scrolling/resizing UITableView

Question

I'll get right to the point.

I have a UIViewController that has two subviews in it. The top one (let's call it HeaderView from now one) is a custom UIView and the bottom one is a UITableView.
I have set them up in InterfaceBuilder so that the HeaderView has 0 margin from the left, top and right, plus it has a fixed height. The UITableView is directly underneath with 0 margin from all sides.

My goal is to achieve a behaviour such that when I start scrolling the UITableView's content the HeaderView will start shrinking and the UITableView becomes higher without scrolling. This should go on until the HeaderView has reached a minimum height. After that the UITableView should start scrolling as normal. When scrolling down the effect should be reversed.

I have initially started this out using a UIScrollView instead of the UITableView and I have achieved the desired result. Here is how:

1. connect the UIScrollView to the outlet
   @IBOutlet weak var scrollView: UIScrollView!

2. set the UIScrollViewDelegate in the controller's viewDidLoad() method self.scrollView.delegate = self and declared the UIViewController to conform to the protocol

3. intercept when the UIScrollView scrolls:
   func scrollViewDidScroll(_ scrollView: UIScrollView) { self.adjustScrolling(offset: scrollView.contentOffset.y, scrollView: scrollView) }

4. in my adjustScrolling(offset:scrollView:) method the "magic" happens

Now let's look at what happens in this method.

    private func adjustScrolling(offset: CGFloat, scrollView: UIScrollView) {

        // bind value between 0 and max header scroll
        let actualOffset: CGFloat = offset < 0 ? 0 : (offset >= self.maxHeaderScroll ? self.maxHeaderScroll : offset)

        // avoid useless calculations
        if (actualOffset == self.currentOffset) {
            return
        }

        /**
         * Apply the vertical scrolling to the header
         */
        // Translate the header up to give more space to the scrollView
        let headerTransform = CATransform3DTranslate(CATransform3DIdentity, 0, -(actualOffset), 0)
        self.header.layer.transform = headerTransform
        // Adjust header's subviews to new size
        self.header.didScrollBy(actualOffset)

        /**
         * Apply the corrected vertical scrolling to the scrollView
         */
        // Resize the scrollView to fill all empty space
        let newScrollViewY = self.header.frame.origin.y + self.header.frame.height
        scrollView.frame = CGRect(
            x: 0,
            y: newScrollViewY,
            width: scrollView.frame.width,
            height: scrollView.frame.height + (scrollView.frame.origin.y - newScrollViewY)
        )
        // Translate the scrollView's content view down to contrast scrolling
        let scrollTransform = CATransform3DTranslate(CATransform3DIdentity, 0, (actualOffset), 0)
        scrollView.subviews[0].layer.transform = scrollTransform
        // Set bottom inset to show content hidden by translation
        scrollView.contentInset = UIEdgeInsets(
            top: 0,
            left: 0,
            bottom: actualOffset,
            right: 0
        )

        self.currentOffset = actualOffset
    }

If I haven't forgotten anything this should be enough to achieve the desired effect. Let me break it down:

1. I calculate the actualOffset binding it between 0 and self.MaxHeaderScroll which is just 67 (I think, it's calculated dynamically but this doesn't really matter)
2. If I see that the actualOffset hasn't changed since the last time this function was called I don't bother to aplly any changes. This avoids some useless calculations.
3. I apply the scrolling to the header by translating it up with a CATransform3DTranslate on just the y axis by negative actualOffset .
4. I call self.header.didScrollBy(actualOffset) so that the HeaderView can apply some visual changes internally. This doesn't concearn the question though.
5. I resize the scrollView so that it keeps 0 margin from top and bottom now that the HeaderView is higher up.
6. I translate down the scrollView's content by the same actualOffset amount to contrast the scrolling. This piece is essential to the correct visual effect that I want to achieve. If I didn't do this, the scrollView would still resize correctly but the content would start scrolling right away, which I don't want. It should only start scrolling once the HeaderView reaches it's minimum height.
7. I now set a bottom inset in the scrollView so that I am able to scroll it all the way to the end. Without this, the last part of the scrollView would be cut off since the scrollView itself would think it reached the end of it's content.
8. Lastly I store the actualOffset for later comparison

As I said, this works fine. The problem arises when I switch from a UIScrollView to a UITableView. I assumed it would work since UITableView inherits from UIScrollView.
The only piece of code that doesn't work is the number 6. I don't really know what is going wrong so I will just list everything I have found out and/or noticed. Hopefully someone will be able to help me out.

• in the case of the UIScrollView, in point 6, the scrollView.subviews[0] refers to a view that holds all the content inside it. When I change to UITableView this subview seems to be of the type UITableViewWrapperView which I could not find any documentation about, nor does XCode recognize it as a valid class. This is already frustrating.
• if in point 6 I also give some translation on the x axis (let's say of 50) I can see an initial very quick translation that is immediately brought back to 0. This only happens when the UITableView starts scrolling, it doesn't go on while scrolling.
• I have tried changing the frame of the subview in point 6 to achieve the desired result. Although the scrolling is correct, the top cells start disappearing as I scroll the UITableView. I thin this is because I am using dequeueReusableCell(withIdentifier:for:) to instatiate the cells and the UITableView thinks that the top cells aren't visible when they actually are. I wasn't able to work around this problem.
• I have tried setting the self.tableView.tableHeaderView to a UIView of the actualOffset height to contrast scrolling but this gave a weird effect where the cells would not scroll correctly and when the UITableView was brought back to the initial position, there would be a gap on top. No clue about this either.

I know there's a lot here so please don't hesitate asking for more details. Thank you in advance.

Answer 1

I made something like this recently, so heres how I achieved it:

Make a UIView with a height constraint constant and link this to your view/VC, have you UITableview constrained to the VC's view full screen behind the UIView.

Now set your UITableViews contentInset top to the starting height of your 'headerView' now, in the scrollViewDidScroll you adjust the constant until the height of the header is at its minimum.

Here is a demo

If you just run it, the blue area is your 'header' and the colored rows are just any cell. You can autolayout whatever you want in the blue area and it should auto size and everything