UICollectionView scrollToItemAtIndexPath 没有作用

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];
}