Swift 泛型应用示例2数组的安全取值

let arr = ["A", "B", "C"]
print(arr.safe[-1]) // nil
print(arr.safe[100])// nil

// 新建泛型结构体SafeCollection 泛型C遵守Collection协议
public struct SafeCollection<C>: Collection where C: Collection {
  public typealias Index = C.Index
  public typealias Element = Optional<C.Element>

  private var collection: C
  
  // SafeCollection.init(self)
  init(_ collection: C) {
    self.collection = collection
  }

  public subscript(position: Index) -> Element {
    guard self.collection.indices.contains(position) else { return nil }
    return self.collection[position]
  }

  public var startIndex: Index {
    return self.collection.startIndex
  }

  public var endIndex: Index {
    return self.collection.endIndex
  }

  public func index(after i: Index) -> Index {
    return self.collection.index(after: i)
  }
}

public struct SafeMutableCollection<C>: MutableCollection where C: MutableCollection {
  public typealias Index = C.Index
  public typealias Element = Optional<C.Element>

  fileprivate var collection: C

  init(_ collection: C) {
    self.collection = collection
  }

  public subscript(position: Index) -> Element {
    get {
      guard self.collection.indices.contains(position) else { return nil }
      return self.collection[position]
    }
    set {
      guard let value = newValue else { return }
      guard self.collection.indices.contains(position) else { return }
      self.collection[position] = value
    }
  }

  public var startIndex: Index {
    return self.collection.startIndex
  }

  public var endIndex: Index {
    return self.collection.endIndex
  }

  public func index(after i: Index) -> Index {
    return self.collection.index(after: i)
  }
}

// arr.safe -> SafeCollection.init(self)
/* arr.safe[0] -> 调用以下方法
public subscript(position: Index) -> Element {
    guard self.collection.indices.contains(position) else { return nil }
    return self.collection[position]
}
*/
public extension Collection {
  public var safe: SafeCollection<Self> {
    return .init(self)
  }
}

public extension MutableCollection {
  public var safe: SafeMutableCollection<Self> {
    get { return .init(self) }
    set { self = newValue.collection }
  }
}