Question

I’m trying to write a simple metronome, by playing a system sound every 0.25 seconds. I use GCD to play the clicks on a separate thread but the playing is uneven with the clicks coming sometimes as two quick beats followed by a slower beat. I Logged the time when the if statement in the loop is executed and its right on 0.25 seconds. I’m hoping I don’t have to use Audio Queue Services. Any suggestions?

- (IBAction)start:(id)sender 
{
    dispatch_queue_t clickQueue; // the queue to run the metronome clicker
    dispatch_queue_t mainQueue; // I access the main queue to demonstrate how to change UIKit items
    //clickQueue = dispatch_queue_create("clickQueue", NULL);
    clickQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    mainQueue = dispatch_get_main_queue();
    dispatch_async(clickQueue, ^{
        double timeWas = [NSDate timeIntervalSinceReferenceDate];
        //delay by a 1/10 of a second so the first few clicks don't bunch up.
        double timeIs = [NSDate timeIntervalSinceReferenceDate]  - 0.1; 
        // playing starts out as NO because it gets switched at the end of the loop
        // and the PlaySystemSound block isn't off the queue yet. There is probably a
        // better way to do this.
        while (playing) {
            timeIs = [NSDate timeIntervalSinceReferenceDate] ;
            if ((timeIs - timeWas) > (60.0/240)) {
                AudioServicesPlaySystemSound(sound);
                timeWas = timeIs;
                // I want to flast the 200 label between orange and black but I have to access
                // user interface objects from the queue that they are running in, usually the
                // main queue.
                dispatch_async(mainQueue, ^{
                    if (flash)
                        [bpm setTextColor:[UIColor orangeColor]];
                    else
                        [bpm setTextColor:[UIColor blackColor]];
                    flash = !flash;
                });
            }
        }
    });
    playing = !playing;
    if (playing) 
        [startButton setTitle:@"Stop" forState:UIControlStateNormal];
    else
        [startButton setTitle:@"Start" forState:UIControlStateNormal];
}

Answer 1

Use an NSTimer for the time, and AVFoundation for the sounds.