UIScrollView 动画到 targetContentOffset 不稳定

Question

我在 UITableViewCell 中实现了 UIScrollView，它使用户能够以与 iOS 邮件应用程序相同的方式左右滚动以显示按钮。明确设置框架和位置的原始实现运行良好，但我重构了代码以在整个过程中使用自动布局。隐藏/显示左侧按钮（辅助按钮）的“容器”的动画效果很好，但是当右侧容器（编辑按钮）在达到所需偏移量之前减慢时，滚动视图停止的动画在进入最终状态之前位置。

所有计算都使用刚刚转换的相同数学（例如，+ 而不是 - 值，> 而不是

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

好的 - 发现错误，这是早期使用 UIScrollView 实验留下的错字。线索是在我之前关于在所需 targetContentOffset 的 10pt 内发生的“捕捉”的评论中......

scrollContentView 宽度约束设置错误如下：

scrollContentView.widthAnchor.constraintEqualToAnchor(contentView.widthAnchor, constant: 10).active = true

在我发现我可以通过设置它的 contentInset 来强制 UIScrollView 滚动之前，我只是让子视图比 UIScrollView 固定到的单元格的 contentView 更大。因为我一直在逐字重构代码以使用新的锚属性，所以旧代码传播了，我得到了我的错误！

所以，不是 iOS 有错……只是我没有注意。学习到教训了！我现在对如何实现一些可能更整洁的东西有一些其他想法。