保持UITableView和UICollectionView滚动位置同步

Question

I have a UIViewController that is able to show its content via a UICollectionView (cv) or via a UITableView (tv).

Both views are added to the view hierarchy via storyboard and I'm just hiding the one that should not be shown. Both the UICollectionView and the UITableView have the same data source. The UICollectionView is showing the content in a two items per row grid, the UITableView obviously in a one item per row grid.

So what I want to happen is if the user decides to switch between the views the newly shown view should scroll to the element that was visible in the other view.

I did some experimenting with scrollToRowAtIndexPath:atScrollPosition:animated: and scrollToItemAtIndexPath:atScrollPosition:animated: with no success.

I understand, that both the UICollectionView and UITableView are able to return their current visible items'/rows' indexPaths, but how can i transform those indexPath to a indexPath of the other view and tell them to scroll to that element?

My current touch event handling of the switch button looks like this:

 - (IBAction)switchView:(id)sender { self.showListView = !self.showListView; UIView *fromView, *toView; NSIndexPath *indexPath; if (self.showListView) { fromView = self.ArticleCollection; toView = self.ArticleList; CGRect visibleRect = (CGRect){.origin = self.ArticleCollection.contentOffset, .size = self.ArticleCollection.bounds.size}; CGPoint visiblePoint = CGPointMake(100, CGRectGetMidY(visibleRect)); indexPath = [self.ArticleCollection indexPathForItemAtPoint:visiblePoint]; } else { fromView = self.ArticleList; toView = self.ArticleCollection; CGRect visibleRect = (CGRect){.origin = self.ArticleList.contentOffset, .size = self.ArticleList.bounds.size}; CGPoint visiblePoint = CGPointMake(100, CGRectGetMidY(visibleRect)); indexPath = [self.ArticleList indexPathForRowAtPoint:visiblePoint]; } NSInteger row = indexPath.row; if (self.showListView) { [self.ArticleList scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:row*2 inSection:0] atScrollPosition:UITableViewScrollPositionTop animated:NO]; } else { [self.ArticleCollection scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:ceil(row/2) inSection:0 atScrollPosition:UICollectionViewScrollPositionCenteredVertically animated:NO]]; } // Here a transitionFromView:ToView:duration:options:completion call follows } 

Thanks in advance for any contribution to my problem.

Answer 1

Appart from a little typo inside the scrollToItemAtIndexPath:atScrollPosition:animated: call:

[self.ArticleCollection scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:row
                                                                        inSection:0]
                                                                 atScrollPosition:UICollectionViewScrollPositionCenteredVertically
                                                                         animated:NO];

I did think indexPath.row would hold different values whether it came from the cv's visible indexPath or the tv's visible indexPath so I calculated the new row by multiplying or dividing through 2. But as it turns out the rows are identical in both cases, so I just needed to use the same value for both cases:

NSInteger row = indexPath.row;

if (self.showListView) {
    [self.ArticleList scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:row inSection:0]
                            atScrollPosition:UITableViewScrollPositionTop
                                    animated:NO];
} else {
    [self.ArticleCollection scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:row
                                                                        inSection:0]
                                                                 atScrollPosition:UICollectionViewScrollPositionCenteredVertically
                                                                         animated:NO];
}

Sorry for the inconvenience.