在开发中遇到了UITableView列表UITableViewCell装载图片但不知Image的宽高问题。采用第三方框架SDWebImage实现对图片异步下载和缓存。

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];
                    }
                }
            }
        }
    }
}