SnapDrag

Snap behavior in UIScrollView in Swift

Needed this to answer a StackOverflow question:
UIAnimator's UISnapBehavior possible with UIScrollview?

Question

What I am Trying to Achieve

UIScrollView to 'snap' to certain point while the user is dragging the scroll view. However, the scrolling has to resume from snapped position without the user having to lift the touch.

Apple seems to achieve this in Photo Editing in their iOS Photos App. (See screenshot below)

Answer

I've tried to mimic iOS Photos app. Here is my logic:

// CALCULATE A CONTENT OFFSET FOR SNAPPING POINT 
let snapPoint = CGPoint(x: 367, y: 0)  

// CHANGE THESE VALUES TO TEST
let minDistanceToSnap = 7.0
let minVelocityToSnap = 25.0
let minDragDistanceToReleaseSnap = 7.0
let snapDuringDecelerating = false

This kind of scrolling needs 3 stages

enum SnapState {
case willSnap
case didSnap
case willRelease
}

1. willSnap: Default state. Decide when to snap. Compare contentOffset distance from SnapPoint with minDistanceToSnap and scrollview velocity with minVelocityToSnap. Change to didSnap state.
2. didSnap: Manually setContentOffset to a provided contextOffset(snapPoint). Calculate dragDistance on scrollView. If user drag more than a certain distance (minDragDistanceToReleaseSnap) change to willRelease state.
3. willRelease: Change to willSnap state again if distance scroll from snapPoint is more than minDistanceToSnap.

extension ViewController: UIScrollViewDelegate {
    func scrollViewDidScroll(scrollView: UIScrollView) {
        switch(snapState) {
            case .willSnap:
                let distanceFromSnapPoint = distance(between: scrollView.contentOffset, and: snapPoint)
                let velocity = scrollView.panGestureRecognizer.velocityInView(view)
                let velocityDistance = distance(between: velocity, and: CGPointZero)
                if distanceFromSnapPoint <= minDistanceToSnap && velocityDistance <= minVelocityToSnap && (snapDuringDecelerating || velocityDistance > 0.0) {
                    startSnapLocaion = scrollView.panGestureRecognizer.locationInView(scrollView)
                    snapState = .didSnap
                }
            case .didSnap:
                scrollView.setContentOffset(snapPoint, animated: false)
                var dragDistance = 0.0
                let location = scrollView.panGestureRecognizer.locationInView(scrollView)
                dragDistance = distance(between: location, and: startSnapLocaion)
                if dragDistance > minDragDistanceToReleaseSnap  {
                    startSnapLocaion = CGPointZero
                    snapState = .willRelease
                }
            case .willRelease:
                let distanceFromSnapPoint = distance(between: scrollView.contentOffset, and: snapPoint)
                if distanceFromSnapPoint > minDistanceToSnap {
                    snapState = .willSnap
                }
        }
    }
}

Helper function

func distance(between point1: CGPoint, and point2: CGPoint) -> Double {
    return Double(hypotf(Float(point1.x - point2.x), Float(point1.y - point2.y)))
}