BeatModel.java

package MVCPattern.DJExample;

import java.util.ArrayList;
import javax.sound.midi.InvalidMidiDataException;
import javax.sound.midi.MetaEventListener;
import javax.sound.midi.MetaMessage;
import javax.sound.midi.MidiEvent;
import javax.sound.midi.MidiSystem;
import javax.sound.midi.MidiUnavailableException;
import javax.sound.midi.Sequence;
import javax.sound.midi.Sequencer;
import javax.sound.midi.ShortMessage;
import javax.sound.midi.Track;

public class BeatModel implements BeatModelInterface, MetaEventListener {
    Sequencer sequencer;
    ArrayList<BeatObserver> beatObservers = new ArrayList<>();
    ArrayList<BPMObserver> bpmObservers = new ArrayList<>();
    int bpm = 90;
    Sequence sequence;
    Track track;

    public void initialize() {
        setUpMidi();
        buildTrackAndStart();
    }

    public void on() {
        sequencer.start();
        setBPM(90);
    }

    public void off() {
        setBPM(0);
        sequencer.stop();
    }

    public void setBPM(int bpm) {
        this.bpm = bpm;
        sequencer.setTempoInBPM(getBPM());
        notifyBPMObservers();
    }

    public int getBPM() {
        return bpm;
    }

    void beatEvent() {
        notifyBeatObservers();
    }

    @Override
    public void registerObserver(BeatObserver o) {
        beatObservers.add(o);
    }

    @Override
    public void removeObserver(BeatObserver o) {
        beatObservers.remove(o);
    }

    @Override
    public void registerObserver(BPMObserver o) {
        bpmObservers.add(o);
    }

    @Override
    public void removeObserver(BPMObserver o) {
        bpmObservers.remove(o);
    }

    // Called by the MIDI sequencer on each meta event; type 47 = end of track (one beat)
    @Override
    public void meta(MetaMessage message) {
        if (message.getType() == 47) {
            beatEvent();
        }
    }

    private void notifyBPMObservers() {
        for (BPMObserver observer : bpmObservers) {
            observer.updateBPM();
        }
    }

    private void notifyBeatObservers() {
        for (BeatObserver observer : beatObservers) {
            observer.updateBeat();
        }
    }

    private void setUpMidi() {
        try {
            // true = connect to the default synthesizer so audio is actually produced
            sequencer = MidiSystem.getSequencer(true);
            sequencer.open();
            sequencer.addMetaEventListener(this);
            sequence = new Sequence(Sequence.PPQ, 4);
        } catch (MidiUnavailableException | InvalidMidiDataException e) {
            e.printStackTrace();
        }
    }

    private void buildTrackAndStart() {
        // Channel 9 is the GM percussion channel; note 35 = bass drum
        int bassDrum = 35;
        try {
            track = sequence.createTrack();

            ShortMessage msg = new ShortMessage();
            msg.setMessage(ShortMessage.NOTE_ON, 9, bassDrum, 100);
            track.add(new MidiEvent(msg, 1));

            msg = new ShortMessage();
            msg.setMessage(ShortMessage.NOTE_OFF, 9, bassDrum, 100);
            track.add(new MidiEvent(msg, 3));

            sequencer.setSequence(sequence);
            // Loop indefinitely; each end-of-track fires meta(47) which triggers beatEvent()
            sequencer.setLoopCount(Sequencer.LOOP_CONTINUOUSLY);
        } catch (InvalidMidiDataException e) {
            e.printStackTrace();
        }
    }
}