SwiftUI: Pin to the top & bottom of a centered element

Question

Using SwiftUI, I am trying to center a View on the screen and then give it a header and/or footer of variable heights.

Using constraints it would look something like this:

let view = ...
let header = ...
let footer = ...

view.centerInParent()
header.pinBottomToTop(of: view)
footer.pinTopToBottom(of: view)

This way, the view would always be centered on the screen, regardless of the size of the header and footer.

I cannot figure out how to accomplish this with SwiftUI. Using any type of HStack or VStack means the sizes of the header and footer push around the view. I would like to avoid hardcoding any heights since the center view may vary in size as well.

Any ideas? New to SwiftUI so advice is appreciated!

Answer 1

If I correctly understood your goal (because, as @nayem commented, at first time seems I missed), the following approach should be helpful.

Code snapshot:

extension VerticalAlignment {
   private enum CenteredMiddleView: AlignmentID {
      static func defaultValue(in dimensions: ViewDimensions) -> CGFloat {
         return dimensions[VerticalAlignment.center]
      }
   }

    static let centeredMiddleView = VerticalAlignment(CenteredMiddleView.self)
}

extension Alignment {
    static let centeredView = Alignment(horizontal: HorizontalAlignment.center, 
                          vertical: VerticalAlignment.centeredMiddleView)
}

struct TestHeaderFooter: View {
    var body: some View {
        ZStack(alignment: .centeredView) {
            Rectangle().fill(Color.clear) // !! Extends ZStack to full screen
            VStack {
                Header()
                Text("I'm on center")
                    .alignmentGuide(.centeredMiddleView) { 
                         $0[VerticalAlignment.center]
                    }
                Footer()
            }
        }
//        .edgesIgnoringSafeArea(.top) // uncomment if needed
    }
}

struct Header: View {
    var body: some View {
        Rectangle()
            .fill(Color.blue)
            .frame(height: 40)
    }
}

struct Footer: View {
    var body: some View {
        Rectangle()
            .fill(Color.green)
            .frame(height: 200)
    }
}

struct SwiftUIView_Previews: PreviewProvider {
    static var previews: some View {
        TestHeaderFooter()
    }
}