[Swift] start-with的なAsyncSequence

subscribeした時に初期値をすぐに発行するAsyncSequence

public struct AsyncStartWithSequence<Base: AsyncSequence>: AsyncSequence {
  
  public struct AsyncIterator: AsyncIteratorProtocol {
    public typealias Element = Base.Element
    
    private var base: Base.AsyncIterator
    private var first: Base.Element?

    init(_ value: Base.AsyncIterator, startWith: Base.Element) {
      self.base = value
      self.first = startWith
    }
    
    public mutating func next() async throws -> Base.Element? {
      if let first = first {
        self.first = nil
        return first
      }
      return try await base.next()
    }
  }
  
  public typealias Element = Base.Element
  
  let base: Base
  let startWith: Base.Element

  init(_ base: Base, startWith: Base.Element) {
    self.base = base
    self.startWith = startWith
  }
  
  public func makeAsyncIterator() -> AsyncIterator {
    return AsyncIterator(base.makeAsyncIterator(), startWith: startWith)
  }
}

extension AsyncStartWithSequence: Sendable where Base.Element: Sendable, Base: Sendable {}