lpd8 + hammerspoon.midi -> new buttons for your computer machine https://images.reverb.com/image/upload/s--TanFLw05--/t_card-square/v1571420337/zzyfza1vf8wsxjynymgr.jpg

lpd8.lua

-- A quick hammerspoon to use the buttons on my Akai LPD8
-- which is a pretty fun little drum pad that can be had
-- for $40-50 bucks if you shop around:
-- https://images.reverb.com/image/upload/s--TanFLw05--/t_card-square/v1571420337/zzyfza1vf8wsxjynymgr.jpg


devices = hs.midi.virtualSources()
lpd8 = hs.midi.new(devices[1])

held = {}


-- row,col mapping of the 8 drum pads on the lpd-8
-- I prefer to use program 3 which by default is
-- this scale.
pads = {
   r1c1=67, r1c2=69, r1c3=71, r1c4=72,
   r2c1=60, r2c2=62, r2c3=64, r2c4=65,
}

--[[ TODO: there are also 8 parameter knobs that could be mapped ]]--

_noteToPad = {}
for pad, note in pairs(pads) do
   _noteToPad[note] = pad
end

-- debug utility
function aa(fmt, ...)
   local args = {...}
   hs.alert.show(string.format(fmt, table.unpack(args)))
end

function noteToPad(note)
--aa("%s, %s", note, _noteToPad[note])
   return _noteToPad[note]
end


-- event name -> pad -> {handler, handler, …}
handlers = {
   padPressed={
      r1c1={(function (event)
         hs.alert.show(event.pad)
      end)}
   }
}

function dispatch(evt)
   handlers[evt.event] = handlers[evt.event] or {}
   handlers[evt.event][evt.pad] = handlers[evt.event][evt.pad] or {}

   for target, handler in ipairs(handlers[evt.event][evt.pad]) do
      handler(evt)
   end
end

lpd8:callback(function (object, deviceName, commandType, description, metadata)
   held[deviceName] = held[deviceName] or {}

   local note, ts = metadata.note, metadata.timestamp

   if commandType == 'noteOff' then
      held[deviceName][note] = false
      dispatch({event='padPressed', pad=noteToPad(note), when=ts})

   elseif commandType == 'noteOn' then
      held[deviceName][note] = ts
   end

   for device, notes in pairs(held) do
      local keyset = {}

      for k,v in pairs(notes) do
         if notes[k] then
            keyset[#keyset + 1] = k
         end
      end

      hs.alert.show(string.format("◊%s → %d [%s]", device, #notes, table.concat(keyset, ",")))
   end

end)