Elixir hot-reloading & MIDI events generation

Lightning talk at ElixirConfEU 2017. Demonstrates how to use Elixir "hot-reloading" feature, together with MIDI events generation.

Video: https://www.youtube.com/watch?v=_VgcUatTilU&feature=youtu.be&t=2m2s
Repo: https://github.com/thbar/demo-elixir-reloading-music

Transcript

1. ELIXIR HOT-RELOADING & MIDI NOTES GENERATION
   https://github.com/thbar/demo-elixir-reloading-music
   

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2. HOW TO IMPLEMENT A VERY SIMPLE ELIXIR
   CODE HOT-RELOADING EXAMPLE?
   

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3. HOW TO MAKE IT INTERESTING?
   

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4. LIVE SOUND EVENTS GENERATION !
   

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5. HOW TO GENERATE ONE NOTE?
   

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6. RENOISE (MUSIC PRODUCTION SYSTEM)
   

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7. PORTMIDI (C LIBRARY)
   

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8. ELIXIR BINDINGS FOR PORTMIDI
   

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9. # Start a process for MIDI event queue
   {:ok, pid} = PortMidi.open(:output, "Renoise MIDI-In")
   note = 48 # C-4
   velocity = 127
   # Send "NOTE ON"
   PortMidi.write(pid, {0x90, note, velocity})
   # Send "NOTE OFF"
   PortMidi.write(pid, {0x80, note})
   

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10. HOW TO BUILD A MUSIC LOOP?
    

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11. ▸ GenServer
    ▸ Process.send_after(xxx)
    

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12. defmodule MidiPlayer do
    use GenServer
    def start_link do
    {:ok, device} = PortMidi.open(:output, "Renoise MIDI-In")
    tick_period = 50
    Process.send_after(:midi, {:tick}, tick_period)
    GenServer.start_link(__MODULE__, %{
    current_tick: -1,
    device: device,
    tick_period: tick_period
    }, name: :midi)
    end
    # SNIP
    end
    

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13. defmodule MidiPlayer do
    def handle_info({:tick}, state) do
    Process.send_after(:midi, {:tick}, state.tick_period)
    current_tick = Map.fetch!(state, :current_tick) + 1
    show_visual_feedback(current_tick)
    play_notes(state.device, current_tick)
    {:noreply, %{state | current_tick: current_tick}}
    end
    end
    

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14. def play_notes(device, current_tick) do
    notes = [0x54, 0x57, 0x5B, 0x60]
    delay = 4
    if rem(current_tick, delay) == 0 do
    index = rem(div(current_tick, delay), Enum.count(notes))
    note = Enum.at(notes, index)
    PortMidi.write(device, {0x90, note, volume})
    Process.send_after(:midi, {:note_off, note}, 50 * 2)
    end
    end
    

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15. I CAN HAZ RELOADING?
    Code.eval_file("music.exs")
    

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16. HOW TO REACT TO FILE CHANGE?
    defmodule Monitor do
    use ExFSWatch,
    dirs: ["music.exs"],
    listener_extra_args: "--latency=0.0"
    def callback(_file_path, _events) do
    Code.eval_file("music.exs")
    end
    end
    Monitor.start
    

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17. AHA MOMENT
    GenServer reloading keeps the state across reloads.
    => We can keep the "current music tick" between reloads.
    

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18. +-----------------+ +------------------------+
    | (reloable) code | + | preserved state (tick) |
    +-----------------+ +------------------------+
    | |
    \ /
    +---------------------+ +----------+ +---------+
    | ex-portmidi process | -> | portmidi | -> | renoise |
    +---------------------+ +----------+ +---------+
    

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19. DEMO
    Youtube Link
    

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