ios - 增加 UIScrollView 橡皮筋阻力

Question

我尝试在我的应用程序中实现一个场景，我希望在您开始强制 UIScrollView 滚动超过其正常内容时表现得更“僵硬”边界。

我的意思是，当你在 ScrollView 的顶部或底部时，如果你点击并继续拖动，你通常可以让 ScrollView 保持滚动超出其边界，但它逐渐建立阻力，直到它通常在 View 边界的中间大约一半处停止。当您抬起手指时，它会弹回滚动区域的边界。

我想要实现的是，我想让“越界”的拖动效果更加沉重，而不是用户拖动 ScrollView 并在中间“触底”通过 ScrollView 边界的方式，它完全停止在超过其滚动边界的 20% 左右。

我一直在尝试在 scrollViewDidScroll: 委托(delegate)方法中覆盖 ScrollView 的 contentOffset，但这似乎不起作用，因为在似乎搞砸了同一方法的进一步委托(delegate)调用。

我的下一个想法是监视与 ScrollView 关联的 UIPanGestureRecognizer，并尝试根据由此产生的事件确定正确的 UIScrollView contentOffset。话虽如此，我认为这可能会开始变得棘手，所以我想在尝试可能会造成困惑的事情之前，我会在这里询问我没有考虑过的任何其他解决方案。

谢谢!

Answer 1

我正在使用 Swift 编写以下代码。这是用于水平滚动，可以很容易地适应垂直滚动。解决方案根据是否启用分页而有所不同。两者都在下面给出。

class ScrollViewDelegate : NSObject, UIScrollViewDelegate
{
    let maxOffset: CGFloat  // offset of the rightmost content (scrollview contentSize.width - frame.width)
    var prevOffset: CGFloat = 0  // previous offset (after adjusting the value)

    var totalDistance: CGFloat = 0  // total distance it would have moved (had we not restricted)
    let reductionFactor: CGFloat = 0.2  // percent of total distance it will be allowed to move (under restriction)
    let scaleFactor: CGFloat = UIScreen.mainScreen().scale  // pixels per point, for smooth translation in respective devices

    init(maxOffset: CGFloat)
    {
        self.maxOffset = maxOffset  // scrollView.contentSize.width - scrollView.frame.size.width
    }

    func scrollViewDidScroll(scrollView: UIScrollView)
    {
        let flipped = scrollView.contentOffset.x >= maxOffset  // dealing with left edge or right edge rubber band
        let currentOffset = flipped ? maxOffset - scrollView.contentOffset.x : scrollView.contentOffset.x  // for right edge, flip the values as if screen is folded in half towards the left

        if(currentOffset <= 0)  // if dragging/moving beyond the edge
        {
            if(currentOffset <= prevOffset)  // if dragging/moving beyond previous offset
            {
                totalDistance += currentOffset - prevOffset  // add the "proposed delta" move to total distance
                prevOffset = round(scaleFactor * totalDistance * reductionFactor) / scaleFactor  // set the prevOffset to fraction of total distance
                scrollView.contentOffset.x = flipped ? maxOffset - prevOffset : prevOffset  // set the target offset, after negating any flipping
            }
            else  // if dragging/moving is reversed, though still beyond the edge
            {
                totalDistance = currentOffset / reductionFactor  // set totalDistance from offset (reverse of prevOffset calculation above)
                prevOffset = currentOffset  // set prevOffset
            }
        }
        else  // if dragging/moving inside the edge
        {
            totalDistance = 0  // reset the values
            prevOffset = 0
        }
    }
}

启用分页后，弹回静止点似乎没有按预期工作。橡皮筋不是停在页面边界，而是越过它并停在非页面偏移处。如果边缘的拉力快速轻弹，即使在您抬起手指后它继续沿该方向移动，然后反转方向并返回到静止点，就会发生这种情况。如果您只是暂停并离开，甚至将其弹回到休息点，它似乎工作正常。为了解决这个问题，在下面的代码中，我尝试确定超调的可能性，并在它返回并尝试越过预期的页面边界时强行停止它。

class PageScrollViewDelegate : NSObject, UIScrollViewDelegate
{
    let maxOffset: CGFloat  // offset of the rightmost content (scrollview contentSize.width - frame.width)
    var prevOffset: CGFloat = 0  // previous offset (after adjusting the value)

    var totalDistance: CGFloat = 0  // total distance it would have moved (had we not restricted)
    let reductionFactor: CGFloat = 0.2  // percent of total distance it will be allowed to move (under restriction)
    let scaleFactor: CGFloat = UIScreen.mainScreen().scale  // pixels per point, for smooth translation in respective devices

    var draggingOver: Bool = false  // finger dragging is over or not
    var overshoot: Bool = false  // is there a chance for page to overshoot page boundary while falling back

    init(maxOffset: CGFloat)
    {
        self.maxOffset = maxOffset  // scrollView.contentSize.width - scrollView.frame.size.width
    }

    func scrollViewWillBeginDragging(scrollView: UIScrollView)
    {
        draggingOver = false  // reset the flags
        overshoot = false
    }

    func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool)
    {
        draggingOver = true  // finger dragging is over
    }

    func scrollViewDidScroll(scrollView: UIScrollView)
    {
        let flipped = scrollView.contentOffset.x >= 0.5 * maxOffset // dealing with left edge or right edge rubber band
        let currentOffset = flipped ? maxOffset - scrollView.contentOffset.x : scrollView.contentOffset.x  // for right edge, flip the values as if screen is folded in half towards the left

        if(currentOffset <= 0)  // if dragging/moving beyond the edge
        {
            if(currentOffset <= prevOffset)  // if dragging/moving beyond previous offset
            {
                overshoot = draggingOver  // is content moving farther away even after dragging is over (caused by fast flick, which can cause overshooting page boundary while falling back)

                totalDistance += currentOffset - prevOffset  // add the "proposed delta" move to total distance
                prevOffset = round(scaleFactor * totalDistance * reductionFactor) / scaleFactor  // set the prevOffset to fraction of total distance
                scrollView.contentOffset.x = flipped ? maxOffset - prevOffset : prevOffset  // set the target offset, after negating any flipping
            }
            else  // if dragging/moving is reversed, though still beyond the edge
            {
                totalDistance = currentOffset / reductionFactor  // set totalDistance from offset (reverse of prevOffset calculation above)
                prevOffset = currentOffset  // set prevOffset
            }
        }
        else  // if dragging/moving inside the edge
        {
            if(overshoot)  // if this movement is a result of overshooting
            {
                scrollView.setContentOffset(CGPointMake(flipped ? maxOffset : 0, scrollView.contentOffset.y), animated: false)  // bring it to resting point and stop further scrolling (this is a patch to control overshooting)
            }

            totalDistance = 0  // reset the values
            prevOffset = 0
        }
    }
}