ios - UICollectionView 仅在 Scroll 上加载图像

Question

我很难在数据解析后加载 UICollectionView 图像。

这是 Apple 的 LazyTable 解决方案，除

CollectionView 图像只有在我滚动时才会出现。其他数据，如标签或其他外观

没有问题的单元格只有图像给我带来了麻烦。

这是我的委托(delegate)中的一个重新加载方法:

    __block ParseOperation *weakParser = parser;

    parser.completionBlock = ^(void) {
        if (weakParser.appRecordList) {
            dispatch_async(dispatch_get_main_queue(), ^{
                CategroyScreenViewController *rootViewController = (CategroyScreenViewController*)[(UINavigationController*)self.window.rootViewController topViewController];

                rootViewController.entries = weakParser.appRecordList;
               [rootViewController.collectionView reloadData];
            });
        }
        self.queue = nil;
    };

    [self.queue addOperation:parser]; 
    self.appListData = nil;
}

这是我的包含 collectionview 的 CategroyScreenViewController:

#define kCustomRowCount     7

@interface CategroyScreemViewController () <UIScrollViewDelegate>
@property (nonatomic, strong) NSMutableDictionary *imageDownloadsInProgress;
@property (nonatomic) NSMutableArray* numbers;
@end
int num = 0;
@implementation CategroyScreemViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
     NSLog(@"view did load");
    [super viewDidLoad];
    self.imageDownloadsInProgress = [NSMutableDictionary dictionary];
    [self.collectionView reloadData];


}

- (CGSize) blockSizeForItemAtIndexPath:(NSIndexPath *)indexPath {


     NSLog(@"Asking for index paths of non-existant cells!! %d", indexPath.row);

    if (indexPath.row == 0) return CGSizeMake(2, 2);    
    return CGSizeMake(1, 1);
}
- (void) viewDidAppear:(BOOL)animated {
     NSLog(@"viewDidAppear load");
    [self.collectionView reloadData];
}
- (UIEdgeInsets)insetsForItemAtIndexPath:(NSIndexPath *)indexPath {
      NSLog(@"insetsForItemAtIndexPath load");
    return UIEdgeInsetsMake(2, 2, 2, 2);
}
- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    NSArray *allDownloads = [self.imageDownloadsInProgress allValues];
    [allDownloads makeObjectsPerformSelector:@selector(cancelDownload)];

    [self.imageDownloadsInProgress removeAllObjects];
}
-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    NSLog(@"numberOfSectionsInCollectionView load");
    return 1;
}

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    NSUInteger count = [self.entries count];
      NSLog(@"numberOfItemsInSection load");
    if (count == 0)
    {
        return kCustomRowCount;
    }
    return count;
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CategoryScreenCell *myCell = [collectionView dequeueReusableCellWithReuseIdentifier:@"MyCell" forIndexPath:indexPath];
    UILabel *label = (UILabel *)[myCell.contentView viewWithTag:10];
    NSUInteger nodeCount = [self.entries count];
    if (nodeCount > 0)
    {
        AppRecord *appRecord = [self.entries objectAtIndex:indexPath.row];
        [label setText:appRecord.appName];
        if (!appRecord.appIcon)
        {

            if (self.collectionView.dragging == NO && self.collectionView.decelerating == NO)
            {

                [self startIconDownload:appRecord forIndexPath:indexPath];
            }
            myCell.imageView.image = [UIImage imageNamed:@"Placeholder.png"];
        }
        else
        {

            myCell.imageView.image = appRecord.appIcon;
        }

    }

    return myCell;
}
- (UIColor*) colorForNumber:(NSNumber*)num {
    return [UIColor colorWithHue:((19 * num.intValue) % 255)/255.f saturation:1.f brightness:1.f alpha:1.f];
}
- (void)startIconDownload:(AppRecord *)appRecord forIndexPath:(NSIndexPath *)indexPath
{
    IconDownloader *iconDownloader = [self.imageDownloadsInProgress objectForKey:indexPath];
    if (iconDownloader == nil)
    {
        iconDownloader = [[IconDownloader alloc] init];
        iconDownloader.appRecord = appRecord;
        [iconDownloader setCompletionHandler:^{

            CategoryScreenCell *myCell = [self.collectionView dequeueReusableCellWithReuseIdentifier:@"MyCell" forIndexPath:indexPath];
            myCell.imageView.image = appRecord.appIcon;
            myCell.imageView.layer.masksToBounds = YES;
            [self.imageDownloadsInProgress removeObjectForKey:indexPath];

        }];
        [self.imageDownloadsInProgress setObject:iconDownloader forKey:indexPath];
        [iconDownloader startDownload];
    }
}
- (void)loadImagesForOnscreenRows
{
     NSLog(@"loadImagesForOnscreenRows");
    if ([self.entries count] > 0)
    {
        NSArray *visiblePaths = [self.collectionView indexPathsForVisibleItems];
        for (NSIndexPath *indexPath in visiblePaths)
        {
            AppRecord *appRecord = [self.entries objectAtIndex:indexPath.item];

            if (!appRecord.appIcon)
                // Avoid the app icon download if the app already has an icon
            {
                [self startIconDownload:appRecord forIndexPath:indexPath];
            }
        }
    }
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    NSLog(@"scrollViewDidEndDragging");

    if (!decelerate)
    {
        NSLog(@"decelerate");
        [self loadImagesForOnscreenRows];
    }
}


- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    NSLog(@"scrollViewDidEndDecelerating");
    [self loadImagesForOnscreenRows];
}

@end

在我改变这个之后就可以了:

        CategoryScreenCell *myCell = [self.collectionView dequeueReusableCellWithReuseIdentifier:@"MyCell" forIndexPath:indexPath];

到这个:

    CategoryScreenCell *myCell = [self.collectionView cellForItemAtIndexPath:indexPath];

程序运行良好，但它给了我这个警告:“不兼容的指针类型正在使用类型表达式初始化‘CategoryScreenCell *’'UICollectionViewCell *'

Answer 1

我在这种方法中看到的问题是，在您的 IconDownloader 的完成处理程序中，您总是使一个单元格出列并填充它，即使它不可见并且可能不会显示。我不确定这是多少开销——这取决于您有多少个单元格以及人们可能滚动的速度。

一种替代方法是在完成处理程序中发布通知，所有单元格都将注册该通知。在通知发生时调用的方法中，单元格将检查通知的 userInfo 中的 AppRecord 是否适用于当前单元格，并更新其图像。

另一个问题是您为滚动经过的每个单元格排队图像下载，因此如果用户快速滚动到 Collection View 的底部，他们必须首先等待上面的所有图像加载。您可以通过几种方式解决此问题:

1) 当您将单元格出列时，为单元格提供对 IconDownloader 的引用。然后在单元格的 prepareForReuse 中，如果仍然挂起则取消下载。

2) 如果您不想取消挂起的下载，但想确保可见单元格尽快更新，您可以将下载放入优先级队列，并增加每个新的优先级 IconDownloader 实例。