Async command parsing in ImageSharp.Web

[wagich]

I have a use case where I need to set a default focal/anchor point (parameter rxy) when processing an image. This focal point data is stored in a database and it would be beneficial to have command parsing be async to allow for async db access.

Since most calls would complete synchronously, the API is returning ValueTask to avoid allocating a task when it's not needed.

public interface SixLabors.ImageSharp.Web.Commands.IRequestParser
{
    // existing API
    // IDictionary<string, string> ParseRequestCommands(HttpContext context);

    // new API
    ValueTask<IDictionary<string, string>> ParseRequestCommandsAsync(HttpContext context);
}

I've implemented it in https://github.com/wagich/ImageSharp.Web/tree/async-command-parsing
Would you be open to a PR for this?

[JimBobSquarePants]

Hi @wagich that sounds like something you would do via the OnParseCommands event.

Do you have a single focal point reused across all images or one per image?

[wagich]

And maybe to give a bit more context:
I'm using a custom IImageProvider/IImageResolver that does the metadata-loading and validation. It also handles cache-invalidation when the metadata changes. Adding commands like this would lead to problems with caching otherwise, I'd think.

[JimBobSquarePants]

Ok... That all makes sense. So it seems like converting the Action OnParseCommands to Func<Task> OnParseCommandsAsync and do the same for other events might be the best approach?

[wagich]

It looks like it. I'll take some time today and change my code to use the On… just to verify that it works.

[JimBobSquarePants]

Excellent. Thanks!

[wagich]

It seems to be working using the callback. I've opened a draft pull request #100 with the changes.
Let me know if I need to change anything or please feel free to close the PR if you prefer a different approach.