Ios UICollectionView单元重用性问题

我正在制作一个简单的gallery iOS程序，对UICollectionView数据源行为有一个问题

在我的代码中，除了当我向上或向下滚动时，每个单元格的图像都会闪烁之外，一切都正常

不可见单元在图像可见之前不会下载图像。 在我的代码中，它是按需下载的。这是最佳实践吗？这就是我得到闪烁的细胞的原因吗

我对细胞的可重用性感到困惑。根据我的观察，当一个细胞变得不可见时，它就失去了自己的形象。我怎样才能防止这种情况？这也是我得到闪烁细胞的原因吗

这是代码

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

        FBPhotoCollectionViewCell *cell = (FBPhotoCollectionViewCell*)[collectionView dequeueReusableCellWithReuseIdentifier:@"PhotoCell" forIndexPath:indexPath];

        __weak FBPhotosVC *weakSelf = self;

        if ([_PhotoLinks count] > 0) {
               dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{


        NSURL* theURL = [NSURL URLWithString:[_PhotoLinks objectAtIndex:(int)indexPath.row]];
        UIImage* tempImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:theURL]];

        if ([weakSelf.theCollectionView.indexPathsForVisibleItems containsObject:indexPath]) {

            if (tempImage)
            {
                dispatch_async(dispatch_get_main_queue(), ^{
                    FBPhotoCollectionViewCell *cell = (FBPhotoCollectionViewCell *)[weakSelf.theCollectionView cellForItemAtIndexPath:indexPath];
                    cell.theImageView.image = tempImage;
                });
            }
            else
            {
                // Download failed.
                NSLog(@"Downloaded image is nil.");
            }
        }
    });
}

cell.backgroundColor = [UIColor grayColor]; <br>
return cell; <br>
}

-（UICollectionViewCell*）collectionView:（UICollectionView*）collectionView cellForItemAtIndexPath:（NSIndexPath*）indexPath{
FBPhotoCollectionViewCell*单元格=（FBPhotoCollectionViewCell*）[collectionView dequeueReusableCellWithReuseIdentifier:@“PhotoCell”forIndexPath:indexPath]；
__弱FBVC*weakSelf=self；
如果（[\u照片链接计数]>0）{
调度异步（调度获取全局队列（调度队列优先级默认为0）^{
NSURL*theURL=[NSURL-URLWithString:[[u PhotoLinks objectAtIndex:（int）indexath.row]]；
UIImage*tempImage=[UIImage imageWithData:[NSData dataWithContentsOfURL:theURL]]；
if（[weakSelf.theCollectionView.indexPathsForVisibleItems包含对象：indexPath]）{
如果（临时图像）
{
dispatch\u async（dispatch\u get\u main\u queue（）^{
FBPhotoCollectionViewCell*单元格=（FBPhotoCollectionViewCell*）[weakSelf.theCollectionView单元格for ItemAtIndexPath:indexPath]；
cell.theImageView.image=tempImage；
});
}
其他的
{
//下载失败。
NSLog（@“下载的图像为零”）；
}
}
});
}
cell.backgroundColor=[UIColor grayColor]；

返回单元格；

}

尝试删除以下内容：

if ([weakSelf.theCollectionView.indexPathsForVisibleItems containsObject:indexPath]) 

您不必检查单元格是否可见。cellForItemAtIndexPath仅在新单元格可见时调用，当该单元格从屏幕上消失时，它将被重用

另外，我不认为您真的需要多线程来加载这样的图像。试着把它拆两下

您可以将已下载的图像存储在数组或字典中，并在每次调用cellForItemAtIndexPath时进行检查

(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    FBPhotoCollectionViewCell cell = (FBPhotoCollectionViewCell)[collectionView dequeueReusableCellWithReuseIdentifier:@"PhotoCell" forIndexPath:indexPath];

    if ([_PhotoLinks count] > 0) 
    { 
        NSString photoName = [_PhotoLinks objectAtIndex:(int)indexPath.row];
        UIImage* tempImage = [self.cacheDictionnary objectForKey:photoName];

        if (!tempImage)
        {
            NSURL* theURL = [NSURL URLWithString:photoName];
            tempImage= [UIImage imageWithData:[NSData dataWithContentsOfURL:theURL]];
        }

        if (tempImage)
        {
            [self.cacheDictionnary setObject:tempImage forKey:photoName];
            cell.theImageView.image = tempImage;
        }
        else
        {
            // Download failed.
            NSLog(@"Downloaded image is nil.");
        }
    }

    cell.backgroundColor = [UIColor grayColor];
    return cell;
}

TrainingGoalsCell单元格=（TrainingGoalsCell）[collectionView dequeueReusableCellWithReuseIdentifier:@“cell”forIndexPath:indexPath]

NSString *thePath=[[NSBundle mainBundle] pathForResource:[arrayVideos objectAtIndex:indexPath.row] ofType:@"mp4"];
NSURL *url = [NSURL fileURLWithPath:thePath];



CGRect attachmentFrame = CGRectMake(2, 2, cell.frame.size.width-4, cell.frame.size.height-4);

if(!isScrolling){
UIView* subView;
// asset is a video
    self.avPlayer = [[AVPlayer alloc] initWithURL:url];
    self.avPlayer.muted = YES;
    self.avPlayerLayer =[AVPlayerLayer playerLayerWithPlayer:self.avPlayer];
    [self.avPlayerLayer setFrame:CGRectMake(0, 0, cell.frame.size.width-4, cell.frame.size.height-4)];

    if(!subView){
    subView = [[UIView alloc]initWithFrame:attachmentFrame];
    [subView.layer addSublayer:self.avPlayerLayer];
    [[cell.contentView subviews] makeObjectsPerformSelector:@selector(removeFromSuperview)];
    [cell.contentView addSubview:subView];
    }

    [self.avPlayer seekToTime:kCMTimeZero];
    self.avPlayerLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
    [self.avPlayer play];

    self.avPlayer.actionAtItemEnd = AVPlayerActionAtItemEndNone;

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(playerItemDidReachEnd:) name:AVPlayerItemDidPlayToEndTimeNotification object:[self.avPlayer currentItem]];
}