dispatch_wait_while.c

void dispatch_wait_while(dispatch_wait_condition_t condition_block, dispatch_time_t interval, dispatch_wait_completion_t completion_block) {
    if ( !condition_block ) return;
	if ( !interval ) return;
	if ( !completion_block ) return;
	
	dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_current_queue());
	dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, interval, 0.25*NSEC_PER_SEC);
	dispatch_source_set_event_handler(timer, ^{
		if (!condition_block()) {
			dispatch_source_cancel(timer);
			completion_block();
		}
	});
	
	dispatch_resume(timer);
}

/*Example*/
dispatch_wait_while(^bool {
    return SomeTypeGetStatus(anObjectOfSomeType) == StatusNotReady;
},
	0.5 *NSEC_PER_SEC,
^{
    OperateOnSomeType(anObjectOfSomeType);
});