dispatch_async（dispatch_get_main_queue（）不適用於iPad，但適用於iPhone

Question

我正在使用SDWebImage庫及其在iPhone上的工作方式。 但是我不知道為什么在iPad中不稱呼它。 我試圖設置斷點，但是它也沒有達到斷點。 我將此方法放在cellForItemAtIndexPath中。

-(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

並發圖像加載實現中存在問題。 調用-collectionView: cellForItemAtIndexPath: ，設備將在主隊列上執行代碼。 假設-downloadImageWithURL:options:progress:completed:方法在后台線程上執行圖像加載並立即返回，我們可以在不使用dispatch_async(dispatch_get_main_queue()...包裝dispatch_async(dispatch_get_main_queue()...情況下調用它，否則我們不能保證完成處理程序在主線程上執行線程，因此代碼應如下所示

-(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;
}

iPhone和iPad上的不同結果可以用測試設備技術規格的體系結構差異來解釋。