As I've continued to play with my async IO stuff, I've found that the programs do get a bit hard to read and maintain. You got handlers here and handlers there, and the flow between them is not easy to disentangle at first.

As I've continued to play with my async IO stuff, I've found that the programs do get a bit hard to read and maintain. You got handlers here and handlers there, and the flow between them is not easy to disentangle at first.

So I got inspired by Boost.Coroutine, which on Windows uses fibers to make "async look sync". The idea is you can have your cake and eat it: you get the benefits of async IO, while being able to write code which looks synchronous.

I just whipped this up in a couple of hours, so naming and implementation could probably do with a bit more polish, but here's the core result:

  inputStream := NewAsyncFileStream(Service, InputFilename, fcOpenExisting, faRead, fsRead);

  serviceContext := NewIOServiceCoroutineContext(Service);
  future := NewIOFuture(serviceContext);

  // queue the async read
  // this will return once the read has completed
  bytesRead := AsyncRead(inputStream, FBuffer, TransferAll(), future);

    if (not future.Result.Success) and (future.Result <> SystemResults.EndOfFile) then
  begin
      future.Result.RaiseException('Reading file');
  end;


So instead of passing a completion handler when performing an async operation, you pass a future. 

This future will switch to a fiber which just runs the IOService, that is waits for IO completion packets and executes the associated handlers.

When a "future completion handler" is executed it switches back to the fiber associated with the future (ie which made the async call) so that the calling code resumes as if the call was synchronous.

Here's the core of the file copy example: http://paste.ie/view/f0f6b7f2

For reference here's core of the old version using completion handlers: http://paste.ie/view/5d173758

Will brush it up and push it to the repo soon, along with TLS support (https, yeah baby yeah!).
http://paste.ie/view/f0f6b7f2