ios - 从 URL 和缓存中一张一张地加载和显示图像

Question

我正在使用 SDWebImage 库在带有页面控件的 ScrollView 中显示图像。

如何一张一张地下载图片并显示出来。假设我有 4 个 URL:url1、url2、url3、url4所以我想先显示 url1 然后...2,3,4..等等...2,3,4 应该在后台加载，直到没有下载占位符图像显示在那里。

下面是我的for循环内部代码，我试过它完美吗？

        NSString* serverImgicon=<url here in a loop>;
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
            NSURL *url=[NSURL URLWithString: serverImgicon];
            //set your image on main thread.
            dispatch_async(dispatch_get_main_queue(),^{
                [img sd_setImageWithURL:url
                       placeholderImage:[UIImage imageNamed:@"placeholderImg"]];
            });
        });

Answer 1

试试下面的代码，它会工作正常

 -(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{

        CollectionViewCell *cell =[collectionView dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];

        myobject *newobject = _myarray[indexPath.item];
        cell.nameLabel.text =newobject.name;

          dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
        dispatch_async(queue, ^{

            [cell.activityIndicatorForImage startAnimating];
            UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:
                                                     [NSURL URLWithString:newobject.image]]];
            dispatch_sync(dispatch_get_main_queue(), ^{
                if(image!= nil){
                    [[cell myimage] setImage:image];
                    [cell.activityIndicatorForImage setHidden:NO];

                }else{
                    UIImage * image = [UIImage imageNamed:@"PLACEHOLDER"];
                    [[cell myimage] setImage:image];
                    [cell.activityIndicatorForImage setHidden:YES];
                    [cell.activityIndicatorForImage stopAnimating];

                }


                [cell setNeedsLayout];
            });
        });

        return cell;
    }