Clear Collection View Cells when new ones are generated (iOS7 - AFNetworking) - ios

Question

I have a Collection View Controller with Cells that get their content from a JSON call. Each Cell has a Scroll View (an image slideshow). I noticed that the slideshows are being loaded on top of the old ones (I could see the slideshow from the old cells appear right before the new slideshow is loaded). Or if I swipe to let's say the third image of my first cell and then scroll the fourth cell, it shows my the third image (instead of the first), but the Page Control will show that that's the first slide.
How can I "clear" the old cells (or at least clear the Scroll View, or stop it from being reused) while the new ones are being generated?
Article.h (NSObject)
#interface Article : NSObject
#property (readonly) NSURL *imageURL;
#property (nonatomic, strong) NSArray *images;
- (instancetype)initWithAttributes:(NSDictionary *)attributes;
+ (void)latestNewsWithBlock:(void (^)(NSArray *news, NSError *error))block;
#end
#pragma mark -
#interface ArticleImage : NSObject
#property (readonly, nonatomic, strong) NSURL *URL;
- (instancetype)initWithAttributes:(NSDictionary *)attributes;
#end
Article.m (NSObject)
#interface Article ()
#property (readwrite, nonatomic, strong) NSURL *URL;
#end
#implementation Article
- (instancetype)initWithAttributes:(NSDictionary *)attributes {
self = [super init];
if (!self) {
return nil;
}
self.URL = [NSURL URLWithString:attributes[#"url"]];
NSMutableArray *mutableImages = [NSMutableArray array];
for (NSDictionary *imageAttributes in attributes[#"photos"]) {
NSDictionary *imageFileAttributes = [imageAttributes valueForKeyPath:#"photo_file.photo_file"];
ArticleImage *image = [[ArticleImage alloc] initWithAttributes:imageFileAttributes];
[mutableImages addObject:image];
}
self.images = mutableImages;
return self;
}
- (NSURL *)imageURL {
return [[self.images firstObject] URL];
}
+ (void)latestNewsWithBlock:(void (^)(NSArray *news, NSError *error))block {
[[DeadstockAPIManager sharedManager] GET:#"articles" parameters:nil success:^(AFHTTPRequestOperation *operation, id JSON) {
NSMutableArray *mutableNews = [NSMutableArray array];
for (NSDictionary *attributes in JSON[#"articles"]) {
Article *news = [[Article alloc] initWithAttributes:attributes];
[mutableNews addObject:news];
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
}
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
if (block) {
block([NSArray arrayWithArray:mutableNews], nil);
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
if (block) {
block(nil, error);
}
}];
}
#end
#pragma mark -
#interface ArticleImage ()
#property (readwrite, nonatomic, strong) NSURL *URL;
#property (readwrite, nonatomic, strong) NSURL *thumbnailURL;
#end
#implementation ArticleImage
- (instancetype)initWithAttributes:(NSDictionary *)attributes {
self = [super init];
if (!self) {
return nil;
}
self.URL = [NSURL URLWithString:[attributes valueForKeyPath:#"thumb.url"]];
return self;
}
Collection View Controller
- (void)setLatestNews:(NSArray *)latestNews {
_latestNews = latestNews;
[self.collectionView reloadData];
}
- (void)loadData:(id)sender {
[Article latestNewsWithBlock:^(NSArray *news, NSError *error) {
self.latestNews = news;
}];
}
#pragma mark - UIViewController
- (void)viewDidLoad {
[super viewDidLoad];
[self.collectionView registerClass:[ArticleCell class] forCellWithReuseIdentifier:#"ArticleCell"];
[self loadData:nil];
}
#pragma mark - UICollectionViewDataSource
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *identifier = #"articleCell";
ArticleCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];
cell.article = [self.latestNews objectAtIndex:indexPath.row];
return cell;
}
Collection View Cell
#interface ArticleCell ()
#property (readwrite, nonatomic, strong) NSArray *pageImages;
#end
#implementation ArticleCell
- (void)setArticle:(Article *)article {
_article = article;
self.pageImages = [self.article.images valueForKeyPath:#"URL"];
NSUInteger pageCount = [self.pageImages count];
self.pageControl.currentPage = 0;
self.pageControl.numberOfPages = pageCount;
self.pageControl.hidden = (pageCount == 1);
for (NSInteger page = 0; page < pageCount; page++) {
CGRect frame = self.scrollView.bounds;
frame.origin.x = frame.size.width * page;
frame.origin.y = 0.0f;
UIImageView *imageView = [[UIImageView alloc] init];
imageView.contentMode = UIViewContentModeScaleAspectFit;
imageView.frame = frame;
[imageView setImageWithURL:self.pageImages[page]];
[self.scrollView addSubview:imageView];
}
CGSize pagesScrollViewSize = self.scrollView.frame.size;
self.scrollView.contentSize = CGSizeMake(pagesScrollViewSize.width * self.pageImages.count, pagesScrollViewSize.height);
}
Thanks.

Answer 1

Example project can be found here: http://www.filedropper.com/collectionviewtest
Implement this as a public method in your ArticleCell:
- (void)cleanForReuse
{
[[self.scrollView subviews]
makeObjectsPerformSelector:#selector(removeFromSuperview)];
self.scrollView.contentSize = CGSizeZero;
}
- (void)prepareForReuse
{
[super prepareForReuse];
self.pageImages = nil;
}
Then update your code to call cleanForReuse before reusing the cell:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *identifier = #"articleCell";
ArticleCell *cell =
[collectionView dequeueReusableCellWithReuseIdentifier:identifier
forIndexPath:indexPath];
[cell cleanForReuse];
cell.article = [self.latestNews objectAtIndex:indexPath.row];
return cell;
}

Answer 2

You're seeing this because the cells are being reused (which is a good thing for performance reasons).
When you have old content that will need to be removed from a cell when it gets reused, you will need to remove it in collectionView:cellForItemAtIndexPath:.
In your case, you will want to call removeFromSuperview on all of the imageViews that you added to the scrollView before adding the new imageViews.
You can also do some reuse preparation in the UICollectionViewCell method prepareForReuse. However, for performance reasons in UITableViewCells Apple recommends that "you should only reset attributes of the cell that are not related to content, for example, alpha, editing, and selection state.". Presumably, this same recommendation applies to UICollectionViewCells.