Return upload progress for fileupload

[MaxSchilling]

It would be great to have a callback to retrieve the upload progress of a file, to show it back to users. Firebase offered a subscription method to the upload progress, which was very helpful and user friendly.

Thanks!

[shyndman]

This would definitely be a useful feature.

For a bit of background, we made the decision to use the http library for two reasons:

1. It's the same library used by graphql, meaning smaller binaries, and a smaller debugging surface
2. It has first-class support for multipart HTTP requests, unlike the library in dart:io

But, unfortunately, the http library does not (directly) support upload progress (dart-lang/http#465).

That said, I'm sure this is possible with a some effort. I'm going to do some research and see what comes up.

[shyndman]

OK, I've looked into how this is implemented elsewhere, and replicating their approach is definitely possible.

It's unclear to me whether the approach I'm seeing in other libraries is right exactly, as they appear to be measuring the progress of data being written to the socket's buffer, and not the network.

But I think I see the piece they're missing. Definitely needs a bit of experimentation, but I think this can be done.

[shyndman]

@MaxSchilling, out of interest, can you provide me a rough estimate of the size of file your customers would be uploading?

[MaxSchilling]

We are uploading medium compressed videos, so roughly 3-10MB, maybe up to 20/50? But not more I think

[MaxSchilling]

Thanks for the research and the support! Not super urgent, but I think a nice convenient feature any way

[shyndman]

This has been in the stable release builds for a couple weeks now.