "No match."
///
/// - Parameters:
/// - lhs: An optional value to compare.
/// - rhs: Another optional value to compare.
public static func != (lhs: Wrapped?, rhs: Wrapped?) -> Bool
}
/// Performs a nil-coalescing operation, returning the wrapped value of an
/// `Optional` instance or a default value.
///
/// A nil-coalescing operation unwraps the left-hand side if it has a value, or
/// it returns the right-hand side as a default. The result of this operation
/// will have the nonoptional type of the left-hand side's `Wrapped` type.
///
/// This operator uses short-circuit evaluation: `optional` is checked first,
/// and `defaultValue` is evaluated only if `optional` is `nil`. For example:
///
/// func getDefault() -> Int {
/// print("Calculating default...")
/// return 42
/// }
///
/// let goodNumber = Int("100") ?? getDefault()
/// // goodNumber == 100
///
/// let notSoGoodNumber = Int("invalid-input") ?? getDefault()
/// // Prints "Calculating default..."
/// // notSoGoodNumber == 42
///
/// In this example, `goodNumber` is assigned a value of `100` because
/// `Int("100")` succeeded in returning a non-`nil` result. When
/// `notSoGoodNumber` is initialized, `Int("invalid-input")` fails and returns
/// `nil`, and so the `getDefault()` method is called to supply a default
/// value.
///
/// - Parameters:
/// - optional: An optional value.
/// - defaultValue: A value to use as a default. `defaultValue` is the same
/// type as the `Wrapped` type of `optional`.
public func ?? <T>(optional: T?, defaultValue: @autoclosure () throws -> T) rethrows -> T
/// Performs a nil-coalescing operation, returning the wrapped value of an
/// `Optional` instance or a default `Optional` value.
///
/// A nil-coalescing operation unwraps the left-hand side if it has a value, or
/// returns the right-hand side as a default. The result of this operation
/// will be the same type as its arguments.
///
/// This operator uses short-circuit evaluation: `optional` is checked first,
/// and `defaultValue` is evaluated only if `optional` is `nil`. For example:
///
/// let goodNumber = Int("100") ?? Int("42")
/// print(goodNumber)
/// // Prints "Optional(100)"
///
/// let notSoGoodNumber = Int("invalid-input") ?? Int("42")
/// print(notSoGoodNumber)
/// // Prints "Optional(42)"
///
/// In this example, `goodNumber` is assigned a value of `100` because
/// `Int("100")` succeeds in returning a non-`nil` result. When
/// `notSoGoodNumber` is initialized, `Int("invalid-input")` fails and returns
/// `nil`, and so `Int("42")` is called to supply a default value.
///
/// Because the result of this nil-coalescing operation is itself an optional
/// value, you can chain default values by using `??` multiple times. The
/// first optional value that isn't `nil` stops the chain and becomes the
/// result of the whole expression. The next example tries to find the correct
/// text for a greeting in two separate dictionaries before falling back to a
/// static default.
///
/// let greeting = userPrefs[greetingKey] ??
/// defaults[greetingKey] ?? "Greetings!"
///
/// If `userPrefs[greetingKey]` has a value, that value is assigned to
/// `greeting`. If not, any value in `defaults[greetingKey]` will succeed, and
/// if not that, `greeting` will be set to the nonoptional default value,
/// `"Greetings!"`.
///
/// - Parameters:
/// - optional: An optional value.
/// - defaultValue: A value to use as a default. `defaultValue` and
/// `optional` have the same type.
public func ?? <T>(optional: T?, defaultValue: @autoclosure () throws -> T?) rethrows -> T?
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