UICollectionView scrollToItemAtIndexPath 没有作用_uicollectionview scrolltoitem completion-优快云博客

本文链接：https://blog.youkuaiyun.com/jeffasd/article/details/53389079

本文讨论了在使用UICollectionView时遇到的问题，即无法确保调整itemSize后正确地滚动到指定位置。作者提供了一段代码示例，并提出了几种解决方案，包括在滚动前调用layoutIfNeeded确保布局更新。

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http://stackoverflow.com/questions/19816184/uicollectionview-scrolltoitematindexpath

I'm trying to scroll to a specific item in a collection view and it seems to happening properly about 90 % of the time. I basically have a collection view whose frame I change via auto layout and then I want my cell be the size of the all of the new frame, so I set a new size. When I put a breakpoint on the scrollToItemAtIndexPath, it seems when it works works the item size have taken effect, but the times it doesn't work, the cell still have the old size. How can I make sure the itemSize has changed before scrolling?

[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];
}];

解决方法：前面加上layoutIfNeeded

Another option is to call layoutIfNeeded on the UICollectionView before you call scrollToItemAtIndexPath. That way you won't perform the scroll operation every time the parent view's subviews are laid out.

或者：

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

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