Question

I’m trying to assemble a graphical representation of a MIDI sequence, and I’d like a vertical line to move across the panel horizontally as the sequence plays, reflecting the actual position in the sequence. I know that I should use something like getTickPosition() to provide the location of the vertical line.

But how do I fire off these events so that the vertical line knows to redraw itself? Do I create a special listener that the ticks somehow trigger?

Answer 1

Establish a Swing Timer to check and update the tick position every NN milliseconds.

So, there’s not any kind of built-in timer in the MidiSystem?

Sure there is. But the point is that all GUI updates should happen on the EDT. By invoking them from a Swing Timer, they are. Also, the MIDI timer is for the use of MIDI API/System, let it do what it does undisturbed, and report back the relevant information when checked from the Swing Timer.

Also, given the nature of the UI component, look to a JProgressBar as seen in the upper right of this GUI.

Update

I adapted the source seen on the Java Sound WIKI into an SSCCE of this approach.

import javax.sound.midi.*;
import javax.swing.*;
import java.awt.event.*;
import java.net.URL;

class PlayMidi {

    public static void main(String[] args) throws Exception {
        URL url = new URL("http://pscode.org/media/EverLove.mid");
        Sequence sequence = MidiSystem.getSequence(url);
        final Sequencer sequencer = MidiSystem.getSequencer();
        sequencer.open();
        sequencer.setSequence(sequence);
        Runnable r = new Runnable() {
            public void run() {
                final JProgressBar progress = new JProgressBar(0,(int)sequencer.getMicrosecondLength()); 
                ActionListener updateListener = new ActionListener(){
                    public void actionPerformed(ActionEvent arg0) {
                        progress.setValue((int)sequencer.getMicrosecondPosition());
                    }
                };
                Timer timer = new Timer(40,updateListener); 
                sequencer.start();
                timer.start();
                JOptionPane.showMessageDialog(null, progress);
                sequencer.close();
                timer.stop();
            }
        };
        SwingUtilities.invokeLater(r);
    }
}