grpc-swift/Sources/GRPCInterceptors/OnFinishAsyncSequence.swift

/*
 * Copyright 2024, gRPC Authors All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
struct OnFinishAsyncSequence<Element: Sendable>: AsyncSequence, Sendable {
  private let _makeAsyncIterator: @Sendable () -> AsyncIterator

  init<S: AsyncSequence>(
    wrapping other: S,
    onFinish: @escaping () -> Void
  ) where S.Element == Element {
    self._makeAsyncIterator = {
      AsyncIterator(wrapping: other.makeAsyncIterator(), onFinish: onFinish)
    }
  }

  func makeAsyncIterator() -> AsyncIterator {
    self._makeAsyncIterator()
  }

  struct AsyncIterator: AsyncIteratorProtocol {
    private var iterator: any AsyncIteratorProtocol
    private var onFinish: (() -> Void)?

    fileprivate init<Iterator>(
      wrapping other: Iterator,
      onFinish: @escaping () -> Void
    ) where Iterator: AsyncIteratorProtocol, Iterator.Element == Element {
      self.iterator = other
      self.onFinish = onFinish
    }

    mutating func next() async throws -> Element? {
      let elem = try await self.iterator.next()

      if elem == nil {
        self.onFinish?()
        self.onFinish = nil
      }

      return elem as? Element
    }
  }
}