从Xcode 4.5以后,我们在Objective-C中使用NS_ENUM和NS_OPTIONS来定义一个枚举,以替代C语言枚举的定义方式。其中NS_ENUM用于定义普通的枚举,NS_OPTIONS用于定义选项类型的枚举。
而到了Swift中,枚举增加了更多的特性。它可以包含原始类型(不再局限于整型)以及相关值。正是由于这些原因,枚举在Swift中得到了更广泛的应用。在Foundation中,Objective-C中的NS_ENUM类型的枚举,都会自动转换成Swift中enum,并且更加精炼。以Collection View的滚动方向为例,在Objective-C中,其定义如下:
typedef NS_ENUM(NSInteger, UICollectionViewScrollDirection) { UICollectionViewScrollDirectionVertical, UICollectionViewScrollDirectionHorizontal };
而在Swift中,其定义如下:
enum UICollectionViewScrollDirection : Int { case Vertical case Horizontal }
精练多了吧,看着舒服多了,还能少码两个字。我们自己定义枚举时,也应该采用这种方式。
不过对于Objective-C中NS_OPTIONS类型的枚举,Swift中的实现似乎就没有那么美好了。
我们再来对比一下UICollectionViewScrollPosition的定义吧,在Objective-C中,其定义如下:
typedef NS_OPTIONS(NSUInteger, UICollectionViewScrollPosition) { UICollectionViewScrollPositionNone = 0, // The vertical positions are mutually exclusive to each other, but are bitwise or-able with the horizontal scroll positions. // Combining positions from the same grouping (horizontal or vertical) will result in an NSInvalidArgumentException. UICollectionViewScrollPositionTop = 1 << 0, UICollectionViewScrollPositionCenteredVertically = 1 << 1, UICollectionViewScrollPositionBottom = 1 << 2, // Likewise, the horizontal positions are mutually exclusive to each other. UICollectionViewScrollPositionLeft = 1 << 3, UICollectionViewScrollPositionCenteredHorizontally = 1 << 4, UICollectionViewScrollPositionRight = 1 << 5 };
而在Swift 2.0中,其定义如下:
struct UICollectionViewScrollPosition : OptionSetType { init(rawValue: UInt) static var None: UICollectionViewScrollPosition { get } // The vertical positions are mutually exclusive to each other, but are bitwise or-able with the horizontal scroll positions. // Combining positions from the same grouping (horizontal or vertical) will result in an NSInvalidArgumentException. static var Top: UICollectionViewScrollPosition { get } static var CenteredVertically: UICollectionViewScrollPosition { get } static var Bottom: UICollectionViewScrollPosition { get } // Likewise, the horizontal positions are mutually exclusive to each other. static var Left: UICollectionViewScrollPosition { get } static var CenteredHorizontally: UICollectionViewScrollPosition { get } static var Right: UICollectionViewScrollPosition { get } }
额,光看代码,不看实现,这也是化简为繁的节奏啊。
为什么要这样做呢?Mattt给了我们如下解释:
Well, the same integer bitmasking tricks in C don’t work for enumerated types in Swift. An enum represents a type with a closed set of valid options, without a built-in mechanism for representing a conjunction of options for that type. An enum could, ostensibly, define a case for all possible combinations of values, but for n > 3, the combinatorics make this approach untenable.
意思是Swift不支持C语言中枚举值的整型掩码操作的技巧。在Swift中,一个枚举可以表示一组有效选项的集合,但却没有办法支持这些选项的组合操作(“&”、”|”等)。理论上,一个枚举可以定义选项值的任意组合值,但对于n > 3这种操作,却无法有效的支持。
为了支持类NS_OPTIONS的枚举,Swift 2.0中定义了OptionSetType协议【在Swift 1.2中是使用RawOptionSetType,相比较而言已经改进了不少】,它的声明如下:
/// Supplies convenient conformance to `SetAlgebraType` for any type /// whose `RawValue` is a `BitwiseOperationsType`. For example: /// /// struct PackagingOptions : OptionSetType { /// let rawValue: Int /// init(rawValue: Int) { self.rawValue = rawValue } /// /// static let Box = PackagingOptions(rawValue: 1) /// static let Carton = PackagingOptions(rawValue: 2) /// static let Bag = PackagingOptions(rawValue: 4) /// static let Satchel = PackagingOptions(rawValue: 8) /// static let BoxOrBag: PackagingOptions = [Box, Bag] /// static let BoxOrCartonOrBag: PackagingOptions = [Box, Carton, Bag] /// } /// /// In the example above, `PackagingOptions.Element` is the same type /// as `PackagingOptions`, and instance `a` subsumes instance `b` if /// and only if `a.rawValue & b.rawValue == b.rawValue`. protocol OptionSetType : SetAlgebraType, RawRepresentable { /// An `OptionSet`'s `Element` type is normally `Self`. typealias Element = Self /// Convert from a value of `RawValue`, succeeding unconditionally. init(rawValue: Self.RawValue) }
从字面上来理解,OptionSetType是选项集合类型,它定义了一些基本操作,包括集合操作(union, intersect, exclusiveOr)、成员管理(contains, insert, remove)、位操作(unionInPlace, intersectInPlace, exclusiveOrInPlace)以及其它的一些基本操作。
作为示例,我们来定义一个表示方向的选项集合,通常我们是定义一个实现OptionSetType协议的结构体,如下所示:
struct Directions: OptionSetType { var rawValue:Int init(rawValue: Int) { self.rawValue = rawValue } static let Up: Directions = Directions(rawValue: 1 << 0) static let Down: Directions = Directions(rawValue: 1 << 1) static let Left: Directions = Directions(rawValue: 1 << 2) static let Right: Directions = Directions(rawValue: 1 << 3) }
所需要做的基本上就是这些。然后我们就可以创建Directions的实例了,如下所示:
let direction: Directions = Directions.Left if direction == Directions.Left { // ... }
如果想同时支持两个方向,则可以如上处理:
let leftUp: Directions = [Directions.Left, Directions.Up] if leftUp.contains(Directions.Left) && leftUp.contains(Directions.Up) { // ... }
如果leftUp同时包含Directions.Left和Directions.Up,则返回true。
这里还有另外一种方法来达到这个目的,就是我们在Directions结构体中直接声明声明Left和Up的静态常量,如下所示:
struct Directions: OptionSetType { // ... static let LeftUp: Directions = [Directions.Left, Directions.Up] static let RightUp: Directions = [Directions.Right, Directions.Up] // ... }
这样,我们就可以以如下方式来执行上面的操作:
if leftUp == Directions.LeftUp { // ... }
当然,如果单一选项较多,而要去组合所有的情况,这种方法就显示笨拙了,这种情况下还是推荐使用contains方法。
总体来说,Swift中的对选项的支持没有Objective-C中的NS_OPTIONS来得简洁方便。而且在Swift 1.2的时候,我们还是可以使用”&“和”|”操作符的。下面这段代码在Swift 1.2上是OK的:
UIView.animateWithDuration(0.3, delay: 1.0, options: UIViewAnimationOptions.CurveEaseIn | UIViewAnimationOptions.CurveEaseOut, animations: { () -> Void in // ... }, completion: nil)
但到了Swift 2.0时,OptionSetType已经不再支持”&“和”|”操作了,因此,上面这段代码需要修改成:
UIView.animateWithDuration(0.3, delay: 1.0, options: [UIViewAnimationOptions.CurveEaseIn, UIViewAnimationOptions.CurveEaseInOut], animations: { () -> Void in // ... }, completion: nil)
不过,慢慢习惯就好。
RawOptionSetType
Exploring Swift 2.0 OptionSetTypes
Notes from WWDC 2015: The Enumerated Delights of Swift 2.0 Option Sets
《100个Swift开发必备Tip》— Tip 66. Options