targetContentOffsetForProposedContentOffset - wrong proposed offset - ios

Question

I have a custom UICollectionViewLayout and it implements targetContentOffsetForProposedContentOffset in order to set paging. The center item in the UICollectionView is the full "large" size, while each other item has a CGAffineTransformScale of the "shrunk" scale value.
My problem is that there appears to be an upper limit on the contentOffset so that I can only scroll to item 5 of 7, and it bounces back. Specifics after the code:
I'm setting the collectionViewContentSize() as follows:
#IBInspectable var shrunkScale: CGFloat = 0.5 // 0.5 in IB
#IBInspectable var largeSize: CGSize = CGSizeZero // 360x490 in IB
#IBInspectable var itemSpacing: CGFloat = 0 // 32 in IB
var widthPerAdditionalItem: CGFloat {
return largeSize.width * shrunkScale + itemSpacing
}
override func collectionViewContentSize() -> CGSize {
guard let collectionView = self.collectionView else {
return CGSizeZero
}
let count = CGFloat(collectionView.numberOfItemsInSection(0))
let width = largeSize.width + (count) * widthPerAdditionalItem
let height = collectionView.bounds.height
let size = CGSize(width: width, height: height)
return size
}
the targetOffset... methods reference a single helper method:
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint) -> CGPoint {
let closestPlace = round(proposedContentOffset.x / widthPerAdditionalItem)
guard let offsetX = offsetXForItemAtIndex(Int(closestPlace)) else {
return proposedContentOffset
}
print("Calculated: \(offsetX), Proposed: \(proposedContentOffset.x), ContentWidth: \(collectionView?.contentSize.width ?? 0 )")
return CGPoint(x: offsetX, y: proposedContentOffset.y)
}
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
return targetContentOffsetForProposedContentOffset(proposedContentOffset)
}
func contentOffsetForItemAtIndexPath(indexPath: NSIndexPath) -> CGPoint? {
guard
let collectionView = self.collectionView,
let offsetX = offsetXForItemAtIndex(indexPath.item)
else {
return nil
}
print("Tap Offset: - \(offsetX) vs. \(collectionView.contentOffset.x)")
return CGPoint(x: offsetX, y: collectionView.contentOffset.y)
}
private func offsetXForItemAtIndex(index: Int) -> CGFloat? {
guard
let count = collectionView?.numberOfItemsInSection(0)
else {
return nil
}
let proposed = CGFloat(index) * widthPerAdditionalItem
let maximum = CGFloat(count) * widthPerAdditionalItem
// bound min = 0, max = count*width
return max( min(maximum, proposed), 0)
}
Here's What I get:
My content Width is 1844.0
I finish dragging the view at offset.x = 1187.5
The targetContentOffsetForProposedContentOffset receives a proposed offset.x = 820.0
I return the "paged" offset.x value of 848.0
The collectionView scrolls to offset.x of 820.0
What I am expecting:
My content Width is 1844.0
I finish dragging the view at offset.x = 1187.5
The targetContentOffsetForProposedContentOffset receives a
proposed offset.x = 1187.5
I return the "paged" offset.x value of 1272.0
The collectionView scrolls to offset.x of 1272.0
Debugging:
If I manually call setContentOffset with the calculated offset of 1272.0 then it scrolls to the correct position. But the instant I try to scroll it snaps back to 820.0

Answer 1

After sitting down and doing some math I figured out the following:
contentWidth = 1844
poposedContentOffset.x = 820
1844 - 820 = 1024 // <--- Width of the screen.
The content was being displayed from the center of the screen for the first item, to the center of the screen for the second item.
This means I needed to add half the collectionView's frame.width so the first item can be centered, and half again so the final item could be centered.
The collectionViewContentSize() now returns
let width = (count-1) * widthPerAdditionalItem + collectionView.bounds.width
let height = collectionView.bounds.height
let size = CGSize(width: width, height: height)