Question

正如我在标题中提到的，我的dispatch_async正在发射10次。我使用它来使GUI更具响应性。但是当它发射10次时，需要很长时间才能完成它必须做的所有事情。这是代码：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    [self calculateAllThatShizzle];
});

calculateAllThatShizzle方法只有大约150行计算（包括很多循环）。

我确实尝试了以下内容：

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    [self calculateAllThatShizzle];
});

但是它似乎只在整个生命周期中使用过一次，我需要在每次显示页面时触发它。

所以问题是：如何强制dispatch_async只触发一次？

任何帮助将不胜感激，谢谢

修改

这些dispatch_async和dispatch_once位于checkAndCalculateIfNecessary方法中。这个方法是从PageControl调用的，如：

- (void)scrollViewDidScroll:(UIScrollView *)sender {
  // We don't want a "feedback loop" between the UIPageControl and the scroll delegate in
  // which a scroll event generated from the user hitting the page control triggers updates from
  // the delegate method. We use a boolean to disable the delegate logic when the page control is used.
  if (pageControlUsed) {
    // do nothing - the scroll was initiated from the page control, not the user dragging
    return;
  }
  // Switch the indicator when more than 50% of the previous/next page is visible
  CGFloat pageWidth = scrollView.frame.size.width;
  int page = floor((scrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
  pageControl.currentPage = page;

  DetailViewController *controller = [viewControllers objectAtIndex:page];
  [controller saveTheValues];

  // load the visible page and the page on either side of it (to avoid flashes when the user starts scrolling)
  [self loadScrollViewWithPage:page - 1];
  [self loadScrollViewWithPage:page];
  [self loadScrollViewWithPage:page + 1];

  if ((page + 1) == kNumberOfPages) {
    [controller checkAndCalculateIfNeccessary];
  }
}

Answer 1

我认为这种情况正在发生，因为scrollViewDidScroll:方法在用户滚动内容时被多次调用，尝试在– scrollViewDidEndDecelerating:中进行计算和/或设置一个bool值来控制计算是否应该触发。

dispatch_async被激活了10次

1 个答案: