iOS简单易用的GCD计时器的实现原理

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRioRITY_DEFAULT, 0);

dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

dispatch_source_set_timer(timer, dispatch_walltime(nil, 0), 10*NSEC_PER_MSEC, 0);

dispatch_source_set_event_handler(timer, ^{
 //do you want....
});

dispatch_resume(timer);

dispatch_suspend(timer);

dispatch_source_cancel(timer);

- (void)appDidEnterBackground{
  [self suspend];
  NSDate *date = [[NSDate alloc] init];
  NSDateFORMatter *format = [[NSDateFormatter alloc] init];
  format.dateFormat = @"yyyy-MM-dd HH:mm:ss:SSS";
  self.appDidEnterBackgroundTime = [date timeIntervalSince1970];
}


- (void)appDidEnterForeground{
  NSDate *date = [[NSDate alloc] init];
  NSDateFormatter *format = [[NSDateFormatter alloc] init];
  format.dateFormat = @"yyyy-MM-dd HH:mm:ss";
  self.appDidEnterForegroundTime = [date timeIntervalSince1970];
  [self reCalculateMinder];
}



- (NSDecimalNumber *)value: (NSTimeInterval)value 
        byOpration: (OMDecimalOprationType)byOpration    
         percision: (NSInteger)percision 
        withValue: (NSTimeInterval)withValue{

  NSDecimalNumber *number = [self numberValueWithString: value];
  NSDecimalNumber *withNumber = [self numberValueWithString: withValue]; 
  NSDecimalNumberHandler *handler = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode: NSRoundPlain scale: percision raiseOnExactness: NO raiseOnOverflow: NO raiseOnUnderflow: NO raiseOnDivideByZero: YES];

switch (byOpration) {
  case OMDecimalOprationTypeAdd:
   return [number decimalNumberByAdding: withNumber withBehavior:handler];
   break;
  case OMDecimalOprationTypeSubtract:
   return [number decimalNumberBySubtracting: withNumber withBehavior: handler];
   break;
  case OMDecimalOprationTypeDivide:
   return [number decimalNumberByDividingBy: withNumber withBehavior: handler];
   break;
 case OMDecimalOprationTypeMultiple:
   return [number decimalNumberByMultiplyingBy: withNumber withBehavior: handler];
   break;
 default:
   break;
   return nil;
}

@property (nonatomic, strong) OMTimer *timer;

self.timer = [[OMTimer alloc] init];
self.timer.timerInterval = 30;
self.timer.precision = 100;
self.timer.isAscend = NO;
self.timer.progressBlock = ^(OMTime *progress) {
   NSLog(@"%@:%@:%@:%@", progress.hour, progress.minute, progress.second, progress.millisecond;
};self.timer.completion = ^{
  NSLog(@"complete done!");
};