Using `dispatch_after` to update UI regularly

dispatch_after_example.m

// clang -framework Foundation dispatch_after_example.m -o dispatch_after_example
// Compiling with gcc requires separating declaration of b from assignment,
// but you can omit the -framework flag for GCC.
/* @author Jeremy W. Sherman
 * @date 2011-06-03
 *
 * Demonstrates use of a recursive block to set up a repeating action
 * using `dispatch_after()`.
 * Assumes 10.6+ or iOS with GCD and blocks (whichever version that was).
 */
#import <Foundation/Foundation.h>

int
main(void)
{
  static int times = 10;

  int64_t repeatAfterSec = 1LL;
  dispatch_queue_t q = dispatch_get_main_queue();
  /* Need __block to have the recursive call work. */
  __block dispatch_block_t b = ^{
    dispatch_time_t when = dispatch_time(
         DISPATCH_TIME_NOW, repeatAfterSec * NSEC_PER_SEC);
    fprintf(stdout, "boom! %d\n", times);
    if (--times >= 0) dispatch_after(when, q, b);
    else exit(0);
  };
  b();
  dispatch_main();
  return 0;
}