可选项响应链（学习笔记）

环境Xcode 11.0 beta4 swift 5.1

• 可选项响应链
  • 可选项响应链是一个用于访问和调用属性、方法、下标的过程，这些属性、方法、下标可能为 nil；如果有值就会调用成功，如果响应链中只要有一处为 nil，则整个链就会失败；Swift 中可选项响应链类似于 Objective-C 中传递消息 nil，但其可用于任何类型，并且可以检查成功不是失败。
• 可选项作为强制解包一种替代方案

  • 示例

    class Person {
        var residence: Residence?
    }
    // 
    class Residence {
        var numberOfRooms = 1
    }
    let john = Person()
    let roomCount = john.residence!.numberOfRooms
    // this triggers a runtime error
    if let roomCount = john.residence?.numberOfRooms {
        print("John's residence has \(roomCount) room(s).")
    } else {
        print("Unable to retrieve the number of rooms.")
    }
    // Prints "Unable to retrieve the number of rooms."
    // 如果你给residence赋值，结果如下
    john.residence = Residence()
    if let roomCount = john.residence?.numberOfRooms {
        print("John's residence has \(roomCount) room(s).")
    } else {
        print("Unable to retrieve the number of rooms.")
    }
    // Prints "John's residence has 1 room(s)."

• 为可选项响应链定义模型类
  • 示例代码块

  class Person {
      var residence: Residence?
  }
  class Residence {
      var rooms = [Room]()
      var numberOfRooms: Int {
          return rooms.count
      }
      subscript(i: Int) -> Room {
          get {
              return rooms[i]
          }
          set {
              rooms[i] = newValue
          }
      }
      func printNumberOfRooms() {
          print("The number of rooms is \(numberOfRooms)")
      }
      var address: Address?
  }
  class Room {
      let name: String
      init(name: String) { self.name = name }
  }
  class Address {
      var buildingName: String?
      var buildingNumber: String?
      var street: String?
      func buildingIdentifier() -> String? {
          if let buildingNumber = buildingNumber, let street = street {
              return "\(buildingNumber) \(street)"
          } else if buildingName != nil {
              return buildingName
          } else {
              return nil
          }
      }
  }

• 通过可选项访问属性
  • 示例代码块

  // 用上面创建的类
  let john = Person()
  if let roomCount = john.residence?.numberOfRooms {
      print("John's residence has \(roomCount) room(s).")
  } else {
      print("Unable to retrieve the number of rooms.")
  }
  // Prints "Unable to retrieve the number of rooms."
  // 通过响应链设置值
  // 由于此之前并没有为residence设值，为nil，因此用someAddress赋值不会成功
  let someAddress = Address()
  someAddress.buildingNumber = "29"
  someAddress.street = "Acacia Road"
  john.residence?.address = someAddress
  // 上面的情况不好观察，下面用函数会比较好观察
  func createAddress() -> Address {
      print("Function was called.")
  //
      let someAddress = Address()
      someAddress.buildingNumber = "29"
      someAddress.street = "Acacia Road"
  //
      return someAddress
  }
  john.residence?.address = createAddress()
  // 可以发现上面的 "Function was called."不会打印

• 通过可选项调用方法
  • 示例代码块

  // 本例中方法 printNumberOfRooms() 有一个隐式返回值 `Void` （或者 `()` 即空元组）
  // 如果是可选的值调用，则返回的是 `Void?`，不是 `Void`
  // 因此可以用 `if` 语句来判断方法是否调用成功
  // 
  if john.residence?.printNumberOfRooms() != nil {
      print("It was possible to print the number of rooms.")
  } else {
      print("It was not possible to print the number of rooms.")
  }
  // Prints "It was not possible to print the number of rooms."
  //
  // 如果属性返回是 `Void?`，也可用同样的判断方法
  if (john.residence?.address = someAddress) != nil {
      print("It was possible to set the address.")
  } else {
      print("It was not possible to set the address.")
  }
  // Prints "It was not possible to set the address."

• 通过可选项访问下标
  • ? 放在 [] 之前，总是紧跟表达式之后

  if let firstRoomName = john.residence?[0].name {
      print("The first room name is \(firstRoomName).")
  } else {
      print("Unable to retrieve the first room name.")
  }
  // Prints "Unable to retrieve the first room name."
  // `?` 的执行是在紧跟 residence 之后， 在 `[]` 调用之前
  john.residence?[0] = Room(name: "Bathroom")
  // 上面的尝试赋值也会失败，因为 residence 仍然是 nil

  let johnsHouse = Residence()
  johnsHouse.rooms.append(Room(name: "Living Room"))
  johnsHouse.rooms.append(Room(name: "Kitchen"))
  john.residence = johnsHouse
  // 
  if let firstRoomName = john.residence?[0].name {
      print("The first room name is \(firstRoomName).")
  } else {
      print("Unable to retrieve the first room name.")
  }
  // Prints "The first room name is Living Room."

  • 可选类型的下标访问，? 要写在 [] 之后

  var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
  testScores["Dave"]?[0] = 91
  testScores["Bev"]?[0] += 1
  testScores["Brian"]?[0] = 72
  // the "Dave" array is now [91, 82, 84] and the "Bev" array is now [80, 94, 81]

• 连接多层级的响应链
  • 如果要获取的值不是可选项，但由于可选响应链，它将会成为可选项
  • 如果要获取的值是可选项，也不会因为可选项响应链变成多层可选（类似：??, ???）

  if let johnsStreet = john.residence?.address?.street {
      print("John's street name is \(johnsStreet).")
  } else {
      print("Unable to retrieve the address.")
  }
  // Prints "Unable to retrieve the address."
  // 如果你给 address 赋一个值
  let johnsAddress = Address()
  johnsAddress.buildingName = "The Larches"
  johnsAddress.street = "Laurel Street"
  john.residence?.address = johnsAddress
  // 
  if let johnsStreet = john.residence?.address?.street {
      print("John's street name is \(johnsStreet).")
  } else {
      print("Unable to retrieve the address.")
  }
  // Prints "John's street name is Laurel Street."

• 方法返回可选项的响应链
  • 示例代码块

  if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
      print("John's building identifier is \(buildingIdentifier).")
  }
  // Prints "John's building identifier is The Larches."
  // 要执行更深层的响应链，如下
  if let beginsWithThe =
      john.residence?.address?.buildingIdentifier()?.hasPrefix("The") {
      if beginsWithThe {
          print("John's building identifier begins with \"The\".")
      } else {
          print("John's building identifier does not begin with \"The\".")
      }
  }
  // Prints "John's building identifier begins with "The"."
  // 注意：`?` 是加在 `()` 之后因为返回值才是可选项，而不是方法名之后