UICollectionView insert cells above maintaining po

Question

By default Collection View maintains content offset while inserting cells. On the other hand I'd like to insert cells above the currently displaying ones so that they appear above the screen top edge like Messages.app do when you load earlier messages. Does anyone know the way to achieve it?

Answer 1

Swift 3 version code: based on James Martin answer

    let amount = 1 // change this to the amount of items to add
    let section = 0 // change this to your needs, too
    let contentHeight = self.collectionView.contentSize.height
    let offsetY = self.collectionView.contentOffset.y
    let bottomOffset = contentHeight - offsetY

    CATransaction.begin()
    CATransaction.setDisableActions(true)

    self.collectionView.performBatchUpdates({
      var indexPaths = [NSIndexPath]()
      for i in 0..<amount {
        let index = 0 + i
        indexPaths.append(NSIndexPath(item: index, section: section))
      }
      if indexPaths.count > 0 {
        self.collectionView.insertItems(at: indexPaths as [IndexPath])
      }
    }, completion: {
       finished in
       print("completed loading of new stuff, animating")
       self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
       CATransaction.commit()
    })