How to draw rapidly refreshed string labels performantly using GCD.

drawTimecodeUsingGCD.m

-(void)drawTimecodeUsingGCD
{
    if( dispatch_semaphore_wait( _timecodeSemaphore, DISPATCH_TIME_NOW ) != 0 )
    {
        return;
    }
    
    dispatch_async( _timecodeQueue, ^()
    {
       float fontSize = ceilf( self.scrubberSize * 0.425f );
       UIFont *font = [ UIFont fontWithName:@"Courier" size:fontSize ];
       
       CGSize labelSize = [ _avPlayerView.timecode sizeWithFont:font ];

       UIGraphicsBeginImageContextWithOptions( labelSize, NO, [ UIScreen mainScreen ].scale );
       
       [[ UIColor blackColor ] set ];
       [ _avPlayerView.timecode drawAtPoint:CGPointZero withFont:font ];
       
       UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
       
       UIGraphicsEndImageContext();
       
       dispatch_async( dispatch_get_main_queue(), ^()
       {
          self.timecodeImageView.image = image;
          
          self.timecodeImageView.frame = CGRectMake( 0.0f, 0.0f, labelSize.width + self.thumbnailPadding, labelSize.height + self.thumbnailPadding );
          [ self.timecodeImageView centreHorizontally ];
          self.timecodeImageView.hidden = self.timecodeHidden;
          
          dispatch_semaphore_signal( _timecodeSemaphore );
        });
    });
}