Behringer X Touch One - Control your system volume!

volume.go

package main

import (
	"log"
	"math"

	"github.com/itchyny/volume-go"
	"github.com/rakyll/portmidi"
)

func main() {
	k, err := NewKnob(0, 1)
	if err != nil {
		log.Fatal(err)
	}
	defer k.Close()

	k.Watch()
}

type VolumeKnob struct {
	out    *portmidi.Stream
	in     *portmidi.Stream
	Volume int
}

func NewKnob(inputID, outputID portmidi.DeviceID) (*VolumeKnob, error) {
	out, err := portmidi.NewOutputStream(outputID, 1024, 0)
	if err != nil {
		return nil, err
	}

	in, err := portmidi.NewInputStream(inputID, 1024)
	if err != nil {
		return nil, err
	}

	v, err := volume.GetVolume()
	if err != nil {
		return nil, err
	}

	return &VolumeKnob{
		out:    out,
		in:     in,
		Volume: v,
	}, nil
}

func (k VolumeKnob) Set(v int) {
	if v < 0 || v > 100 {
		return
	}

	volume.SetVolume(v)

	k.out.WriteShort(144, 111, 127)
	k.out.WriteShort(231, 0, intToMidi(v))
	k.out.WriteShort(144, 111, 0)
}

func (k VolumeKnob) Close() {
	k.out.Close()
	k.in.Close()
}

func (k VolumeKnob) Watch() {
	ch := k.in.Listen()
	for event := range ch {
		switch event.Status {
		case 144:
			// Data1 111
			// Data2 on/off
			if event.Data2 == 0 {
				k.Set(k.Volume)
			}
			break
		case 231:
			// Data2 midiLevel
			k.Volume = midiToInt(event.Data2)
			break
		}
	}
}

func init() {
	portmidi.Initialize()
}

func intToMidi(v int) int64 {
	return int64(math.Round(float64(v) * 1.27))
}

func midiToInt(v int64) int {
	return int(math.Round(float64(v) / 1.27))
}