How to connect UIPageControl to UICollectionView (Swift)

Question

I'm currently using a UICollectionView to display 3 images (each image spans the entire cell). I also have a UIPageControl that I placed on top of the UICollectionView. What I want to happen is to have the UIPageControl show the number of images (which in this case is 3), and also which image the user is currently viewing. The effect I am trying to go for is that of the Instagram app.

The way that I am currently using to achieve this effect is by placing the updating of the UIPageControl within the UICollectionView's willDisplay function, like so:

func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    pictureDots.currentPage = indexPath.item
}

This manages to correctly hook up the paging effect between the collection view and page control. However, the problem that I have is that the UIPageControl starts off saying the user is on the third image, even though it is displaying the first image.

Does anyone know why this is happening and how to fix this problem?

Answer 1

Firstly add your UIPageControl into your storyboard with your UICollectionView, then connect them as outlets to your view controller.

@IBOutlet var pageControl: UIPageControl!
@IBOutlet var collectionView: UICollectionView!

Adjust your numberOfItemsInSection method in UICollectionViewDataSource to set the count of the page control to always be equal to the number of cells in the collection view.

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {

    let count = ...

    pageControl.numberOfPages = count
    pageControl.isHidden = !(count > 1)

    return count
}

Lastly, using the UIScrollViewDelegate, we can tell which cell the UICollectionView stops on. If you are not using a UICollectionViewController, you may have to add the delegate protocol.

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {

    pageControl?.currentPage = Int(scrollView.contentOffset.x) / Int(scrollView.frame.width)
}

func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {

    pageControl?.currentPage = Int(scrollView.contentOffset.x) / Int(scrollView.frame.width)
}

This is possible because a UICollectionView is in fact a UIScrollView under the hood.