UITableView加载优化_懒加载图片

2017年11月apple给出了个懒加载图片的demo， 下载地址
实现的效果如下图所示：

主要实现的原理，就是当tableView停止时候渲染网络图片，思路为：
1.当滑动进行中时候，如果之前图片已经存在，则直接加载渲染，否则加载占位图
2.当滑动停止时，如果之前图片已经存在则直接渲染网络图片，否则从网络获取图片。
其中网络图片加载的小优化，如果网络图片和本地需要渲染的大小不一致时，则重绘图片：

if (image.size.width != kAppIconSize || image.size.height != kAppIconSize)
  {
         CGSize itemSize = CGSizeMake(kAppIconSize, kAppIconSize);
         UIGraphicsBeginImageContextWithOptions(itemSize, NO, 0.0f);
         CGRect imageRect = CGRectMake(0.0, 0.0, itemSize.width, itemSize.height);
         [image drawInRect:imageRect];
         self.appRecord.appIcon = UIGraphicsGetImageFromCurrentImageContext();
         UIGraphicsEndImageContext();
 }

对于加载处理部分，在- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath方法中的实现为:

// Only load cached images; defer new downloads until scrolling ends
if (!appRecord.appIcon)
{
         if (self.tableView.dragging == NO && self.tableView.decelerating == NO)
         {
         [self startIconDownload:appRecord forIndexPath:indexPath];
         }
         // if a download is deferred or in progress, return a placeholder image
         cell.imageView.image = [UIImage imageNamed:@"Placeholder.png"];            
}
else
 {
         cell.imageView.image = appRecord.appIcon;
}

当tableView拖拽停止和滑动停止的时候，此时如果图片未下载，则需要下载图片进行渲染， 具体代码在UIScrollViewDelegate的- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate和- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView中实现

// -------------------------------------------------------------------------------
//	scrollViewDidEndDragging:willDecelerate:
//  Load images for all onscreen rows when scrolling is finished.
// -------------------------------------------------------------------------------
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (!decelerate)
	{
        [self loadImagesForOnscreenRows];
    }
}

// -------------------------------------------------------------------------------
//	scrollViewDidEndDecelerating:scrollView
//  When scrolling stops, proceed to load the app icons that are on screen.
// -------------------------------------------------------------------------------
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self loadImagesForOnscreenRows];
}

// -------------------------------------------------------------------------------
//	loadImagesForOnscreenRows
//  This method is used in case the user scrolled into a set of cells that don't
//  have their app icons yet.
// -------------------------------------------------------------------------------
- (void)loadImagesForOnscreenRows
{
    if (self.entries.count > 0)
    {
        NSArray *visiblePaths = [self.tableView indexPathsForVisibleRows];
        for (NSIndexPath *indexPath in visiblePaths)
        {
            AppRecord *appRecord = (self.entries)[indexPath.row];
            
            if (!appRecord.appIcon)
            // **Avoid the app icon download if the app already has an icon**
            {
                [self startIconDownload:appRecord forIndexPath:indexPath];
            }
        }
    }
}

其中在dealloc方法中，终止图片的下载

// -------------------------------------------------------------------------------
//	dealloc
//  If this view controller is going away, we need to cancel all outstanding downloads.
// -------------------------------------------------------------------------------
- (void)dealloc
{
    // terminate all pending download connections
    [self terminateAllDownloads];
}

参考：
iOS性能优化–tableView懒加载图片