dispatch_async(dispatch_get_main_queue() 不适用于 iPad，但适用于 iPhone

Question

我正在使用 SDWebImage 库及其在 iPhone 上的工作。但是我不知道为什么iPad中没有调用这个。我试图设置断点，但它也没有达到断点。我把这个方法放在 cellForItemAtIndexPath.

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
     static NSString* cellIdentifier = @"CustomCollectionViewCell";
     CustomCollectionViewCell* cell = (CustomCollectionViewCell*)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
     [cell.downloadButton setTag:indexPath.row];
     [cell.downloadButton addTarget:self action:@selector(deleteButtonPressed:) forControlEvents:UIControlEventTouchUpInside];
     catalogModel = [catalogArray objectAtIndex:indexPath.item];
     cell.cellDescription.text = catalogModel.catalogName;
     SDWebImageManager *manager = [SDWebImageManager sharedManager];
     NSString *urlStr = catalogModel.imageNameThumbnail;
     NSURL *url = [NSURL URLWithString:urlStr];
     dispatch_async(dispatch_get_main_queue(), ^{
             if ([self.catalogCollectionView.indexPathsForVisibleItems containsObject:indexPath])
             {
                  catalogModel = [catalogArray objectAtIndex:indexPath.item];
                  [manager downloadImageWithURL:[NSURL URLWithString:catalogModel.imageNameThumbnail] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {}completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL)
                  {
                       cell.cellImageView.image = image;
                       NSLog(@"%@",catalogModel.catalogName);
                  }];
             };

        });
     return cell;
}

Answer 1

并发图片加载实现有问题。当 -collectionView: cellForItemAtIndexPath: 调用时，设备在主队列上执行代码。假设 -downloadImageWithURL:options:progress:completed: 方法在后台线程上执行图像加载并且 returns 立即我们可以在没有 dispatch_async(dispatch_get_main_queue()... 包装的情况下调用它。否则我们不能保证完成处理程序在主线程上执行，所以代码应该是这样的

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
     static NSString* cellIdentifier = @"CustomCollectionViewCell";

     CustomCollectionViewCell* cell = (CustomCollectionViewCell*)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];

     ...

     SDWebImageManager *manager = [SDWebImageManager sharedManager];

     // methods with completion block usually return instantly
     [manager downloadImageWithURL:aURL options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize){} completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
         // ensure ui is updating on main thread and for visible cell
         dispatch_async(dispatch_get_main_queue(), ^{
             if ([collectionView.indexPathsForVisibleItems containsObject:indexPath]) {
                 cell.cellImageView.image = image;
             }
         });
     }];

     return cell;
}

而iPhone和iPad的不同结果可以用测试设备技术规格的架构差异来解释。

dispatch_async(dispatch_get_main_queue() 不适用于 iPad，但适用于 iPhone

dispatch_async(dispatch_get_main_queue() is not working on iPad, but working on iPhone

iphone

objective-c

ipad

ios

sdwebimage