ios - 频繁重新加载 collectionView 时崩溃

Question

在一个 iOs 应用程序中，我使用了一个 Collection View ，当用户点击屏幕上不同 collectionView 中的单元格时，该 View 将经常更新。 Collection View 一次接收一行数据并重新加载 Collection View 。

这里是选择中的代码:

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{
    if (collectionView == self.datePickerCollectionView) {


        [[self.PickerCollectionView cellForItemAtIndexPath:[NSIndexPath indexPathForItem:self.crtDelta inSection:0]] setSelected:NO];

        if (self.crtDelta == indexPath.item) {
            // in case the user taps on the day he allready is on
            // don't do anything
            return;
        }

        self.crtDelta = indexPath.item;

            [self.attriburesRectDict removeAllObjects];
            [self.CollectionView.collectionViewLayout invalidateLayout];
            [self.CollectionView reloadData];
            [self.TableView reloadData];
            self.DataDict = [NSMutableDictionary new];
      }

// get data from web manager here
}

如您所见，数据字典已重置。

- (void)didReceiveDataForSection:(NSInteger)section withDataArray:(NSArray*)data andRectsArray:(NSArray*)rects
{
   self.programsDict[@(index)] = data;
    self.attriburesRectDict[@(index)] = rects;
   [self.programsCollectionView reloadData];
}

这里接收到1行数据，添加到相关词典中使用。这是布局代码:

@interface MultipleLineLayout()

@property (nonatomic, assign) NSInteger numRows;

@end


@implementation MultipleLineLayout {
    CGFloat itemHeight;
}

-(id)init {
    if (self = [super init]) {
        self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
        itemHeight = 0.0;
    }
    return self;
}

-(CGSize)collectionViewContentSize {
    NSInteger xSize = self.contentSizeWidth;
    NSInteger ySize = [self.collectionView numberOfSections] * (itemHeight + PROGRAMS_CELL_SPACING);
    return CGSizeMake(xSize, ySize);
}

-(void)prepareLayout
{
    [super prepareLayout];
    [self.collectionView setBounces:NO];
    if (self.collectionView) {
        self.numRows = [self.collectionView numberOfSections];
    }
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path {
    UICollectionViewLayoutAttributes *a = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:path];
    a.frame = [self.dataSource rectForAttributesForRow:path.row inSection:path.section];
    return a;
}

-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray* attributes = [NSMutableArray array];
    if (itemHeight == 0.0) {
        itemHeight = [self.dataSource heightOfProgramCell];
    }

    NSUInteger startRow = floorf(rect.origin.y / itemHeight);
    NSUInteger endRow =   ceilf(CGRectGetMaxY(rect) / (CGFloat)itemHeight);
    if (endRow >= self.numRows) {
        endRow = self.numRows ;
    }
    for (NSUInteger r = startRow; r < endRow; r++)
    {
        NSUInteger noProgs = [self.dataSource numberOfElementsInSection:r];
        for (NSUInteger c = 0; c <noProgs; c++)
        {
            UICollectionViewLayoutAttributes* o = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:c inSection:r]];

            if(o.size.width > 0) {
                [attributes addObject:o];
            }
            else {
                break;
            }
        }
    }

    return attributes;
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
    return NO;
}

@end

问题是，有时如果您非常频繁地切换数据(点击第一个代码片段中显示的 Collection View 代码)(例如每秒点击 2 次)，应用程序会崩溃:

*** Assertion failure in -[UICollectionViewData validateLayoutInRect:], /SourceCache/UIKit/UIKit-2903.23/UICollectionViewData.m:341
2014-08-21 12:14:33.052 du View[365:60b] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'UICollectionView recieved layout attributes for a cell with an index path that does not exist: <NSIndexPath: 0x18c1be10> {length = 2, path = 0 - 0}'
*** First throw call stack:
(0x2dd4cf4b 0x385c36af 0x2dd4ce25 0x2e6f4fe3 0x3051c953 0x3051c063 0x3051b8c7 0x304bfda3 0x30146c6b 0x3014247b 0x3014230d 0x30141d1f 0x30141b2f 0x3013b85d 0x2dd181cd 0x2dd15b71 0x2dd15eb3 0x2dc80c27 0x2dc80a0b 0x32981283 0x30524049 0x14c529 0x38acbab7)
libc++abi.dylib: terminating with uncaught exception of type NSException

我相信这是 didSelectCell 中的重新加载和 did 接收数据中的重新加载之间的某种干扰。我确实在切换时取消了所有数据请求，但不知何故，过了一会儿， Collection View 似乎进入了这种困惑状态并崩溃了。

我怎样才能避免这种崩溃？

Answer 1

我想我明白了。问题出在 numberOfCellsInSection 中。如果没有数据可用，我会返回 0，但显然该方法调用的频率不够高，因此当数据可用时，它仍然期望 0。将其更改为返回 100，并且再也没有看到崩溃。