Rework NativeShell/AppHost

[heyhippari]

This issue is to track and discuss a rewrite of NativeShell/AppHost into a system that makes more sense and is well-documented, well-thought-out, and just generally better made.

Here is a preliminary idea of what it could look like, based on a 5min discussion between @dkanada and me.

Jellyfin
 -> Platform
        -> getPlugins
        -> getFeatures
        -> deviceName
        -> deviceId
 -> Device
        -> fullscreen
        -> openUrl
        -> rotate
 -> Storage
        -> download
        -> isCached
        -> localDirectory

[dkanada]

We would have one major update with backwards compatibility to allow time for clients to update their global uses, and then drop the hacks. That would let us add a compatibility layer just before an update, and immediately revert it after a release since we use release branches.

[Maxr1998]

I agree, this is a good idea. In the device section, there's missing a function that supplies the current media metadata though (e.g. for a notification).

We should also ask ourselves whether the new API methods should be synchronous or asynchronous. The new Android app uses WebView.addJavascriptInterface which registers itself as a window object with a collection of synchronous functions. Supporting async calls/callbacks there is possible, but kinda hacky - currently, I have to register the callbacks in a map on the Javascript side, which I can later call from Java with WebView.loadUrl().

[cromefire]

If it's synchronous it doesn't work with jf desktop at all, best just wrap any call with Promise.resolve() so it's irrelevant if it's sync or async

[Maxr1998]

Yeah, that'll need JavaScript code though. Should that go into jf-web or jf-android?

[cromefire]

So it's quite easy to write a wrapper that web (or Android) could used to call native shell methods an convert anything in to a promise (so catch all errors and pass them to Promise.reject and pass the result to Promise.resolve) it's like 10 lines or so

[cromefire]

I'm also happy to help with this but only after web is ts because otherwise I'm sure there'll be quite a bunch of issues (and it's a pain to work with).