UIScrollView animation to targetContentOffset erratic - ios

Question

I've implemented a UIScrollView within a UITableViewCell that enables the user to scroll left and right to reveal buttons in the same fashion as the iOS Mail app. The original implementation that set frames and positions explicitly worked well but I've refactored the code to use autolayout throughout. Animation to hide/reveal the 'container' for the buttons on the left (accessory buttons) works well but the animation that brings the scrollview to rest when the right container (edit buttons) slows just before reaching the desired offset before jerking into its final position.
All calculations use the same math just transformed (e.g. + rather than - value, > rather than < in tests) depending on the side the container is located and the values displayed by logging are correct. I can't see any obvious code errors and there are no constraints for the cells set up in IB. Is this a bug or is there something obvious I've missed through staring at the code for the last hour?
class SwipeyTableViewCell: UITableViewCell {
// MARK: Constants
private let thresholdVelocity = CGFloat(0.6)
private let maxClosureDuration = CGFloat(40)
// MARK: Properties
private var buttonContainers = [ButtonContainerType: ButtonContainer]()
private var leftContainerWidth: CGFloat {
return buttonContainers[.Accessory]?.containerWidthWhenOpen ?? CGFloat(0)
}
private var rightContainerWidth: CGFloat {
return buttonContainers[.Edit]?.containerWidthWhenOpen ?? CGFloat(0)
}
private var buttonContainerRightAnchor = NSLayoutConstraint()
private var isOpen = false
// MARK: Subviews
private let scrollView = UIScrollView()
// MARK: Lifecycle methods
override func awakeFromNib() {
super.awakeFromNib()
// Initialization code
scrollView.delegate = self
scrollView.showsHorizontalScrollIndicator = false
scrollView.showsVerticalScrollIndicator = false
contentView.addSubview(scrollView)
scrollView.translatesAutoresizingMaskIntoConstraints = false
scrollView.topAnchor.constraintEqualToAnchor(contentView.topAnchor).active = true
scrollView.leftAnchor.constraintEqualToAnchor(contentView.leftAnchor).active = true
scrollView.rightAnchor.constraintEqualToAnchor(contentView.rightAnchor).active = true
scrollView.bottomAnchor.constraintEqualToAnchor(contentView.bottomAnchor).active = true
let scrollContentView = UIView()
scrollContentView.backgroundColor = UIColor.cyanColor()
scrollView.addSubview(scrollContentView)
scrollContentView.translatesAutoresizingMaskIntoConstraints = false
scrollContentView.topAnchor.constraintEqualToAnchor(scrollView.topAnchor).active = true
scrollContentView.leftAnchor.constraintEqualToAnchor(scrollView.leftAnchor).active = true
scrollContentView.rightAnchor.constraintEqualToAnchor(scrollView.rightAnchor).active = true
scrollContentView.bottomAnchor.constraintEqualToAnchor(scrollView.bottomAnchor).active = true
scrollContentView.widthAnchor.constraintEqualToAnchor(contentView.widthAnchor, constant: 10).active = true
scrollContentView.heightAnchor.constraintEqualToAnchor(contentView.heightAnchor).active = true
buttonContainers[.Accessory] = ButtonContainer(type: .Accessory, scrollContentView: scrollContentView)
buttonContainers[.Edit] = ButtonContainer(type: .Edit, scrollContentView: scrollContentView)
for bc in buttonContainers.values {
scrollContentView.addSubview(bc)
bc.widthAnchor.constraintEqualToAnchor(contentView.widthAnchor).active = true
bc.heightAnchor.constraintEqualToAnchor(scrollContentView.heightAnchor).active = true
bc.topAnchor.constraintEqualToAnchor(scrollContentView.topAnchor).active = true
bc.containerToContentConstraint.active = true
}
scrollView.contentInset = UIEdgeInsetsMake(0, leftContainerWidth, 0, rightContainerWidth)
}
func closeContainer() {
scrollView.contentOffset.x = CGFloat(0)
}
}
extension SwipeyTableViewCell: UIScrollViewDelegate {
func scrollViewWillEndDragging(scrollView: UIScrollView, withVelocity velocity: CGPoint,
targetContentOffset: UnsafeMutablePointer<CGPoint>) {
let xOffset: CGFloat = scrollView.contentOffset.x
isOpen = false
for bc in buttonContainers.values {
if bc.isContainerOpen(xOffset, thresholdVelocity: thresholdVelocity, velocity: velocity) {
targetContentOffset.memory.x = bc.offsetRequiredToOpenContainer()
NSLog("Target offset \(targetContentOffset.memory.x)")
isOpen = true
break /// only one container can be open at a time so cn exit here
}
}
if !isOpen {
NSLog("Closing container")
targetContentOffset.memory.x = CGFloat(0)
let ms: CGFloat = xOffset / velocity.x /// if the scroll isn't on a fast path to zero, animate it closed
if (velocity.x == 0 || ms < 0 || ms > maxClosureDuration) {
NSLog("Animating closed")
dispatch_async(dispatch_get_main_queue()) {
scrollView.setContentOffset(CGPointZero, animated: true)
}
}
}
}
/**
Defines the position of the container view for buttons assosicated with a SwipeyTableViewCell
- Edit: Identifier for a UIView that acts as a container for buttons to the right of the cell
- Accessory: Identifier for a UIView that acts as a container for buttons to the left of the vell
*/
enum ButtonContainerType {
case Edit, Accessory
}
extension ButtonContainerType {
func getConstraints(scrollContentView: UIView, buttonContainer: UIView) -> NSLayoutConstraint {
switch self {
case Edit:
return buttonContainer.leftAnchor.constraintEqualToAnchor(scrollContentView.rightAnchor)
case Accessory:
return buttonContainer.rightAnchor.constraintGreaterThanOrEqualToAnchor(scrollContentView.leftAnchor)
}
}
func containerOpenedTest() -> ((scrollViewOffset: CGFloat, containerFullyOpenWidth: CGFloat, thresholdVelocity: CGFloat, velocity: CGPoint) -> Bool) {
switch self {
case Edit:
return {(scrollViewOffset: CGFloat, containerFullyOpenWidth: CGFloat, thresholdVelocity: CGFloat, velocity: CGPoint) -> Bool in
(scrollViewOffset > containerFullyOpenWidth || (scrollViewOffset > 0 && velocity.x > thresholdVelocity))
}
case Accessory:
return {(scrollViewOffset: CGFloat, containerFullyOpenWidth: CGFloat, thresholdVelocity: CGFloat, velocity: CGPoint) -> Bool in
(scrollViewOffset < -containerFullyOpenWidth || (scrollViewOffset < 0 && velocity.x < -thresholdVelocity))
}
}
}
func transformOffsetForContainerSide(containerWidthWhenOpen: CGFloat) -> CGFloat {
switch self {
case Edit:
return containerWidthWhenOpen
case Accessory:
return -containerWidthWhenOpen
}
}
}
/// A UIView subclass that acts as a container for buttongs associated with a SwipeyTableCellView
class ButtonContainer: UIView {
private let scrollContentView: UIView
private let type: ButtonContainerType
private let maxNumberOfButtons = 3
let buttonWidth = CGFloat(65)
private var buttons = [UIButton]()
var containerWidthWhenOpen: CGFloat {
// return CGFloat(buttons.count) * buttonWidth
return buttonWidth // TODO: Multiple buttons not yet implements - this will cause a bug!!
}
var containerToContentConstraint: NSLayoutConstraint {
return type.getConstraints(scrollContentView, buttonContainer: self)
}
var offsetFromContainer = CGFloat(0) {
didSet {
let delta = abs(oldValue - offsetFromContainer)
containerToContentConstraint.constant = offsetFromContainer
if delta > (containerWidthWhenOpen * 0.5) { /// this number is arbitary - can it be more formal?
animateConstraintWithDuration(0.1, delay: 0, options: UIViewAnimationOptions.CurveEaseOut, completion: nil) /// ensure large changes are animated rather than snapped
}
}
}
// MARK: Initialisers
init(type: ButtonContainerType, scrollContentView: UIView) {
self.type = type
self.scrollContentView = scrollContentView
super.init(frame: CGRectZero)
backgroundColor = UIColor.blueColor()
translatesAutoresizingMaskIntoConstraints = false
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
// MARK: Public methods
func isContainerOpen(scrollViewOffset: CGFloat, thresholdVelocity: CGFloat, velocity: CGPoint) -> Bool {
let closure = type.containerOpenedTest()
return closure(scrollViewOffset: scrollViewOffset, containerFullyOpenWidth: containerWidthWhenOpen, thresholdVelocity: thresholdVelocity, velocity: velocity)
}
func offsetRequiredToOpenContainer() -> CGFloat {
return type.transformOffsetForContainerSide(containerWidthWhenOpen)
}
}

Answer 1

OK - found the error and it was a typo left from earlier experimentation with UIScrollView. The clue was in my earlier comment about the 'snap' occurring within 10pt of the desired targetContentOffset...
The scrollContentView width constraint was set incorrectly as follows:
scrollContentView.widthAnchor.constraintEqualToAnchor(contentView.widthAnchor, constant: 10).active = true
Before I found out that I could force a UIScrollView to scroll by setting its contentInset, I just made the subview larger that the cell's contentView that the UIScrollView was pinned to. As I've been refactoring code verbatim to use the new anchor properties, the old code propagated and I got my bug!
So, not iOS at fault...just me not paying attention. Lesson learnt! I now have some other ideas on how to implement things that might be a little tidier.