Returning values from streams

[Malibu]

Hi there, there are many things about streams that are still perplexing to me, for example:

Future<AuthError?> connect(String user, String pw) {

    connectStream.take(1).listen((_) { return null; }, onError: (error) { return error;});

    

    return resultOfStream?

}

What is the accepted way to return null from the function if there is a single result or the error if there is an error?  I know I can return await Completer().future at the end of the function but that seems rather messy.  Is that the only way?

I was trying to figure out how to rework the stream so that it put null into a Future on a good result and put the error into a future on a bad result and return the result of the connect stream directly but couldn't figure that out.

[Malibu]

To clarify, this is basically what I want to accomplish but without using a completer:

Future<AuthError?> connect(String user, String pw) async {

    connectCompletion = Completer()

    connectStream.take(1).listen((_) { connectCompletion.complete(null); }, onError: (error) { connectCompletion.complete(error);});

    return connectCompletion.future

[Bob Nystrom]

I'm not an expert on streams, but I would try:

Future<AuthError?> connect(String user, String pw) async {

  try {
    await connectStream.first;
  } on AuthError catch (error) {
    return error;
  }
 
  // If we get here, we got a value.
  return null;
}

– bob

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[Bob Nystrom]

Stream.first returns a Future and it's specified that any errors produced by the stream will be routed through that returned Future.

– bob 

On Wed, Mar 22, 2023 at 11:27 AM Malibu <mali...@gmail.com> wrote:

Yes, that worked.  Thank you!

So I had tried the same thing but with:

await connectStream.listen( (_) { } );

instead of:

await connectStream.first;

But in my case, the exception did not propagate up to the try/catch block.  It stayed within the stream and reported an uncaught exception.  Do you know why the .first causes different exception handling?

[Malibu]

Yes, that worked.  Thank you!

So I had tried the same thing but with:

await connectStream.listen( (_) { } );

instead of:

await connectStream.first;

But in my case, the exception did not propagate up to the try/catch block.  It stayed within the stream and reported an uncaught exception.  Do you know why the .first causes different exception handling?

On Wednesday, 22 March 2023 at 14:39:20 UTC-3 Bob Nystrom wrote:

[Malibu]

Ach, I read the page on Futures and Error handling five times and it only talks about future functions for error handling.  It would have been nice if they started with try-catch on a future.  But that is on the page about 'Asyncrony support'.  When I read 'errors will be routed through that returned future' I thought that meant you had to use a future function.

So I guess whenever you want an exception from a stream to propagate to the outside you need to end the stream with a future somehow.

Thank you for helping me learn this.