滚动时UICollectionViewCell消失

Question

嘿伙计有人曾经遇到一个问题，UICollectionViewCell在滚动UICollectionView时消失了。我实现了一个自定义UICollectionViewLayout，一切似乎都运行良好。但是当我滚动（快速）时，有时单元格会消失，这主要发生在我在UICollectionView的顶部或底部边缘滚动时。

我想指出的另一个问题是当UICollectionDataCell被用来呈现图像时UICollectioView的滚动性能，如下所示。如何改进这种滚动性能？

CustomUICollectionLayoutFile

@implementation MellowFeedViewLayout

- (void)prepareLayout {

    _cellAttributes  = [[NSMutableArray alloc] init];
    float firstColX  = 10.0f;
    float secondColX = firstColX + ((MellowFeedViewCellData *)self.data[0]).size.width + _spacingBetweenCols;

    float firstColY  = 10.0f;
    float secondColY = 10.0f;
    for (int i=0; i<[self.data count]; i++) {

        MellowFeedViewCellData *data = self.data[i];
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];

        UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

        if (i % 2  == 1) {
            // First Col
            attributes.frame = CGRectMake(firstColX, firstColY, data.size.width, data.size.height);
            firstColY += data.size.height + 10.0f;
        } else {
            // Second Col
            attributes.frame = CGRectMake(secondColX, secondColY, data.size.width, data.size.height);
            secondColY += data.size.height + 10.0f;
        }

        [_cellAttributes addObject:attributes];
    }

    _maxHeight = MAX(firstColY, secondColY);
}

- (CGSize)collectionViewContentSize {
    return CGSizeMake(UIScreen.mainScreen.bounds.size.width, _maxHeight);
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    return _cellAttributes;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    return _cellAttributes[indexPath.item];
}

CustomUICollectionManagerFile

@implementation MellowFeedViewController

- (void)viewDidLoad
{
    self.view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    MellowFeedViewLayout *layout = [[MellowFeedViewLayout alloc] init];
    layout.spacingBetweenCols = 10.0f;
    layout.data = self.data;

    _collectionView = [[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:layout];

    [_collectionView setShowsHorizontalScrollIndicator:NO];
    [_collectionView setShowsVerticalScrollIndicator:NO];

    [_collectionView setDataSource:self];
    [_collectionView setDelegate:self];

    [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellIdentifier"];
    [_collectionView setBackgroundColor:[UIColor colorWithRed:232.0/255.0f green:232.0/255.0f blue:232.0/255.0f alpha:1.0f]];

    [self.view addSubview:_collectionView];

    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

#pragma mark - CHTCollectionViewDelegateWaterfallLayout

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    MellowFeedViewCellData *data = (MellowFeedViewCellData *)self.data[indexPath.item];
    return CGSizeMake(data.size.width, data.size.height);
}

//- (UIEdgeInsets)collectionView:
//(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section {
//    return _borderMargin;
//}

//- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*) minimumInteritemSpacingForSectionAtIndex:(NSInteger)section {
//    return UIEdgeInsetsMake(0, 0, 0, 0);
//}

#pragma mark - UICollectionViewDataSource

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return [self.data count];
}

// The cell that is returned must be retrieved from a call to -dequeueReusableCellWithReuseIdentifier:forIndexPath:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];
    //TODO::Make my own layout http://www.rahuljiresal.com/2014/03/understanding-chtcollectionviewwaterfalllayout/
    MellowFeedViewCellData *data = (MellowFeedViewCellData *)self.data[indexPath.item];
    [cell setBackgroundView:data.cellView];
    return cell;
}
@end

数据文件

@implementation MellowFeedViewCellData

- (instancetype)initWithImage:(NSString *)feedImage {
    self = [super init];
    if (!self) return nil;

    _feedImage = feedImage;
    int rndIndex = arc4random() % 2;
    if (rndIndex == 0) {
        _size  = CGSizeMake(145.0f, 230.0f);
    } else if(rndIndex == 1) {
        _size  = CGSizeMake(145.0f, 150.0f);
    }

    // Set Cell View
    _cellView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, _size.width, _size.height)];

    // Adds Shadow
    [_cellView.layer setShadowColor:[UIColor blackColor].CGColor];
    [_cellView.layer setShadowOpacity:0.5];
    [_cellView.layer setShadowRadius:1.0];
    [_cellView.layer setShadowOffset:CGSizeMake(1.0, 1.0)];

    // Add rounder corner container
    UIView *cellViewContentContainer = [[UIView alloc] initWithFrame:CGRectMake(0, 0, _size.width, _size.height)];
    [cellViewContentContainer setBackgroundColor:[UIColor whiteColor]];
    cellViewContentContainer.layer.cornerRadius = 5;
    cellViewContentContainer.layer.masksToBounds = YES;
    [_cellView addSubview:cellViewContentContainer];

    // Set bgImage
    UIImageView *bgImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, _size.width, _size.height)];
    bgImageView.contentMode = UIViewContentModeScaleAspectFill;
    [bgImageView setImage:[UIImage imageNamed:_feedImage]];
    [bgImageView setClipsToBounds:YES];
    [cellViewContentContainer addSubview:bgImageView];

    // Set title
    UILabel *title = [[UILabel alloc] initWithFrame:CGRectMake(0.0f, _size.height - 60.0f, _size.width, 60.0f)];
    [title setText:@"Breaking news, very important."];
    [title setNumberOfLines:2];
    [title setTextColor:[UIColor whiteColor]];
    [title setFont:[UIFont systemFontOfSize:13]];
    [title setTextAlignment:NSTextAlignmentCenter];
    [title setBackgroundColor:[UIColor colorWithRed:0.0f/255.0f green:0.0f/255.0f blue:0.0f/255.0f alpha:0.5f]];
    [cellViewContentContainer addSubview:title];
    return self;
}