Question

I have a program for iPad iOS 5 that reads in a MIDI and then displays the notes on a keyboard in time with the music. It works fine but I am attempting to add a “Repeat Section” function where it would repeat a section from timestamp A to timestamp B over and over again.

I have been able to get the timestamps to work as the bounds of my repeat section but I am having trouble getting the repetion to work properly. When I try and repeat a section, I don’t get my keyboard animation anymore. I have a feeling this problem is going to require hyperthreading but I’m not sure. I’ve outlined what I’d like to do below in pseudocode.

//Start Repeat Method
while (the repeat switch is toggled) {
     Stop music player.

     Set music player to the start point of the repeat.

     while (the current play point is before the end point of the repeat) {
          Check the current play point.
     }
}
//End Repeat Method

So basically, what I want to do is to have a method triggered when a user hits a switch that will be called over and over until they switch it off. In the method, it will stop the player, set it to the start of the repeat, play the repeat until it sees it’s at the end of the repeat, and then start the method over again.

I didn’t think this part was going to be as tricky as it has been. Another problem I have is that when I hook this up to a switch, it doesn’t allow me to switch it off, it just goes forever.

Thanks in advance for the advice.

**EDIT

Here is what I have so far. It allows me to loop my section but my animations are being displayed and I can’t interact with the UI and I have to terminate the program with the stop button in Xcode.

- (IBAction)playRepeat:(id)sender {
     if (repeatToggle.on) {
          MusicPlayerStop(player);
          playerIsPlaying = NO;

          MusicPlayerSetTime(player, sequenceRepeatStartTime);
          moviePlayerViewController.moviePlayer.currentPlaybackTime = rollRepeatStartTime;

          MusicPlayerStart(player);
          [moviePlayerViewController.moviePlayer play];
          playerIsPlaying = YES;

          float difference = rollRepeatEndTime - rollRepeatStartTime;
          [NSThread sleepForTimeInterval:difference];

          MusicPlayerStop(player);
          playerIsPlaying = NO;
          [moviePlayerViewController.moviePlayer pause];

          [self playRepeat:sender];
     }
     else if (!repeatToggle.on) {
          MusicPlayerStop(player);
          playerIsPlaying = NO;
     }
}

Answer 1

Your while loop would exhaust the CPU, because it keeps running and waits for nothing.
Putting this in separated thread might help, but you would need locking mechanism if your player is not thread-safe.

Not that without the notification from the player itself, it would be hard to repeat it at the correct time. You should check if your MIDI player supports any notification or delegate callback that you can use for getting notified when the play reach the point you specified.

Anyway, I would provide the way out that might be adequate for you. You can use timer to check on the player, maybe every 100ms by doing something like this.

-(void) repeatCheck {
    if (the repeat switch is ON) {
        if (the current play point is NOT before the end point of the repeat) {
            Stop music player.
            Set music player to the start point of the repeat.
        }
    }
    [self performSelector:_cmd withObject:nil afterDelay:0.1];
}

-(IBAction) repeatSwitchToggled {
    if (the repeat switch is ON) {
        [self repeatCheck];
    }
    else {
        [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(repeatCheck) object:nil];
    }
}