我正在开发一个将从网络读取数据的驱动程序。它不知道响应中有多少，除了当它尝试读取并返回 0 字节时，它就完成了。所以我的阻塞 Swift 代码看起来很天真：
func readAllBlocking() -> [Byte] {
var buffer: [Byte] = []
var fullBuffer: [Byte] = []
repeat {
buffer = read() // synchronous, blocking
fullBuffer.append(buffer)
} while buffer.count > 0
return fullBuffer
}
我如何将其重写为将继续运行直到读取整个结果的承诺？在试图将我的大脑包裹起来之后，我仍然被困在这里：
func readAllNonBlocking() -> EventLoopFuture<[Byte]> {
///...?
}
我应该补充一点，我可以重写 read() 而不是返回 [Byte] 返回 EventLoopFuture<[Byte]>
通常，同步编程中的循环被转换为递归，以获得与使用期货的异步编程相同的效果（以及在函数式编程中）。
所以你的函数可能看起来像这样：
func readAllNonBlocking(on eventLoop: EventLoop) -> EventLoopFuture<[Byte]> {
// The accumulated chunks
var accumulatedChunks: [Byte] = []
// The promise that will hold the overall result
let promise = eventLoop.makePromise(of: [Byte].self)
// We turn the loop into recursion:
func loop() {
// First, we call `read` to read in the next chunk and hop
// over to `eventLoop` so we can safely write to `accumulatedChunks`
// without a lock.
read().hop(to: eventLoop).map { nextChunk in
// Next, we just append the chunk to the accumulation
accumulatedChunks.append(contentsOf: nextChunk)
guard nextChunk.count > 0 else {
promise.succeed(accumulatedChunks)
return
}
// and if it wasn't empty, we loop again.
loop()
}.cascadeFailure(to: promise) // if anything goes wrong, we fail the whole thing.
}
loop() // Let's kick everything off.
return promise.futureResult
}
不过我想补充两点：
首先，您在上面实现的是简单地读取所有内容，直到您看到 EOF，如果该软件暴露在互联网上，您绝对应该添加一个最大字节数限制在内存中。
其次，SwiftNIO 是一个事件驱动的系统，所以如果你用 SwiftNIO 读取这些字节，程序实际上看起来会略有不同。如果您对在 SwiftNIO 中简单地累积所有字节直到 EOF 的样子感兴趣，那就是：
struct AccumulateUntilEOF: ByteToMessageDecoder {
typealias InboundOut = ByteBuffer
func decode(context: ChannelHandlerContext, buffer: inout ByteBuffer) throws -> DecodingState {
// `decode` will be called if new data is coming in.
// We simply return `.needMoreData` because always need more data because our message end is EOF.
// ByteToMessageHandler will automatically accumulate everything for us because we tell it that we need more
// data to decode a message.
return .needMoreData
}
func decodeLast(context: ChannelHandlerContext, buffer: inout ByteBuffer, seenEOF: Bool) throws -> DecodingState {
// `decodeLast` will be called if NIO knows that this is the _last_ time a decode function is called. Usually,
// this is because of EOF or an error.
if seenEOF {
// This is what we've been waiting for, `buffer` should contain all bytes, let's fire them through
// the pipeline.
context.fireChannelRead(self.wrapInboundOut(buffer))
} else {
// Odd, something else happened, probably an error or we were just removed from the pipeline. `buffer`
// will now contain what we received so far but maybe we should just drop it on the floor.
}
buffer.clear()
return .needMoreData
}
}
如果你想用 SwiftNIO 制作一个完整的程序，这里有一个例子，它是一个服务器，它接受所有数据，直到它看到 EOF，然后实际上只是写回接收到的字节数:)。当然，在现实世界中，您永远不会保留所有接收到的字节来计算它们（您可以只添加每个单独的部分），但我想它可以作为示例。
import NIO
let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
defer {
try! group.syncShutdownGracefully()
}
struct AccumulateUntilEOF: ByteToMessageDecoder {
typealias InboundOut = ByteBuffer
func decode(context: ChannelHandlerContext, buffer: inout ByteBuffer) throws -> DecodingState {
// `decode` will be called if new data is coming in.
// We simply return `.needMoreData` because always need more data because our message end is EOF.
// ByteToMessageHandler will automatically accumulate everything for us because we tell it that we need more
// data to decode a message.
return .needMoreData
}
func decodeLast(context: ChannelHandlerContext, buffer: inout ByteBuffer, seenEOF: Bool) throws -> DecodingState {
// `decodeLast` will be called if NIO knows that this is the _last_ time a decode function is called. Usually,
// this is because of EOF or an error.
if seenEOF {
// This is what we've been waiting for, `buffer` should contain all bytes, let's fire them through
// the pipeline.
context.fireChannelRead(self.wrapInboundOut(buffer))
} else {
// Odd, something else happened, probably an error or we were just removed from the pipeline. `buffer`
// will now contain what we received so far but maybe we should just drop it on the floor.
}
buffer.clear()
return .needMoreData
}
}
// Just an example "business logic" handler. It will wait for one message
// and just write back the length.
final class SendBackLengthOfFirstInput: ChannelInboundHandler {
typealias InboundIn = ByteBuffer
typealias OutboundOut = ByteBuffer
func channelRead(context: ChannelHandlerContext, data: NIOAny) {
// Once we receive the message, we allocate a response buffer and just write the length of the received
// message in there. We then also close the channel.
let allData = self.unwrapInboundIn(data)
var response = context.channel.allocator.buffer(capacity: 10)
response.writeString("\(allData.readableBytes)\n")
context.writeAndFlush(self.wrapOutboundOut(response)).flatMap {
context.close(mode: .output)
}.whenSuccess {
context.close(promise: nil)
}
}
func errorCaught(context: ChannelHandlerContext, error: Error) {
print("ERROR: \(error)")
context.channel.close(promise: nil)
}
}
let server = try ServerBootstrap(group: group)
// Allow us to reuse the port after the process quits.
.serverChannelOption(ChannelOptions.socket(.init(SOL_SOCKET), .init(SO_REUSEADDR)), value: 1)
// We should allow half-closure because we want to write back after having received an EOF on the input
.childChannelOption(ChannelOptions.allowRemoteHalfClosure, value: true)
// Our program consists of two parts:
.childChannelInitializer { channel in
channel.pipeline.addHandlers([
// 1: The accumulate everything until EOF handler
ByteToMessageHandler(AccumulateUntilEOF(),
// We want 1 MB of buffering max. If you remove this parameter, it'll also
// buffer indefinitely.
maximumBufferSize: 1024 * 1024),
// 2: Our "business logic"
SendBackLengthOfFirstInput()
])
}
// Let's bind port 9999
.bind(to: SocketAddress(ipAddress: "127.0.0.1", port: 9999))
.wait()
// This will never return.
try server.closeFuture.wait()
演示：
$ echo -n "hello world" | nc localhost 9999
11