Use unique port identifiers

[Boddlnagg]

Ports should not be identified by index (for calls to port_name and connect), but rather by some backend-specific identifier that does not change when the order of devices changes.

These identifiers do not need to be persistent across sessions (e.g. to write them to a configuration file), because that would be quite a bit harder and can be added later.

The following identifiers could work for each platform:

Backend	Port Identifier
ALSA	snd_seq_addr_t (numeric client + port IDs)
CoreMIDI	MIDIEndpointRef, or maybe MIDIObjectFindByUniqueID
JACK	Port name (string)
WinMM	Maybe a combination of port name (as returned by GetDevCaps) and interface name (another string, see https://docs.microsoft.com/de-de/windows-hardware/drivers/audio/obtaining-a-device-interface-name). This requires a linear search when calling connect(), but I think that's not a problem.
WinRT	DeviceInformation::Id (string)

The platform-specific representation could be wrapped in a PortId struct.

[Boscop]

Why is it not enough to identify ports by names? :)

What I always do is find the port by name first, before calling connect(), so in my app config I store only the port name...

pub fn open_input(port_name: &str) -> Result<(MidiInputConnection<()>, Receiver<ChanMsg>), Box<Error + Send + Sync>> {
	let mut midi = MidiInput::new("")?;
	midi.ignore(Ignore::None); // All
	let port = (|| {
		for i in 0..midi.port_count() {
			if let Ok(name) = midi.port_name(i) {
				if name == port_name {
					return Some(i);
				}
			}
		}
		return None;
	})().ok_or(format!("midi input port '{}' not found", port_name))?;

	let (tx_midi_to_daw, rx_midi_to_daw) = channel();
	Ok((midi.connect(port, "", move |_stamp, msg, _| {
		if let Some(msg) = LowLvlMsg::parse(msg) {
			tx_midi_to_daw.send(ChanMsg::from(msg)).unwrap();
		}
	}, ()).map_err(|e| e.kind())?, rx_midi_to_daw))
}

pub fn open_output(port_name: &str) -> Result<MidiOutputConnection, Box<Error + Send + Sync>> {
	let midi = MidiOutput::new("")?;
	let port = (|| {
		for i in 0..midi.port_count() {
			if let Ok(name) = midi.port_name(i) {
				if name == port_name {
					return Some(i);
				}
			}
		}
		return None;
	})().ok_or(format!("midi output port '{}' not found", port_name))?;

	Ok(midi.connect(port, "").map_err(|e| e.kind())?)
}

[Boddlnagg]

@Boscop Because multiple devices of the same type may have the same name. 😞 There are better ways to identify ports, as you can see in the table above, but they differ by backend, and therefore need to be wrapped in a cross-platform way. I have a prototype implementation for Windows in https://github.com/Boddlnagg/midir/tree/port-ids, if you want to see how this looks (have a look at the changes in src/common.rs to see the API changes).

For configuration you can store the port name ... there's not really a good way to deal with multiple devices of the same name/type, because their internal device ID will change when they are plugged out and back in again. But that is tracked by #34.

[Boddlnagg]

Status update: The port-ids branch contains an implementation of this change for all backends except CoreMIDI (which I didn't have the time to do yet).

[Boddlnagg]

Implemented in #38

[Boscop]

@Boddlnagg I just updated my midir dep from 0.3 to 0.5. So what is different exactly regarding ports? The connect function still has the same signature.
What should I pass as port_name argument? I've always passed "" and the port number that I got from finding the port number in the list of available ports based on my app's cfg.port_name.
Does connect nowadays use the port_name argument to find the right port to connect to? So if I pass "" it won't work?

Btw, now I'm starting to use multiple midi controllers of the same kind, who all have the same port names, so I need to extend my config to be able to specify which one is meant (e.g. I could add a nth param, as in "choose the nth one with name", would that be recommended?).
How does this PR relate to this use case? :)

[Boddlnagg]

Actually, version 0.5 does not yet contain this change. Don't ask me why, but there hasn't been a release for a long while now. I'll try to release a new version soon™.

You can try to work with a git dependency directly if you want to try it out before that.

[Boddlnagg]

Btw, now I'm starting to use multiple midi controllers of the same kind, who all have the same port names, so I need to extend my config to be able to specify which one is meant (e.g. I could add a nth param, as in "choose the nth one with name", would that be recommended?).
How does this PR relate to this use case? :)

It helps, because you can now implement that logic to "choose thenth one with name". It would help even more if we had #34 ...