writeDataUnsupported in ChannelInboundHandler (Swift-NIO) - swift

Question

I am trying to make a simple echo UDP server that sends back all incoming datagrams prefixed with a UTF8 string.
In my attempts to reach this goal, I succeeded in sending back the incoming data, but when I try to prefix this data with the string: "You sent: ", I get an error writeDataUnsupported
This is my code:
I made a ChannelInboundHandler called Echo all it does is: For each incoming datagram, it sends the string "You sent: " and then the data of the incoming datagram.
final class Echo: ChannelInboundHandler {
typealias InboundIn = ByteBuffer
typealias OutboundOut = ByteBuffer
var wroteResponse = false
static let response = "You sent: ".data(using: .utf8)!
func channelRead(ctx: ChannelHandlerContext, data: NIOAny) {
if !wroteResponse {
var buffer = ctx.channel.allocator.buffer(capacity: Echo.response.count)
buffer.write(bytes: Echo.response)
ctx.write(self.wrapOutboundOut(buffer), promise: nil)
wroteResponse = true
}
ctx.write(data, promise: nil)
}
func channelReadComplete(ctx: ChannelHandlerContext) {
ctx.flush()
wroteResponse = false
}
}
Then I made a single threaded event loop group and assigned a datagram bootsrap to it. Then I bound the bootstrap to port 4065.
let 🔂 = MultiThreadedEventLoopGroup(numThreads: 1)
let bootstrap = DatagramBootstrap(group: 🔂)
.channelOption(ChannelOptions.socket(SocketOptionLevel(SOL_SOCKET), SO_REUSEADDR), value: 1)
.channelInitializer { $0.pipeline.add(handler: Echo()) }
defer {
try! 🔂.syncShutdownGracefully()
}
try bootstrap
.bind(host: "127.0.0.1", port: 4065)
.wait()
.closeFuture
.wait()
Why do I always get this writeDataUnsupported while trying to send the string: "You sent: "?

Answer 1

For DatagramChannel you need to wrap your ByteBuffer into an AddressEnvelope. Which also means your ChannelInboundHandler should operate on AddressedEnvelope<ByteBuffer>.

Answer 2

To make the ChannelInboundHandler operate on AddressedEnvelope<ByteBuffer>, as Norman Maurer suggests, you can rewrite Echo so it looks more like:
final class Echo: ChannelInboundHandler {
typealias InboundIn = AddressedEnvelope<ByteBuffer>
typealias OutboundOut = AddressedEnvelope<ByteBuffer>
static let response = "You sent: ".data(using: .utf8)!
func channelRead(ctx: ChannelHandlerContext, data: NIOAny) {
var incomingEnvelope = unwrapInboundIn(data)
var buffer = ctx.channel.allocator.buffer(capacity: Echo.response.count + incomingEnvelope.data.readableBytes)
buffer.write(bytes: Echo.response)
buffer.write(buffer: &incomingEnvelope.data)
let envelope = AddressedEnvelope(remoteAddress: incomingEnvelope.remoteAddress, data: buffer)
ctx.write(wrapOutboundOut(envelope), promise: nil)
}
func channelReadComplete(ctx: ChannelHandlerContext) {
ctx.flush()
}
}