UICollectionView scrollToItemAtIndexPath

Question

我正在尝试滚动到集合视图中的特定项目,它似乎在90%的时间内正常发生.我基本上有一个集合视图,其框架我通过自动布局更改,然后我希望我的单元格是所有新框架的大小,所以我设置了一个新的大小.当我在scrollToItemAtIndexPath上放置一个断点时,它似乎在工作时项目大小已经生效,但是它不起作用的时间,单元格仍然具有旧的大小.如何在滚动之前确保itemSize已更改？

[weakSelf.view removeConstraints:@[weakSelf.verticalSpace, weakSelf.collectionViewBottomSpace, weakSelf.bottomLayoutTopCollectionView]];
[weakSelf.view addConstraints:@[weakSelf.collectionViewToTop, weakSelf.imageHeight, weakSelf.youLabelToTop]];
[UIView animateWithDuration:animated ? .5 : 0.0
                                      animations:^{
                                          [weakSelf.view layoutIfNeeded];
                                      }
                                      completion:^(BOOL finished) {
  UICollectionViewFlowLayout * layout = (UICollectionViewFlowLayout *)self.currentUserCollectionView.collectionViewLayout;

  layout.itemSize = weakSelf.currentUserCollectionView.frame.size;

  [weakSelf.currentUserCollectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:[self getSelectedIndex:selectItem] inSection:0]
                                         atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
                                                 animated:NO];
}];

Answer 1

确保集合视图首先布置其子视图以及在滚动之前调整单元格的大小.我建议将滚动移动到一个你确定所有布局都已完成的地方,例如:

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];
    NSIndexPath *indexPath = // compute some index path

    [self.collectionView scrollToItemAtIndexPath:indexPath
                                atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
                                        animated:YES];
}

Answer 2

另一种方法是调用layoutIfNeeded在UICollectionView你打电话之前scrollToItemAtIndexPath.这样,每次布置父视图的子视图时,您都不会执行滚动操作.