Caching with UIImage and downloaded images

问题

I have a class method which fetches images with a completion block. This fetched UIImage is added to an NSCache with a relevant key. This seems to work as expected, however, in the method which fetches images I am using a UIImage's imageWithData: method, which I have discovered does not cache it's data, only imageNamed: does.

I am understandably getting memory warnings because of this, how do I make sure the images loaded with UIImage's imageWithData: method are removed from memory when not needed anymore?

EDIT

Here is the code for the method which downloads the images.

- (void)imageForFootageSize:(FootageSize)footageSize withCompletionHandler:(void (^)(UIImage *image))completionBlock
{
    if (completionBlock) {
        __block UIImage *image;

        //  Try getting local image from disk.
        //
        __block NSURL *imageURL = [self localURLForFootageSize:footageSize];

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            image = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageURL]];

            dispatch_async(dispatch_get_main_queue(), ^{
                if (image) {
                    completionBlock(image);
                } else {
                    //
                    //  Otherwise try getting remote image.
                    //
                    imageURL = [self remoteURLForFootageSize:footageSize];

                    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                        NSData *imageData = [NSData dataWithContentsOfURL:imageURL];

                        dispatch_async(dispatch_get_main_queue(), ^{
                            image = [UIImage imageWithData:imageData];

                            if (image) {
                                //
                                //  Save remote image to disk
                                //
                                NSURL *photoDirectoryURL = [Footage localURLForDirectory];

                                //      Create the folder(s) where the photos are stored.
                                //
                                [[NSFileManager defaultManager] createDirectoryAtPath:[photoDirectoryURL path] withIntermediateDirectories:YES attributes:nil error:nil];

                                //      Save photo
                                //
                                NSString *localPath = [[self localURLForFootageSize:footageSize] path];
                                [imageData writeToFile:localPath atomically:YES];
                            }

                            completionBlock(image);
                        });
                    });
                }
            });
        });
    }
}

EDIT 2

Methods which use the above class method to fetch and process the UIImage in the completionHandler.

Method inside UICollectionViewCell subclass.

- (void)setPhoto:(Photo *)photo withImage:(UIImage *)image
{
    [self setBackgroundColor:[UIColor blackColor]];
    [self.imageView setBackgroundColor:[UIColor clearColor]];

    if (photo && !image) {
        [photo imageForFootageSize:[Footage footageSizeThatBestFitsRect:self.bounds]
             withCompletionHandler:^(UIImage *image) {
                 if ([self.delegate respondsToSelector:@selector(galleryPhotoCollectionViewCell:didLoadImage:)]) {
                     [self.delegate galleryPhotoCollectionViewCell:self didLoadImage:image];
                 }

                 image = nil;
             }];
    }

    [self.imageView setImage:image];

    BOOL isPhotoAvailable = (BOOL)(image);

    [self.imageView setHidden:!isPhotoAvailable];
    [self.activityIndicatorView setHidden:isPhotoAvailable];
}

Method in UICollectionView data source delegate

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    DIGalleryPhotoCollectionViewCell *photoCell = [collectionView dequeueReusableCellWithReuseIdentifier:photoCellIdentifier forIndexPath:indexPath];

    [photoCell setDelegate:self];

    Footage *footage = [self footageForIndexPath:indexPath];
    Photo *photo = ([footage isKindOfClass:[Photo class]]) ? (Photo *)footage : nil;

    if (photo) {
        //
        //  Photo
        //
        [photoCell setPhoto:photo withImage:[self.galleryCache objectForKey:photo.footageID]];
    }

    return photoCell;
}

Here are the other relevant methods:

- (void)galleryPhotoCollectionViewCell:(DIGalleryPhotoCollectionViewCell *)cell didLoadImage:(UIImage *)image
{
    NSIndexPath *indexPath = [self.galleryCollectionView indexPathForCell:cell];

    Footage *footage = [self footageForIndexPath:indexPath];

    if ([footage isKindOfClass:[Footage class]]) {
        Photo *photo = (Photo *)footage;

        UIImage *cachedImage = [self.galleryCache objectForKey:photo.footageID];

        if (!cachedImage) {
            cachedImage = image;

            [self.galleryCache setObject:image forKey:photo.footageID];
        }

        [cell setPhoto:photo withImage:image];
    }
}

And also my getter method for the NSCache property galleryCache

- (NSCache *)galleryCache
{
    if (!_galleryCache) {
        _galleryCache = [[NSCache alloc] init];
    }

    return _galleryCache;
}

回答1:

Instead of rolling your own image downloading and caching solution you might be better off using SDWebImage. Then you don't have to worry about the downloading, caching or anything. SDWebImage also using disk caching so you don't have to worry about freeing memory.

SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadWithURL:imageURL options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize)
    {
        // progression tracking code
    } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished)
    {
        if (image)
        {
            // do something with image
        }
    }];

I'm not sure but you also might have a retain cycle:

__weak typeof(self) weakSelf = self;
[photo imageForFootageSize:[Footage footageSizeThatBestFitsRect:self.bounds] withCompletionHandler:^(UIImage *image) {
    if ([weakSelf.delegate respondsToSelector:@selector(galleryPhotoCollectionViewCell:didLoadImage:)])
    {
        [weakSelf.delegate galleryPhotoCollectionViewCell:weakSelf didLoadImage:image];
    }
    image = nil;
}];

来源：https://stackoverflow.com/questions/23033475/caching-with-uiimage-and-downloaded-images