How to scroll UICollectionViewCell programmaticall

问题:

I have a vertical UICollectionView with each cell taking up the entire self.view.frame I can easily swipe upwards to page to the next cell, but I would like to do the same thing with the press of a button.

I've tried using:

- (void)setContentOffset:(CGPoint)contentOffset animated:(BOOL)animated

And:

- (void)scrollRectToVisible:(CGRect)rect animated:(BOOL)animated

They work but they temporarily "White-Out" the currentCell to present the nextCell, while the swiping shows both cells during the transition.

Ideally I would like to use:

- (void)scrollToItemAtIndexPath:(NSIndexPath *)indexPath atScrollPosition:(UICollectionViewScrollPosition)scrollPosition animated:(BOOL)animated

But I don't know how to access the nextCell's indexPath... I've tried:

NSIndexPath *nextIndexPath = [_collectionView indexPathForItemAtPoint:CGPointMake(25, (_collectionView.frame.size.height-25)*(_currentIndexRowForCellOnScreen+1))];

Where _currentIndexRowForCellOnScreen is the indexPath.row of the UICollectionView's first appearance on screen in:

- (UICollectionViewCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath

But when I put it into:

- (NSIndexPath *)indexPathForCell:(UICollectionViewCell *)cell

it returns NULL after the first Cell, and does not animate....

Any direction would be greatly appreciated. Thank you for your time.

回答1:

Assuming your collectionView contains only 1 section, and given that each item occupies the whole frame then you could do something like this;

  NSArray *visibleItems = [self.collectionView indexPathsForVisibleItems];
  NSIndexPath *currentItem = [visibleItems objectAtIndex:0];
  NSIndexPath *nextItem = [NSIndexPath indexPathForItem:currentItem.item + 1 inSection:currentItem.section];
  [self.collectionView scrollToItemAtIndexPath:nextItem atScrollPosition:UICollectionViewScrollPositionTop animated:YES];

回答2:

Here are the Swift versions

Swift 2.x:

let visibleItems: NSArray = self.collectionView.indexPathsForVisibleItems()
let currentItem: NSIndexPath = visibleItems.objectAtIndex(0) as! NSIndexPath
let nextItem: NSIndexPath = NSIndexPath(forRow: currentItem.item + 1, inSection: 0)
self.collectionView.scrollToItemAtIndexPath(nextItem, atScrollPosition: .Top, animated: true)

Swift 3.x

let visibleItems: NSArray = self.collectionView.indexPathsForVisibleItems as NSArray
let currentItem: IndexPath = visibleItems.object(at: 0) as! IndexPath
let nextItem: IndexPath = IndexPath(item: currentItem.item + 1, section: 0)
self.collectionView.scrollToItem(at: nextItem, at: .top, animated: true)

回答3:

By far this is the best approach i have encountered, the trick is to scroll to the frame which is containing next objects. Please refer the code

@IBAction func actionPreviousFriends(_ sender: Any) {

    let collectionBounds = self.collectionView.bounds
    let contentOffset = CGFloat(floor(self.collectionView.contentOffset.x - collectionBounds.size.width))
    self.moveToFrame(contentOffset: contentOffset)
}

/* -------------- display next friends action ----------------*/
@IBAction func actionNextFriends(_ sender: Any) {

    let collectionBounds = self.collectionView.bounds
    let contentOffset = CGFloat(floor(self.collectionView.contentOffset.x + collectionBounds.size.width))
    self.moveToFrame(contentOffset: contentOffset)
}

func moveToFrame(contentOffset : CGFloat) {

    let frame: CGRect = CGRect(x : contentOffset ,y : self.collectionView.contentOffset.y ,width : self.collectionView.frame.width,height : self.collectionView.frame.height)
    self.collectionView.scrollRectToVisible(frame, animated: true)
}

回答4:

Swift 4.1 answer

    let visibleItems = self.collectionView.indexPathsForVisibleItems
    let currentItem = visibleItems.first
    let nextRow = (currentItem?.row)! + visibleItems.count
    if nextRow < elementArray.count {
        let nextIndexPath = IndexPath.init(row: nextRow, section: (currentItem?.section)!)
        self.collectionView.scrollToItem(at: nextIndexPath, at: UICollectionViewScrollPosition.left, animated: true)
    } else {
        let nextIndexPath = IndexPath.init(row: 0, section: (currentItem?.section)!)
        self.collectionView.scrollToItem(at: nextIndexPath, at: UICollectionViewScrollPosition.right, animated: true)
    }

Change the scroll position according to your requirement and also the row count you want to skip (Here i have skipped visibleItems.count)