UICollectionView: current index path for page control - ios

Question

I'm using a UICollectionView with a flow layout to show a list of cells, I also have a page control to indicate current page, but there seems to be no way to get current index path, I know I can get visible cells:
UICollectionView current visible cell index
however there can be more than one visible cells, even if each of my cells occupies full width of the screen, if I scroll it to have two halves of two cells, then they are both visible, so is there a way to get only one current visible cell's index?
Thanks

Answer 1

You can get the current index by monitoring contentOffset in scrollViewDidScroll delegate
it will be something like this
-(void)scrollViewDidScroll:(UIScrollView *)scrollView
{
NSInteger currentIndex = self.collectionView.contentOffset.x / self.collectionView.frame.size.width;
}

Answer 2

Get page via NSIndexPath from center of view.
Works even your page not equal to width of UICollectionView.
func scrollViewDidScroll(scrollView: UIScrollView) {
let xPoint = scrollView.contentOffset.x + scrollView.frame.width / 2
let yPoint = scrollView.frame.height / 2
let center = CGPoint(x: xPoint, y: yPoint)
if let ip = collectionView.indexPathForItemAtPoint(center) {
self.pageControl.currentPage = ip.row
}
}

Answer 3

Definitely you need catch the visible item when the scroll movement is stopped. Use next code to do it.
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
if let indexPath = myCollectionView.indexPathsForVisibleItems.first {
myPageControl.currentPage = indexPath.row
}
}

Answer 4

Swift 5.1
The easy way and more safety from nil crash
func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
if collectionView == newsCollectionView {
if newsPager.currentPage == indexPath.row {
guard let visible = newsCollectionView.visibleCells.first else { return }
guard let index = newsCollectionView.indexPath(for: visible)?.row else { return }
newsPager.currentPage = index
}
}
}

Answer 5

Place PageControl in your view or set by Code.
Set UIScrollViewDelegate
In Collectionview-> cellForItemAtIndexPath (Method) add the below
code for calculate the Number of pages,
int pages = floor(ImageCollectionView.contentSize.width/ImageCollectionView.frame.size.width);
[pageControl setNumberOfPages:pages];
Add the ScrollView Delegate method,
#pragma mark - UIScrollViewDelegate for UIPageControl
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
CGFloat pageWidth = ImageCollectionView.frame.size.width;
float currentPage = ImageCollectionView.contentOffset.x / pageWidth;
if (0.0f != fmodf(currentPage, 1.0f))
{
pageControl.currentPage = currentPage + 1;
}
else
{
pageControl.currentPage = currentPage;
}
NSLog(#"finishPage: %ld", (long)pageControl.currentPage);
}

Answer 6

I had similar situation where my flow layout was set for UICollectionViewScrollDirectionHorizontal and I was using page control to show the current page.
I achieved it using custom flow layout.
/------------------------
Header file (.h) for custom header
------------------------/
/**
* The customViewFlowLayoutDelegate protocol defines methods that let you coordinate with
*location of cell which is centered.
*/
#protocol CustomViewFlowLayoutDelegate <UICollectionViewDelegateFlowLayout>
/** Informs delegate about location of centered cell in grid.
* Delegate should use this location 'indexPath' information to
* adjust it's conten associated with this cell.
* #param indexpath of cell in collection view which is centered.
*/
- (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout cellCenteredAtIndexPath:(NSIndexPath *)indexPath;
#end
#interface customViewFlowLayout : UICollectionViewFlowLayout
#property (nonatomic, weak) id<CustomViewFlowLayoutDelegate> delegate;
#end
/------------------- Implementation file (.m) for custom header -------------------/
#implementation customViewFlowLayout
- (void)prepareLayout {
[super prepareLayout];
}
static const CGFloat ACTIVE_DISTANCE = 10.0f; //Distance of given cell from center of visible rect
static const CGFloat ITEM_SIZE = 40.0f; // Width/Height of cell.
- (id)init {
if (self = [super init]) {
self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
self.minimumInteritemSpacing = 60.0f;
self.sectionInset = UIEdgeInsetsZero;
self.itemSize = CGSizeMake(ITEM_SIZE, ITEM_SIZE);
self.minimumLineSpacing = 0;
}
return self;
}
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
return YES;
}
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
NSArray *attributes = [super layoutAttributesForElementsInRect:rect];
CGRect visibleRect;
visibleRect.origin = self.collectionView.contentOffset;
visibleRect.size = self.collectionView.bounds.size;
for (UICollectionViewLayoutAttributes *attribute in attributes) {
if (CGRectIntersectsRect(attribute.frame, rect)) {
CGFloat distance = CGRectGetMidX(visibleRect) - attribute.center.x;
// Make sure given cell is center
if (ABS(distance) < ACTIVE_DISTANCE) {
[self.delegate collectionView:self.collectionView layout:self cellCenteredAtIndexPath:attribute.indexPath];
}
}
}
return attributes;
}
Your class containing collection view must conform to protocol 'CustomViewFlowLayoutDelegate' I described earlier in custom layout header file. Like:
#interface MyCollectionViewController () <UICollectionViewDataSource, UICollectionViewDelegate, CustomViewFlowLayoutDelegate>
#property (strong, nonatomic) IBOutlet UICollectionView *collectionView;
#property (strong, nonatomic) IBOutlet UIPageControl *pageControl;
....
....
#end
There are two ways to hook your custom layout to collection view, either in xib OR in code like say in viewDidLoad:
customViewFlowLayout *flowLayout = [[customViewFlowLayout alloc]init];
flowLayout.delegate = self;
self.collectionView.collectionViewLayout = flowLayout;
self.collectionView.pagingEnabled = YES; //Matching your situation probably?
Last thing, in MyCollectionViewController implementation file, implement delegate method of 'CustomViewFlowLayoutDelegate'.
- (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout cellCenteredAtIndexPath:(NSIndexPath *)indexPath {
self.pageControl.currentPage = indexPath.row;
}
I hope this would be helpful. :)

Answer 7

Note - I have found andykkt's answer useful but since it is in obj-c converted it to swift and also implemented logic in another UIScrollView delegate for a smoother effect.
func updatePageNumber() {
// If not case to `Int` will give an error.
let currentPage = Int(ceil(scrollView.contentOffset.x / scrollView.frame.size.width))
pageControl.currentPage = currentPage
}
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
// This will be call when you scrolls it manually.
updatePageNumber()
}
func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
// This will be call when you scrolls it programmatically.
updatePageNumber()
}

Answer 8

for swift 4.2
#IBOutlet weak var mPageControl: UIPageControl!
#IBOutlet weak var mCollectionSlider: UICollectionView!
private var _currentIndex = 0
private var T1:Timer!
private var _indexPath:IndexPath = [0,0]
private func _GenerateNextPage(){
self._currentIndex = mCollectionSlider.indexPathForItem(at: CGPoint.init(x: CGRect.init(origin: mCollectionSlider.contentOffset, size: mCollectionSlider.bounds.size).midX, y: CGRect.init(origin: mCollectionSlider.contentOffset, size: mCollectionSlider.bounds.size).midY))?.item ?? 0
self.mPageControl.currentPage = self._currentIndex
}
func scrollViewDidScroll(_ scrollView: UIScrollView) {
_SetTimer(AutoScrollInterval)
_GenerateNextPage()
}
#objc private func _AutoScroll(){
self._indexPath = IndexPath.init(item: self._currentIndex+1, section: 0)
if !(self._indexPath.item < self.numberOfItems){
_indexPath = [0,0]
}
self.mCollectionSlider.scrollToItem(at: self._indexPath, at: .centeredHorizontally, animated: true)
}
private func _SetTimer(_ interval:TimeInterval){
if T1 == nil{
T1 = Timer.scheduledTimer(timeInterval: interval , target:self , selector: #selector(_AutoScroll), userInfo: nil, repeats: true)
}
}
you can skip the function _SetTimer() , thats for auto scroll

Answer 9

With UICollectionViewDelegate methods
func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
pageControl.currentPage = indexPath.row
}
func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
if pageControl.currentPage == indexPath.row {
pageControl.currentPage = collectionView.indexPath(for: collectionView.visibleCells.first!)!.row
}
}

Answer 10

Swift 5.0
extension youriewControllerName:UIScrollViewDelegate{
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
let pageWidth = self.collectionView.frame.size.width
pageControl.currentPage = Int(self.collectionView.contentOffset.x / pageWidth)
}
}

Answer 11

(void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGFloat pageWidth = _cvImagesList.frame.size.width;
float currentPage = _cvImagesList.contentOffset.x / pageWidth;
_pageControl.currentPage = currentPage + 1;
NSLog(#"finishPage: %ld", (long)_pageControl.currentPage);
}