gistfile1.txt

#import <Foundation/Foundation.h>
#import <libkern/OSAtomic.h>

typedef struct {
    BOOL scheduled;
    OSSpinLock lock;
} PAScheduleOnceToken;

static char PAScheduleOnceTokenKey;

static void PAScheduleOnce(dispatch_queue_t queue, void (^block)())
{
    PAScheduleOnceToken* tokenPointer = (PAScheduleOnceToken*)dispatch_queue_get_specific(queue, &PAScheduleOnceTokenKey);
    if (!tokenPointer) {
        tokenPointer = (PAScheduleOnceToken*)malloc(sizeof(PAScheduleOnceToken));
        *tokenPointer = (PAScheduleOnceToken){.scheduled = NO, .lock = OS_SPINLOCK_INIT};
    }
    
    if (tokenPointer && block) {
        OSSpinLockLock(&tokenPointer->lock);
        BOOL scheduled = tokenPointer->scheduled;
        OSSpinLockUnlock(&tokenPointer->lock);
        
        if (!scheduled) {
            dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
            if (timer) {
                OSSpinLockLock(&tokenPointer->lock);
                tokenPointer->scheduled = YES;
                OSSpinLockUnlock(&tokenPointer->lock);
                
                dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW, 0 * NSEC_PER_SEC), 0 * NSEC_PER_SEC, (1ull * NSEC_PER_SEC) / 10);
                dispatch_source_set_event_handler(timer, ^ {
                    block();
                    dispatch_source_cancel(timer);
                    
                    OSSpinLockLock(&tokenPointer->lock);
                    tokenPointer->scheduled = NO;
                    OSSpinLockUnlock(&tokenPointer->lock);
                });
                dispatch_resume(timer);
            }
        }
    }
}