ios - 缓存图像和 UICollectionView

Question

我是 Objective-C 的新手，所以希望这一切都有意义。我已经从服务器下载图像，并在 collectionView cellForItemAtIndexPath: 方法的 ImageView 中显示它们。我面临的问题是图像似乎没有缓存。看起来每次重复使用一个单元格时，都会重新下载来自服务器的关联图像。

在我的 viewDidLoad 方法中，我正在创建一个 NSMutableDictionary:

imageDictionary = [[NSMutableDictionary alloc]initWithCapacity:50.0];

通过阅读文档和查看类似问题的答案，我认为这个加上下面的代码就足够了。我已经这样做了几天，我知道我遗漏了一些东西或者我没有掌握的概念。

#pragma mark - UICollectionView Data Source
- (NSInteger)collectionView:(UICollectionView *)view numberOfItemsInSection:(NSInteger)section;{
    NSLog(@"Begin retrieving photos");
    return [self.photos count];
}

-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView{
    return 1;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath;{
    CollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"MY_CELL" forIndexPath:indexPath];
    cell.imageView.image = nil;

    if (cell.imageView.image == nil) {
    dispatch_queue_t downloadQueue = dispatch_queue_create("image downloader", NULL);
    dispatch_async(downloadQueue, ^{
        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[[self.photos objectAtIndex:indexPath.row] objectForKey:@"fullimage"]]];
        UIImage *image = [UIImage imageWithData:data];
        [imageDictionary setObject:image forKey:@"Image"];

        dispatch_async(dispatch_get_main_queue(), ^{
            cell.imageView.image = [imageDictionary objectForKey:@"Image"];
            [cell setNeedsDisplay];
        });
    });
    }
    return cell;
}

如有任何帮助，我们将不胜感激。提前致谢。

Answer 1

@yuf - 再次感谢您的指导。 NSCache 似乎让我得到了我想要的结果。这是正常工作的代码。如果有人遇到类似问题，您可以将以下内容与我原来的问题进行比较。

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

    NSString *imageName = [[self.photos objectAtIndex:indexPath.row] objectForKey:@"fullimage"];
    UIImage *image = [imageCache objectForKey:imageName];

    if(image){

        cell.imageView.image = image;
    }

    else{

    cell.imageView.image = nil;

    dispatch_queue_t downloadQueue = dispatch_queue_create("image downloader", NULL);
    dispatch_async(downloadQueue, ^{

        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[[self.photos objectAtIndex:indexPath.row] objectForKey:@"fullimage"]]];
        UIImage *image = [UIImage imageWithData:data];

        dispatch_async(dispatch_get_main_queue(), ^{

            cell.imageView.image = image;

        });

        [imageCache setObject:image forKey:imageName];
    });
    }

    return cell;
}