在开发中遇到了UITableView列表 UITableViewCell装载图片但不知Image的宽高 问题。

在解决该问题的时候，首先想到的是异步加载图片 采用第三方框架SDWebImage 实现对图片异步下载和缓存

以下是我采用的方法几个关键地方

1.计算UITableView的高度

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{

NSString \*imgURL  = self.electionPictureArray.count > indexPath.row ? self.electionPictureArray\[indexPath.row\] :nil;  
if (imgURL) {  
    //根据当前Row的ImageUrl作为Key获取图片缓存  
    UIImage \*img = \[\[SDImageCache sharedImageCache\] imageFromDiskCacheForKey: imgURL \];  
    if (!img) {  
        img = \[UIImage resizedImageWithName:@"childshow\_placeholder"\];;

    }  
    CGFloat height = img.size.height \*Main\_Screen\_Width/img.size.width;//Image宽度为屏幕宽度 ，计算宽高比求得对应的高度  
    NSLog(@"----------------return Height:%f",height);  
    return height;

}  
return 0;  

}

2.在UITableViewCell中实现图片的下载，回调下载完成刷新页面代理

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

ElectronicBookCell \*cell = \[ElectronicBookCell cellWithTableView:tableView\];  
cell.imageUrl = self.electionPictureArray.count > indexPath.row ? self.electionPictureArray\[indexPath.row\] :nil;  
cell.selectionStyle = UITableViewCellSelectionStyleNone;  
return cell;

}

在cell中的setImageUrl中进行下载图片

-(void) setImageUrl:(NSString *)imageUrl{
if (imageUrl) {
_imageUrl = imageUrl;
UIImage *cachedImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:imageUrl];
// 没有缓存图片
if (!cachedImage) {
__weak typeof(self) target = self;
// 利用 SDWebImage 框架提供的功能下载图片
[[SDWebImageDownloader sharedDownloader] downloadImageWithURL:[NSURL URLWithString:imageUrl] options:SDWebImageDownloaderUseNSURLCache progress:^(NSInteger receivedSize, NSInteger expectedSize) {

        } completed:^(UIImage \*image, NSData \*data, NSError \*error, BOOL finished) {  
            // 保存图片  
            \[\[SDImageCache sharedImageCache\] storeImage:image forKey:imageUrl toDisk:YES\]; // 保存到磁盘  
            if (imageUrl == target.imageUrl) {  
                 \[target configPreviewImageViewWithImage:image\];  
            }  
            if (\[self.delegate respondsToSelector:@selector(reloadCellAtIndexPathWithUrl:)\]) {  
                \[self.delegate reloadCellAtIndexPathWithUrl:imageUrl\];  
            }  
        }\];  
    }else  
    {  
        \[self configPreviewImageViewWithImage:cachedImage\];  
    }

}  

}

/**
* 加载图片成功后设置image's frame
*/

• (void)configPreviewImageViewWithImage:(UIImage *)image
  {
  _previewWidth = Main_Screen_Width;
  _previewHeight = image.size.height *Main_Screen_Width/image.size.width;
  CGRect rect = _previewImageView.frame;
  rect.size.width = _previewWidth;// image.size.width;
  rect.size.height = _previewHeight;
  _previewImageView.frame = rect;
  _previewImageView.image = image;
  [self resetLayoutByPreviewImageView];
  }

3.在Controller中实现代理方法，

-(void)reloadCellAtIndexPathWithUrl:(NSString *)url{

if (url) {  
    for (int i = 0; i< self.electionPictureArray.count; i++) {  
        //遍历当前数据源中并找到ImageUrl  
        NSString \*imgURL  =   self.electionPictureArray.count >i ? self.electionPictureArray\[i\] :nil;  
        if (\[imgURL isEqualToString:url\]) {  
            //获取当前可见的Cell NSIndexPaths  
            NSArray \*paths  = self.tableView.indexPathsForVisibleRows;  
            //判断回调的NSIndexPath 是否在可见中如果存在则刷新页面  
            NSIndexPath \*pathLoad = \[NSIndexPath indexPathForItem:i inSection:0\];  
            for (NSIndexPath \*path in paths) {  
                if (path && path == pathLoad ) {  
                    \[self.tableView reloadData\];  
                }  
            }  
        }  
    }  
}  

}

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