Swift Enum 枚举底层（转载）

Swift Enumerations or how to annoy Tom

• In the dark fetid implementation mists behind the slick city Swift streets lies a secret world where enumerations are merely ints with pretensions. In more objective terms, Swift enums provide a complete and finite ordered listing of the possible members in a fixed collection. They basically come in three flavors.

1、Basic Enumerations.

• First, there’s a common variety of “related things” that form a family.

  enum Coin {case Heads, Tails}

• These basic enumerations provide a fixed vocabulary about possible states you may encounter (if rollValue == .Heads or if collection.contains(.Tails)). Collections with enumerations can contain repetitions. For example, [.Tails, .Tails, .Heads] represents a series of enumeration states, perhaps detailing the history of coin tosses.

• Avoid basic enumerations for bit flags as there’s a specific RawOptionSet solution for flags.

2、Fixed Values.

• A second flavor of enums offer an associated a raw value. The following example uses natural ordering, starting with 1. Tails’ rawValue is 2.

  enum Coin: Int {case Heads = 1, Tails}

• These values can be continuous, as in this example, or discrete {case Heads = 5, Tails = 23} but the type, which is declared after the enumeration name, is always homogenous.

• You can use other types, such as Strings, but there’s always a one-to-one correspondence between the enumeration and its value. So a .King enumeration may always equate to 12 or “King”. Think of these as a look-up table.

3、Associated Payloads.

• And there’s the kind that packs payloads. The most commonly used implementation of these is optionals (case .None, case .Some(T)) but you can build your own as well. Cases can use any types, and those types may include tuples.

  enum Coin {case Heads(NSDate, Bool, String); case Tails}

4、The Dark Underbelly of the Enum

• To better understand enumerations, it helps to poke at them with a sharp stick. For obvious reasons, don’t use the following material for production code. Or really for any code. That said, I found this exercise extremely valuable for understanding how enums work.

• Enums are typically one byte long.

  sizeof(Coin); // 1

• If you want to get very very silly, you can build an enumeration with hundreds of cases, in which case the enum takes up 2 or more bytes depending on the minimum bit count needed. If you’re building enumerations with more than 256 cases, you probably should reconsider why you’re using enumerations.

• Basic enumerations (the first two cases) are Hashable, that is, they provide an integer hashValue that is unique to each value. Unsurprisingly, the hash values for enumerations start at zero and increase monotonically. (Yes, this is an implementation detail. Alarm bell. Warning blare.)

  enum Planets {case Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus, Pluto} 
  print(Planets.Mars.hashValue) // 3
  print(Planets.Mercury.hashValue) // 0

• Enumerations with basic associated values are also raw representable. The rawValue follows from whatever default or explicit assignment you’ve made. All raw values are of the same type, in this case Int:

  enum Foo : Int {case i = 1, j = 5, k = 9}
  Foo.j.hashValue // 1
  Foo.j.rawValue // 5

5、Moving Forward

• So given these details of how enumerations work:

  • How do you create instances from rawValues?
  • How do you create instances from hashValues?
  • How do you query an enumeration about its members?

• I warn you in the strongest possible terms against continuing to read. Tom does not approve.

6、Creating instances from Raw Values

• Okay, I lied. This one, he does approve of. That’s because it’s legal.

  enum Foo : Int {case i = 1, j = 5, k = 9}
  Foo(rawValue: 5) // .j

• You create instances using the built-in constructor, supplying the raw value.

7、Enhancing Enums

• Now we wander into the shadow of the valley of doom, so you should start to fear some evil. Swift is no longer with you, and unsafe bitcasts lie ahead. The goal is to create this protocol, with default implementations that supplies the following features to all enumerations of the first two types.

  public protocol EnumConvertible: Hashable {
      init?(hashValue hash: Int)
      static func countMembers() -> Int
      static func members() -> [Self]
  }

8、Building Enums from Hash Values

• Since it’s a given that an enumeration is basically an Int8 that stores the hash value, you can build a really simple initializer. Just cast the Int8 that’s initialized with a hash value to the enumeration type.

  let member = unsafeBitCast(UInt8(index), Self.self)

• This line doesn’t check for safe values, so you probably want to use some sort of check that the value is within the membership limit, and create a failable initializer instead.

  internal static func fromHash(
      hashValue index: Int) -> Self {
      let member = unsafeBitCast(UInt8(index), Self.self)
      return member
  }
  
  public init?(hashValue hash: Int) {
      if hash >= Self.countMembers() {return nil}
      self = Self.fromHash(hashValue: hash)
  }

• Once added to the protocol, you can construct an instance from its hash value (countable, starting with 0) and look up its raw value:

  Foo(hashValue: 1)!.rawValue // 5

• Boom done. And Tom turns away, disapproval writ large upon his face.

9、Counting Members

• The hash-value-based init depends on there being some way to count enumeration members. If you know you’ll always deal with one-byte enumerations, this is super easy. Adding support for two bytes isn’t much harder.

  static public func countMembers() -> Int {
      let byteCount = sizeof(self)
      if byteCount == 0 {return 1}
      if byteCount byteCount > 2 {
          fatalError("Unable to process enumeration")}
      let singleByte = byteCount == 1
      let minValue = singleByte ? 2 : 257
      let maxValue = singleByte ? 2 << 8 : 2 << 16
      for hashIndex in minValue..<maxValue {
          switch singleByte {
              case true:
                  if unsafeBitCast(UInt8(hashIndex), self).hashValue == 0 {
                      return hashIndex
                  }
              case false:
                  if unsafeBitCast(UInt16(hashIndex), self).hashValue == 0 {
                      return hashIndex
                  }
          }
      }
      return maxValue
  }

• This approach uses a simple iteration to construct values until a hashValue look-up fails. It’s pretty brain dead although it knows that 2-byte enums cannot contain fewer than 256 values.

• Unfortunately, protocol implementation doesn’t allow you to create storage so you end up re-computing this value all the time (or would if you used this, which you won’t because Tom would not approve).

10、Enumerating Enumerations

• The final goal lies in creating a collection that allows you to enumerate through your enumerations to cover all available cases. For that, you need to return all members.

  static public func members() -> [Self] {
      var enumerationMembers = [Self]()
      let singleByte = sizeof(self) == 1
      for index in 0..<Self.countMembers() {
          switch singleByte {
              case true:
                  let member = unsafeBitCast(UInt8(index), self)
                  enumerationMembers.append(member)
              case false:
                  let member = unsafeBitCast(UInt16(index), self)
                  enumerationMembers.append(member)
          }
      }
      return enumerationMembers
  }

• As with the membership count, this is something that would benefit either from being built-in (well, of course), or from implementation that prevents it being computed more than once.

• Once you add this, you can perform tasks like “show me a function as it relates to each member of an enumeration”. Although actual sequencing is an illusion — enumeration members may not be built upon any intrinsic sequence semantics — it can be super handy to be able to access items in this way.

11、Wrap-Up

• You can see an example of why this function would be particularly helpful in this gist, which I wrote in response to a post on devforums. Someone was looking for a probability-weighted enum and it was their post that led me to start exploring this whole question.

• I gisted my answer here. It is based on a far less elegant solution for collecting members but it showcases why the use-case is valid and compelling.

• The entire protocol discussed in this post is at this gist and awaits your feedback, insight, and suggestions. Please tweet or leave comments here because Github doesn’t notify by email.

• Finally. Sorry, Tom.

本文转载自 Erica Sadun 的博客