Add Reactor API

[eNV25]

#73

test program:

package main

import (
	"log"
	"time"

	"github.com/fhs/gompd/v2/mpd"
)

func main() {
	r, err := mpd.NewReactor("tcp", "localhost:6600", "", func(c *mpd.Client, intr interface{}, subsystems []string, err error) {
		log.Println(subsystems, intr, err)
		status, err := c.CurrentSong()
		log.Println(status, err)
	})
	if err != nil {
		panic(err)
	}
	defer r.Close()
	t := time.After(5 * time.Second)
	err = r.Interrupt(<-t)
	if err != nil {
		log.Println(err)
	}
	select {}
}

[eNV25]

After trying to implement this and trying to use it, I don't like it. The API is inflexible, and I don't think it's worth the trouble.

I think it would be better to provide safe wrappers to idle and noidle. We could have an idling atomic boolean in Client and use that to disallow all commands other than noidle. Users can then do whatever they want.

In my software, I currently copy gompd using bundle to access the unexported idle and noIdle methods.

https://pkg.go.dev/golang.org/x/tools/cmd/bundle

$ bundle -o mpd_bundle.go github.com/fhs/gompd/v2/mpd