由于項目需要，在ViewController里面獲取到圖片地址數組之后，需要批量下載并在下載完后，返回圖片數組和相關信息。

//pics - 圖片地址數組
//complete - 完成之后的回調block
+ (void)startDownloadingPics:(NSArray<NSString *> *)pics complete:(void (^)(NSMutableArray<PicRatioModel *> * picRatioArray))complete {
    
    NSMutableArray<PicRatioModel *> *picsRatio = [[NSMutableArray alloc] init];
    // 合并匯總結果
    dispatch_group_t pics_download_queue = dispatch_group_create();
    
    for (NSString *picUrl in pics) {
        
        dispatch_group_async(pics_download_queue, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
            UIImage *image = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:picUrl];
            if (!image) {
                image = [self getImageFromURL:picUrl];
            }
            
            PicRatioModel *pModel = [[PicRatioModel alloc] initWithPicUrlString:picUrl];
            
            if (image) {
                pModel.whRatio = image.size.width/image.size.height;
                [[SDImageCache sharedImageCache] storeImage:image forKey:picUrl];
            }
            
             [picsRatio addObject:pModel];
            
        });
        
    }
    dispatch_group_notify(pics_download_queue, dispatch_get_main_queue(), ^{
        // 匯總結果
        if (complete && picsRatio) {
            complete(picsRatio);
        }
    });
}

寫完之后，貌似看上去很好，其實，這里埋下了一個bug。
經過debug之后，會出現picsRatio 數組為nil ，當調用 [picsRatio addObject:pModel]; 導致奔潰的現象。

后來經過研究發現，數組picsRatio 是在主線程的，然而代碼中
picsRatio的addObject:方法卻不在主線程，這樣就有問題了，于是在addObject方法調用的時候，在主線程操作：

+ (void)startDownloadingPics:(NSArray<NSString *> *)pics complete:(void (^)(NSMutableArray<PicRatioModel *> * picRatioArray))complete {
    
    
    NSMutableArray<PicRatioModel *> *picsRatio = [[NSMutableArray alloc] init];
    // 合并匯總結果
    dispatch_group_t pics_download_queue = dispatch_group_create();
    
    for (NSString *picUrl in pics) {
        
        dispatch_group_async(pics_download_queue, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
            UIImage *image = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:picUrl];
            if (!image) {
                image = [self getImageFromURL:picUrl];
            }
            
            PicRatioModel *pModel = [[PicRatioModel alloc] initWithPicUrlString:picUrl];
            
            if (image) {
                pModel.whRatio = image.size.width/image.size.height;
                [[SDImageCache sharedImageCache] storeImage:image forKey:picUrl];
            }
            //返回到主線程添加數據
            dispatch_async(dispatch_get_main_queue(), ^{
                [picsRatio addObject:pModel];
            });
            
        });
        
    }
    dispatch_group_notify(pics_download_queue, dispatch_get_main_queue(), ^{
        // 匯總結果
        if (complete && picsRatio) {
            complete(picsRatio);
        }
    });
}

不知道我這樣解決和思路是不是正確，如果有錯誤，望大神指出，萬分感謝。