ScrollView inertia effect manually, iOS, Swift - ios

Question

I have UICollectionView which I'm dragging from code (don't ask me why it's very long story:)).
And my code is working pretty well:
func move(prevPoint: CGPoint, curPoint: CGPoint) {
let xDiff = curPoint.x - prevPoint.x
let yDiff = curPoint.y - prevPoint.y
let xSign = xDiff == 0 ? 1 : (xDiff / abs(xDiff))
let ySign = yDiff == 0 ? 1 : (yDiff / abs(yDiff))
let x = max(min(abs(xDiff), maxPickerStep), minPickerStep) * -xSign * xMultiplier
let y = max(min(abs(yDiff), maxPickerStep), minPickerStep) * -ySign
let offset = CGPoint(x: collectionView.contentOffset.x + x, y: collectionView.contentOffset.y)
let cell = (collectionView.visibleCells.first as? ColorsCollectionViewCell)
let innerOffset = cell?.colorCollectionView.contentOffset ?? .zero
let inset = (cell?.colorCollectionView.contentInset.top ?? 0) * 2
let innerYContentOffset = min(max(innerOffset.y + y, -inset), (cell?.colorCollectionView.contentSize.height ?? 0) - inset)
cell?.colorCollectionView.contentOffset = CGPoint(x: innerOffset.x, y: innerYContentOffset)
collectionView.contentOffset = offset
}
But in addition to scrolling, I want to achieve the same effect as in UICollectionView when scrollView moves by inertia after user takes away finger. Thanks.

Answer 1

First thing first, I think that moving the scroll view manually is most certainly a thing I would avoid.
Probably there is something much simpler to fulfill the behavior you need.
So I highly suggest you, and any other reader, to not go further in the reading of this post and, instead, go ahead and try to solve the problem that guided you here in the first place.
You could also ask another question here on Stack Overflow to maybe get help to try to avoid you to manually update the scrollView position.
So if you are still reading, this article is probably the way to go with implementing something that really feels like a UIScrollView. Doing anything else will probably really look and feel awful.
Basically it consists of using UIKit Dynamics to control the inertia.
So you can create an object that conforms to UIDynamicItem (with a non-zero CGRect), and change its center instead of the scrollView contentOffset, than use a UIDynamicAnimator and its UIDynamicBehavior to set up the inertia and to connect the changes during the animation to the corresponding contentOffset in the scrollView using the UIDynamicBehavior's action block.
Assuming that you have an item that is a UIDynamicItem, and an animator that is a UIDynamicAnimator, the handling of the panGesture recognizer would look something like this:
func handlGestureRecognizer(panGesture: UIPanGestureRecognizer) {
switch panGesture.state {
case .began:
self.animator.removeAllBehaviors()
case .changed:
// Update scroll view position
break
case .ended:
var velocity = panGesture.velocity(in: panGesture.view!)
velocity.x = -velocity.x
velocity.y = -velocity.y
// You probably need to check for out of bound velocity too, and also put velocity.x to 0 if the scroll is only scrolling vertically
// This is done to just save the current content offset and then change it alongside the animation from this starting point
item.center = scrollView.contentOffset
let decelerationBehavior = UIDynamicItemBehavior(items: [item])
decelerationBehavior.addLinearVelocity(velocity, for: item)
decelerationBehavior.resistance = 2.0
decelerationBehavior.action = {
// Connect the item center to the scroll contentOffset. Probably something like this:
scrollView.contentOffset = item.center
}
self.animator.addBehavior(decelerationBehavior)
default:
break
}
}
You than just need to play up with the values of the behavior and be careful with the velocity you put into the behavior having extra care in looking at the edge cases (if you scroll over the min/max for example)
PS: After all I've written, I still believe you should strongly consider not doing this and, instead, go with the standard scrollView scrolling, avoiding manual updates.

Answer 2

You can try to play with decelerationRate and see if it satisfies your needs.
collectionView.decelerationRate = UIScrollView.DecelerationRate(rawValue: 1)