QTMovie Tips

Thanks to the QTKit, playing songs in Cocoa has become rather trivial. But there are a few methods that are less than trivial, and other methods that Apple simply left out.

One common thing a developer may want to know: is the song playing? You would imagine there would be a "isPlaying" method that returns a BOOL, but unfortunately there's not. Instead, use the rate, and write your own method like this:

- (BOOL)isPlaying
{
  return (movie != nil) && ([movie rate] != 0);
}

Moving on to something a little more complicated: playing songs over the internet. It seems easy enough... Just use the initWithURL:error: method and then call the play method. But when you do this the song doesn't play! What's going on here? Well, when you immediately call play, QuickTime essentially says, "Play what? I haven't downloaded any data yet." So the trick is to listen to the load state, and immediately call the play method when QuickTime has stated that it's downloaded enough of the song to start playing it. Like this:

// First you need to register for the proper notifications
// Probably put this in your init method
[[NSNotificationCenter defaultCenter] addObserver:self
    selector:@selector(movieLoadStateDidChange:)
    name:QTMovieLoadStateDidChangeNotification
    object:nil];

// Create our QTMovie
movie = [[QTMovie alloc] initWithURL:myURL error:nil];

// Immediately call play
// This will only work if the song at myURL is cached
[movie play];

- (void)movieLoadStateDidChange:(NSNotification *)notification
{
  // First make sure that this notification is for our movie.
  if([notification object] == movie)
  {
    if([movie rate] == 0)
    {
      if([[movie attributeForKey:QTMovieLoadStateAttribute] longValue] >= kMovieLoadStatePlaythroughOK)
      {
        [movie play];
      }
    }
}

For more information on the load state see the document titled "Monitoring the Load State." It should be right in the Xcode documentation.

What if we want to know how much of the song has been played? This one's not that hard, you just have to know what a QTTime variable is. I find it's helpful to add comments defining what these hard-to-remember types are.

- (float)playProgress
{
  if(movie != nil)
  {
    // typedef struct { long long timeValue; long timeScale; long flags; } QTTime
    QTTime qtCurrentTime = [movie currentTime];
    QTTime qtDuration    = [movie duration];

    long long currentTime = qtCurrentTime.timeValue;
    long long duration = qtDuration.timeValue;

    if(duration > 0)
      return ((float)currentTime) / ((float)duration);
    else
      return 0;
  }
  else {
    return 0;
  }
}

What if we want to know how much of the song has been loaded? You might guess it has something to do with the load state. But then you obviously haven't read the "Monitoring the Load State" document that I told you about, and you'd be wrong. :) (Hey, that's what I thought at first too!) This functionality is availble but it's not in QTKit, it's in QuickTime. We're looking for the GetMaxLoadedTimeInMovie method. Here's a method that returns the load progress as a percentage:

- (float)loadProgress
{
  if(movie != nil)
  {
    // typedef long TimeValue;
    TimeValue loadProgress;
    GetMaxLoadedTimeInMovie([movie quickTimeMovie], &loadProgress);

    // typedef struct { long long timeValue; long timeScale; long flags; } QTTime
    QTTime qtDuration    = [movie duration];
    long long duration = qtDuration.timeValue;

    if(duration > 0)
      return ((float)loadProgress) / ((float)duration);
    else
      return 0;
  }
  else {
    return 0;
  }
}

And speaking of playing songs over the Internet, how would you like to connect to the music libraries of your friends, play their music, and download any songs you want to your computer? Sound cool? Check out Mojo!