异步加载图像,单元格中的图像错误

Question

我有一个UICollectionView我正在加载多个图像.从我读过的内容来看,为了将正确的图像与每个单元格匹配,我需要子类化UIImageView并在那里获得图像.因为每次我collectionView reloadData,一些图像重复,它们都是乱序.但我不确定如何做到这一点,并没有找到任何教程.我正在使用Parse作为数据库.

 - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
albumImageCell *cell = (albumImageCell *) [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];

     if (cell == nil) {
         cell = [[albumImageCell alloc]init];
     }

     PFObject *temp = [_dataArray objectAtIndex:indexPath.row];
     PFFile *file = [temp objectForKey:@"imageThumbnail"];

     if (![cell.hasImage isEqualToString:@"YES"]) {
         dispatch_async(imageQueue, ^{
             NSData *data = [file getData];
                 if (data) {
                     UIImage *image = [UIImage imageWithData:data];

                     dispatch_async(dispatch_get_main_queue(), ^(void){
                         cell.imageView.image = image;
                         cell.hasImage = @"YES";
                     });

                 }
         });
     }

     return cell;
}

Answer 1

解决此问题的一种方法是在您返回主队列后再次查询单元的集合视图.这段代码应该有效:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
albumImageCell *cell = (albumImageCell *) [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];

     if (cell == nil) {
         cell = [[albumImageCell alloc]init];
     }

     PFObject *temp = [_dataArray objectAtIndex:indexPath.row];
     PFFile *file = [temp objectForKey:@"imageThumbnail"];

     if (![cell.hasImage isEqualToString:@"YES"]) {
         dispatch_async(imageQueue, ^{
             NSData *data = [file getData];
                 if (data) {
                     UIImage *image = [UIImage imageWithData:data];

                     dispatch_async(dispatch_get_main_queue(), ^(void){
                         // cellAgain will be the actual cell at that index path, if it is visible. 
                         // If it is not visible, cellAgain will be nil.
                         albumImageCell *cellAgain = [collectionView cellForItemAtIndexPath:indexPath];
                         cellAgain.imageView.image = image;
                         cellAgain.hasImage = @"YES";
                     });

                 }
         });
     }

     return cell;
}

Answer 2

我最终做的是子类化UIImaveView，然后将图像文件传递进去cellForRow

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
albumImageCell *cell = (albumImageCell *) [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];

    if (cell == nil) {
        cell = [[albumImageCell alloc]init];
    }

    PFObject *temp = [_dataArray objectAtIndex:indexPath.row];
    PFFile *file = [temp objectForKey:@"imageThumbnail"];

    [cell.imageView setFile:file];

    return cell;
}  

然后在customImageView-

- (void) setFile:(PFFile *)file {

    NSString *requestURL = file.url; // Save copy of url locally (will not change in block)
    [self setUrl:file.url]; // Save copy of url on the instance
    self.image = nil;
    [file getDataInBackgroundWithBlock:^(NSData *data, NSError *error) {
        if (!error) {
            UIImage *image = [UIImage imageWithData:data];
            if ([requestURL isEqualToString:self.url]) {
                [self setImage:image];
                [self setNeedsDisplay];
            }
        } else {
            NSLog(@"Error on fetching file");
        }
    }];
}  

但是每次用户滚动到新单元格时都会获取数据。所以我仍在尝试找出如何将特定图像与单元格匹配，而不是每次都获取数据。