UICollectionView: current index path for page cont

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

1. Place PageControl in your view or set by Code.
2. Set UIScrollViewDelegate

3. 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];
   

4. 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 2

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 3

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 4

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 5

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 6

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
    }
}