在启用分页的同时使两个UICollectionView的滚动同步

Question

Is it possible to make two collectionviews scroll synchronously, and if yes, how?

I have this class, containing two collectionviews - one with a date, and another, containing a tableview, with elements matching the date from the first collectionview. When i scroll either the date, or the elements, i want them to follow each other.

Here's some code, and what i've tried so far, but without any luck:

I added the ScrollViewDelegate to my class, and tried this delegate-method:

func scrollViewDidScroll(scrollView: UIScrollView) {
    if scrollView.isEqual(dayCollectionView) {
        var offset = contentCollectionView.contentOffset
        offset.x = dayCollectionView.contentOffset.x
        contentCollectionView.setContentOffset(offset, animated: true)
    } else {
        var offset = dayCollectionView.contentOffset
        offset.x = contentCollectionView.contentOffset.x
        dayCollectionView.setContentOffset(offset, animated: true)
    }
}

This method works some how, but the scrollview's are just going mental, and cant seem to stop scrolling, when they're first starts rolling..

I also tried this:

func scrollViewDidEndDecelerating(scrollView: UIScrollView) {

    if scrollView.isEqual(dayCollectionView) {
        // Days
        for cell in self.dayCollectionView.visibleCells() {
            if let indexPath = dayCollectionView.indexPathForCell(cell) {
                contentCollectionView.scrollToItemAtIndexPath(indexPath, atScrollPosition: .None, animated: true)
            }
        }
    } else {
        // Classes
        for cell in self.contentCollectionView.visibleCells() {
            if let indexPath = contentCollectionView.indexPathForCell(cell) {
                dayCollectionView.scrollToItemAtIndexPath(indexPath, atScrollPosition: .None, animated: true)
            }
        }
    }
}

which is the best solution so far - but it's still not exactly what I want - this one scrolls, when the other collectionview is done scrolling.. But I want them to scroll synchronously :-)

Any suggestions?

Answer 1

Indeed, the method scrollViewDidScroll(_:) is the best method for handling such an experience.

What seems to be happening here is that the delegate method scrollViewDidScroll gets called for the first CollectionView and when the contentOffset is applied to the other CollectionView, the delegate method gets called again and scroll is applied to the original collectionView. You need to recognize which of the collectionViews is being scrolled at the time.

For this, you can make use of the isDragging property of UIScrollView.

func scrollViewDidScroll(scrollView: UIScrollView) {
    if scrollView.isEqual(dayCollectionView), scrollView.isDragging {
        var offset = contentCollectionView.contentOffset
        offset.x = dayCollectionView.contentOffset.x
        contentCollectionView.setContentOffset(offset, animated: true)
    } else if scrollView.isEqual(contentCollectionView), scrollView.isDragging {
        var offset = dayCollectionView.contentOffset
        offset.x = contentCollectionView.contentOffset.x
        dayCollectionView.setContentOffset(offset, animated: true)
    }
}