map(keyPath) 其中 keyPath 是一个变量

Question

let arr = [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5)]
arr.map(\.0) // [1, 2, 3, 4, 5]

效果很好。但是下面的代码无法编译：

let keyPath = \(Int, Int).0
arr.map(keyPath)

Cannot convert value of type 'WritableKeyPath<(Int, Int), Int>' to expected argument type '((Int, Int)) throws -> T'.
Generic parameter 'T' could not be inferred.

Answer 1

Array.map 需要一个带有签名 (Element) throws -> T.

的闭包

在 Swift 5.2 中，键路径被允许作为 functions/closures 传入（这里是一个 evolution proposal），但是 只能作为文字 （至少，根据提案，它说“暂时”，所以也许这个限制会被取消）。

为了克服这个问题，您可以在 Sequence 上创建一个接受关键路径的扩展：

extension Sequence {
   func map<T>(_ keyPath: KeyPath<Element, T>) -> [T] {
      return map { [=10=][keyPath: keyPath] }
   }
}

^{（记入：https://www.swiftbysundell.com/articles/the-power-of-key-paths-in-swift/）}

那你就可以随心所欲了：

let keyPath = \(Int, Int).0
arr.map(keyPath)

Answer 2

evolution proposal 展示了如何使用运算符来完成它，但您也可以使用相同的 [] 或 () 语法，无论是否为 partially-applied，因为下标和函数不需要参数。

let oneTo5 = 1...5
let keyPath = \(Int, Int).0
XCTAssert(
  zip(oneTo5, oneTo5).map(keyPath[]).elementsEqual(oneTo5)
)

let keyPath = \Double.isZero
XCTAssertFalse(keyPath[1.0]())

public extension KeyPath {
  /// Convert a `KeyPath` to a partially-applied get accessor.
  subscript() -> (Root) -> Value {
    { [=12=][keyPath: self] }
  }

  /// Convert a `KeyPath` to a get accessor.
  subscript(root: Root) -> () -> Value {
    { root[keyPath: self] }
  }
}

public extension ReferenceWritableKeyPath {
  /// Convert a `KeyPath` to a partially-applied get/set accessor pair.
  subscript() -> (Root) -> Computed<Value> {
    { self[[=12=]] }
  }

  /// Convert a `KeyPath` to a get/set accessor pair.
  subscript(root: Root) -> Computed<Value> {
    .init(
      get: self[root],
      set: { root[keyPath: self] = [=12=] }
    )
  }
}



/// A workaround for limitations of Swift's computed properties.
///
/// Limitations of Swift's computed property accessors:
/// 1. They are not mutable.
/// 2. They cannot be referenced as closures.
@propertyWrapper public struct Computed<Value> {
  public typealias Get = () -> Value
  public typealias Set = (Value) -> Void

  public init(
    get: @escaping Get,
    set: @escaping Set
  ) {
    self.get = get
    self.set = set
  }

  public var get: Get
  public var set: Set

  public var wrappedValue: Value {
    get { get() }
    set { set(newValue) }
  }

  public var projectedValue: Self {
    get { self }
    set { self = newValue }
  }
}

//MARK:- public
public extension Computed {
  init(
    wrappedValue: Value,
    get: @escaping Get = {
      fatalError("`get` must be assigned before accessing `wrappedValue`.")
    },
    set: @escaping Set
  ) {
    self.init(get: get, set: set)
    self.wrappedValue = wrappedValue
  }
}

map(keyPath) 其中 keyPath 是一个变量

map(keyPath) where keyPath is a variable

swift

swift5

keypaths