iphone - 简单的节拍器

Question

我正在尝试通过每0.25秒播放一次系统声音来编写一个简单的节拍器。我使用GCD在单独的线程上播放点击，但是播放不均匀，点击有时是两次快速拍打，然后是较慢的拍打。我记录了执行循环中的if语句的时间以及它在0.25秒时的正确时间。我希望我不必使用音频队列服务。有什么建议么？

- (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

时间使用NSTimer，声音使用AVFoundation。