iOS 即使使用调度也会出现延迟

Question

我有一个壁纸应用程序，在主视图上有一个分页 ScrollView 。 ScrollView 的每一页显示 9 个图像。当 View 滚动时，我将加载接下来的 10 页图像，并将已加载的前 10 页 uiimages 设置为 nil 以防止出现内存警告。

问题是当 View 滚动并且调用以下 ScrollView 方法时，即使我已将加载新图像的 block 代码放入dispatch_async， View 滚动之前也会有几秒钟的延迟。当我用调度内容注释掉整个代码部分时，没有延迟。

如果有人知道为什么会发生这种情况。请让我知道。

非常感谢大家

- (void)scrollViewDidScroll:(UIScrollView *)scrollV
{
float fractionalPage = scrollView.contentOffset.x / self.view.frame.size.width;

if (curPageNum != lround(fractionalPage)) {

    curPageNum = lround(fractionalPage);        

    //display the page number
    pageNumLabel.text = [NSString stringWithFormat:@"%d",curPageNum+1];
    pageNumLabel.alpha = 1.0;
    [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(dismissPageNum) userInfo:nil repeats:NO];


    //loading more pages with 9 image in each
    lastLoadedPageNum = MIN(lastLoadedPageNum + 10, numPages - 1);

    dispatch_queue_t getImages = dispatch_queue_create("Getting Images", nil);
    dispatch_async(getImages, ^{

        for (int i = curPageNum + 4 ; i <= lastLoadedPageNum ; i++) {

            int numPicsPerPage = 9;
            if (picsNames.count%9 && i == numPages-1) {

                numPicsPerPage = picsNames.count%9;
            }

            for (int j = 0 ; j < numPicsPerPage ; j++) {

                UIImage *image = [brain imageWith:[picsNames objectAtIndex:(i*9) + j]];

                dispatch_async(dispatch_get_main_queue(), ^{

                    //loading the image to imageview
                    UIImageView *imageView = (UIImageView *)[scrollView viewWithTag:IMAGE_VIEWS_TAG + (i*9) + j];
                    imageView.image = image;
                });
            }
        }
    });

    //setting old imageview images to nil to release memory
    int oldFirstLoadedPageNum = firtLoadedPageNum;
    firtLoadedPageNum = MAX(curPageNum - 4, 0);
    for (int i = oldFirstLoadedPageNum ; i < firtLoadedPageNum ; i++) {

        int numPicsPerPage = 9;
        if (picsNames.count%9 && i == numPages-1) {

            numPicsPerPage = picsNames.count%9;
        }

        for (int j = 0 ; j < numPicsPerPage ; j++) {

            UIImageView *imageView = (UIImageView *)[scrollView viewWithTag:IMAGE_VIEWS_TAG + (i*9) + j];
            imageView.image = nil;
            [((UIActivityIndicatorView *)[imageView viewWithTag:ACTIVITY_INDICATOR_TAG]) startAnimating];
        }            
    }
}
}

脑法图像:

-(UIImage *)imageWith:(NSString *)imageName
{
NSString *imagePath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0] stringByAppendingPathComponent:imageName];
UIImage *image = [UIImage imageWithContentsOfFile:imagePath];

if (!image && [self connected]) {

    image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@"%@/%@", picsURL, imageName]]]];

    if (image) {

        [UIImagePNGRepresentation(image) writeToFile:imagePath atomically:YES];
    }

}

return image;
}

Answer 1

显然您的代码正在循环，这导致了延迟。我认为由于调度位于 for 循环内部，因此仅在一定的迭代之后才会调用它，因此在这里使用多线程没有真正的好处。