How to consume ServiceStack's ServerEventsClient OnMessage asynchronously

I'm looking for a way for this part of the code to be asynchronous.

var sse = new ServerEventsClient(apiUrl)
{
  OnMessage = HandleResponse
};

I've looked at "Using C# Async/Await friendly API’s" here https://docs.servicestack.net/csharp-server-events-client#using-c-asyncawait-friendly-apis and I'm not sure if the code provided only works for one future message or every message that will come. I'll be sending a lot of requests in which I expect server responses via SSE so I'd like every response to be handled.

Solution

OnMessage is a synchronous event which fires as soon as it receives a message, it doesn't have an async alternative callback.

If you want to handle the messages asynchronously you can decouple the producer from its consumers which can do using a BlockingCollection where you can the SSE Client capturing messages as soon as they're sent, e.g:

using var bc = new BlockingCollection<ServerEventMessage>();
using var sse = new ServerEventsClient(apiUrl) {
  OnMessage = bc.Add
};
await sse.Connect();
//...

Then you can have multiple sync & async consumers handling messages at their own pace independent of how fast they're being received with Take, TryTake or GetConsumingEnumerable APIs, e.g:

await Task.Run(async () => {
    foreach (var msg in bc.GetConsumingEnumerable())
    {
        // handle msg
        await Task.Delay(100);
    }
});

When you want to stop handling messages (e.g. when disposing ServerEventsClient) you can notify and short-circuit consumers with:

bc.CompleteAdding();