ios - 如何使用自定义布局修复滚动到 UICollectionView 中的可见单元格？

Question

我正在研究受 UIPickerView 启发的自定义值选择器。它看起来像这样:

如您所见，此选择器的主要特征之一是中央单元格，它应该比其他单元格更宽，以使其在中央框架旁边可见。当您使用平移手势滚动选择器时，它应该动态更改中心值并根据上述逻辑调整单元格。它工作得非常完美:

问题出在点击手势上。当用户通过点击它来选择选择器上的任何项目时，选择器会尝试滚动到该项目。但是由于自定义布局更改了它的偏移量，因此 UIScrollView 滚动到错误的点。它看起来像这样:

当我尝试滚动到屏幕外的单元格时，一切正常 - 该单元格不受布局影响，并且它的坐标是正确的。该问题仅针对可见单元格出现。我完全不知道如何解决这个问题。你可以在这里找到整个项目:Carousel Collection View Test Project

请在下面找到一些重要的代码。

// CarouselLayout.m

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *array = [super layoutAttributesForElementsInRect:rect];
    CGRect visibleRect;
    visibleRect.origin = self.collectionView.contentOffset;
    visibleRect.size = self.collectionView.bounds.size;

    CGRect center = CGRectMake(CGRectGetMidX(visibleRect) - 1.0, 0.0, 2.0, CGRectGetHeight(visibleRect));
    CGFloat coreWidth = CGRectGetWidth(self.centralFrame) / 3.0;

    for (UICollectionViewLayoutAttributes *attributes in array) {
        if (CGRectIntersectsRect(attributes.frame, rect)){
            CGFloat distance = CGRectGetMidX(visibleRect) - attributes.center.x;
            CGFloat offset = 0.0;
            CGRect coreFrame = CGRectMake(attributes.center.x - (coreWidth / 2.0), 0.0, coreWidth, CGRectGetHeight(self.centralFrame));
            if (CGRectIntersectsRect(center, coreFrame)) {
                if (attributes.indexPath.item % 2 == 0) {
                    self.centralItemOffset = (CGRectGetWidth(self.centralFrame) - CGRectGetWidth(attributes.frame) - 4.0) / 2.0;
                    if ([self.collectionView.delegate respondsToSelector:@selector(collectionView:layout:didChangeCentralItem:)]) {
                        [(id <CarouselLayoutDelegate>)self.collectionView.delegate collectionView:self.collectionView layout:self didChangeCentralItem:attributes.indexPath];
                    }
                }
            }
            offset = (distance > 0) ? -self.centralItemOffset : self.centralItemOffset;
            attributes.center = CGPointMake(attributes.center.x + offset, attributes.center.y);
        }
    }
    return array;
}

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {
    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2.0);
    CGRect targetRectHorizontal = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

    NSArray *array = [super layoutAttributesForElementsInRect:targetRectHorizontal];
    for (UICollectionViewLayoutAttributes *attributes in array) {
        if (attributes.indexPath.item % 2 == 1) {
            continue;
        }
        CGFloat itemHorizontalCenter = attributes.center.x;
        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemHorizontalCenter - horizontalCenter;
        }
    }

    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

// ViewController.m

- (void)collectionView:(UICollectionView *)collectionView didHighlightItemAtIndexPath:(NSIndexPath *)indexPath {
    if (indexPath.item % 2 == 1) {
        return;
    }
    NSString *nextValue = [self valueAtIndexPath:indexPath];
    [self scrollToValue:nextValue animated:YES];
    self.currentValue = nextValue;
}

- (void)scrollToValue:(NSString *)value animated:(BOOL)animated {
    NSIndexPath *targetPath = nil;
    NSIndexPath *currentPath = nil;
    for (NSString *item in self.itemsArray) {
        if (!targetPath && [value isEqualToString:item]) {
            targetPath = [NSIndexPath indexPathForItem:([self.itemsArray indexOfObject:item] * 2) inSection:0];
        }
        if (!currentPath && [self.currentValue isEqualToString:item]) {
            currentPath = [NSIndexPath indexPathForItem:([self.itemsArray indexOfObject:item] * 2) inSection:0];
        }
        if (targetPath && currentPath) {
            break;
        }
    }
    if (targetPath && currentPath) {
        [self.itemsCollectionView scrollToItemAtIndexPath:targetPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:animated];
    }
}

如果还不够，请在评论中索取额外的代码。

Answer 1

在 scrollToValue 方法中更改:

if (targetPath && currentPath) {
    [self.itemsCollectionView scrollToItemAtIndexPath:targetPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:animated];
}

进入:

if (targetPath && currentPath) {
    if (targetPath.row < currentPath.row) {
        [self.itemsCollectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:self.itemsArray.count inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:animated];
        [self.itemsCollectionView scrollToItemAtIndexPath:targetPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:animated];
    } else {
        [self.itemsCollectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:animated];
        [self.itemsCollectionView scrollToItemAtIndexPath:targetPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:animated];

    }
}

它会起作用。