如何将使用 SwiftNIO AsyncHTTPClient 下载的文件写入文件系统？

Question

我想用基于SwiftNIO 的AsyncHTTPClient 库下载一个大文件（数百兆字节）。我希望将此文件流式传输到文件系统，同时消耗尽可能少的 RAM（理想情况下，它不应该将整个文件保留在 RAM 中），并且还能够通过简单的print 输出报告下载进度显示完成百分比。

据我了解，我需要实现一个HTTPClientResponseDelegate，但是我应该使用什么确切的 API 来进行文件写入？文件写入可以被阻塞，同时仍然允许 HTTP 客户端进行吗？在这种情况下，委托代码的外观如何？

Answer 1

事实证明，HTTPClientResponseDelegate 允许在其函数中返回一个未来，以使其能够正确处理背压。将此方法与NonBlockingFileIO 和NIOFileHandle 结合使用，在下载文件时将文件写入磁盘并提供进度报告的委托如下所示：

import AsyncHTTPClient
import NIO
import NIOHTTP1

final class FileDownloadDelegate: HTTPClientResponseDelegate {
  typealias Response = (totalBytes: Int?, receivedBytes: Int)

  private var totalBytes: Int?
  private var receivedBytes = 0

  private let handle: NIOFileHandle
  private let io: NonBlockingFileIO
  private let reportProgress: (_ totalBytes: Int?, _ receivedBytes: Int) -> ()

  private var writeFuture: EventLoopFuture<()>?

  init(
    path: String,
    reportProgress: @escaping (_ totalBytes: Int?, _ receivedBytes: Int) -> ()
  ) throws {
    handle = try NIOFileHandle(path: path, mode: .write, flags: .allowFileCreation())
    let pool = NIOThreadPool(numberOfThreads: 1)
    pool.start()
    io = NonBlockingFileIO(threadPool: pool)

    self.reportProgress = reportProgress
  }

  func didReceiveHead(
    task: HTTPClient.Task<Response>,
    _ head: HTTPResponseHead
  ) -> EventLoopFuture<()> {
    if let totalBytesString = head.headers.first(name: "Content-Length"),
      let totalBytes = Int(totalBytesString) {
      self.totalBytes = totalBytes
    }

    return task.eventLoop.makeSucceededFuture(())
  }

  func didReceiveBodyPart(
    task: HTTPClient.Task<Response>,
    _ buffer: ByteBuffer
  ) -> EventLoopFuture<()> {
    receivedBytes += buffer.readableBytes
    reportProgress(totalBytes, receivedBytes)

    let writeFuture = io.write(fileHandle: handle, buffer: buffer, eventLoop: task.eventLoop)
    self.writeFuture = writeFuture
    return writeFuture
  }

  func didFinishRequest(task: HTTPClient.Task<Response>) throws -> Response {
    writeFuture?.whenComplete { [weak self] _ in
      try? self?.handle.close()
      self?.writeFuture = nil
    }
    return (totalBytes, receivedBytes)
  }
}

使用此代码，下载和写入文件的过程对于下载约 600MB 的文件不会消耗超过 5MB 的 RAM。