生成新单元格时清除集合视图单元格（iOS7 - AFNetworking）

Question

我有一个集合视图控制器，其中包含从 JSON 调用获取其内容的单元格。每个单元格都有一个滚动视图（图像幻灯片）。我注意到幻灯片正在加载在旧幻灯片之上（我可以看到旧单元格中的幻灯片出现在新幻灯片加载之前）。或者，如果我滑动到第一个单元格的第三个图像，然后滚动第四个单元格，它会显示我的第三个图像（而不是第一个），但页面控件会显示这是第一张幻灯片。

如何在生成新单元格时“清除”旧单元格（或至少清除滚动视图，或阻止其重复使用）？

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;
}

集合视图控制器

- (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;
}

集合视图单元格

@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);
}

谢谢。

Answer 1

示例项目可以在这里找到：http://www.filedropper.com/collectionviewtest

在您的 ArticleCell 中将此作为公共方法实现：

- (void)cleanForReuse
{
   [[self.scrollView subviews]
               makeObjectsPerformSelector:@selector(removeFromSuperview)];
   self.scrollView.contentSize = CGSizeZero;
}

- (void)prepareForReuse
{
   [super prepareForReuse];
   self.pageImages = nil;
}

然后在重用单元格之前更新您的代码以调用 cleanForReuse：

- (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

您看到这一点是因为单元正在被重复使用（出于性能原因，这是一件好事）。

如果您有旧内容需要在重用时从单元格中删除，您需要在 collectionView:cellForItemAtIndexPath: 中将其删除。

在您的情况下，您需要在添加新的 imageView 之前对添加到 scrollView 的所有 imageView 调用 removeFromSuperview。

你也可以在 UICollectionViewCell 方法 prepareForReuse 中做一些复用准备。但是，出于 UITableViewCells 中的性能原因，Apple 建议“您应该只重置与内容无关的单元格属性，例如 alpha、编辑和选择状态。”。据推测，同样的建议也适用于 UICollectionViewCells。