Add MacOS backend, switch to callback model

[jpochyla]

Hi!

There're a couple of unrelated but intertwined things in this PR, I'm sorry. We can split it if it's a problem.

• Adds MacOS NowPlayable backend.
• Extends the Playback model from bool to enum (added just a Stopped variant for now).
• Makes the metadata fields optional.
• Switches from explicit polling into a callback-based approach. Modifies example to show how to queue events.

I've tried to adjust the Windows code to the best of my knowledge, but I can't really compile or test it :( Is that OK?

Thanks for the library!

[jpochyla]

Btw the PR branch is writable, in case you want to change something, please feel free!

[Sinono3]

Hi, sorry for taking so long. I actually was about to commit some changes to the branch, nothing big really, just got it working on Windows again. This weekend I'll pull your new changes and get it working again (hopefully). I actually had written a few notes about some things, and I'll post them this weekend as well.

One thing I'd like to discuss now though is if it's necessary or useful (at least in your platform) to have every media controls function return a Result. Shouldn't all actions with the media controls be infallible? In Windows I don't see a reason it could fail. If I'm correct, originally the Windows MediaControls I wrote returns Result but honestly at the time I actually didn't put much thought into it. So, the question is, what are possible errors that could occur during a change in the media controls (metadata, playback state, etc).

Now that I think about it, in Linux (MPRIS), there could be a point of failure in the D-Bus if it doesn't respond or maybe if it isn't running at all. So maybe it is necessary, though I don't see use of the Results in the macOS implementation. We'll most probably use them anyway for cross-platform compatibility.

Great job on the simplification! Everything is much neater now. You were right on that, the Ext traits felt off, especially importing them for MediaControls creation functions.

[Sinono3]

I don't see why we shouldn't merge this now, my only concern was that detach wasn't actually enforced, thus the Drop implementation. I have yet to test this last part on Windows.

[jpochyla]

Hi @Sinono3, I'm sorry about the delays as well, I've been vacationing AFK and will be back this weekend. Thanks for the fixing the Windows backend, and sorry that the changeset is so big! The idea behind returning a Result is to keep the same interface between the platforms, yes.

[jpochyla]

Alright I'm back! Are you OK with merging this?

[Sinono3]

Yes! Merging.

[jpochyla]

Thank you! ❤️