Allow describedBy to be an async function

[fcollonval]

Follow-up of #322 (comment)

cc @bollwyvl

[afshin]

Maybe instead of special-casing describedBy to be either a Promise or a CommandFunc, we should have all command functions potentially return a promise?

export type CommandFunc<T> = (args: ReadonlyPartialJSONObject) => T | Promise<T>;

Or maybe that's a bad idea ... commands need to run synchronously, so maybe this special-casing is sound.

What do you think?

[fcollonval]

Good question. I'm a bit reluctant to do that because most of them are for displaying information (in particular to build menus or command buttons). So I would rather not apply it as very slow asynchronous attributes may prevent displaying part of the UI or render in a weird way with popping e.g..

[afshin]

Thank you!