SwiftUI - disclosure group expanding behaviour - keep top item static - swift

Question

I'm trying to create a drop down menu using swift UI. I'm actually implementing this into a UIKit project.
The basic functionality I am going for should be that the user clicks on a label with a certain data unit on it and a list expands with other units of measure that can be selected.
Here is my SwiftUI code:
import SwiftUI
#available(iOS 14.0.0, *)
struct DataUnitDropDown: View {
var units = ["ltr", "usg", "impg"]
#State private var selectedDataUnit = 0
#State private var isExpanded = false
var body: some View {
VStack(alignment: .leading, spacing: 15) {
DisclosureGroup(units[selectedDataUnit], isExpanded: $isExpanded) {
VStack {
ForEach(0 ..< units.count, id: \.self) { index in
Text("\(units[index])")
.font(.title3)
.padding(.all)
.onTapGesture {
self.selectedDataUnit = index
withAnimation {
self.isExpanded.toggle()
}
}
}
}
}
}
}
}
So I have a couple of questions here. Firstly, ideally I wanted to place this into an existing horizontal UIStackView. However, the issue I have is that obviously once the dropdown expands, it increases the height of the stack, which is not what I want. Here is a screen shot of the component (see 'ltr'):
When expanded in the stackView:
So I reluctantly placed it in the main view adding some autolayout constraints:
if #available(iOS 14.0.0, *) {
let controller = UIHostingController(rootView: DataUnitDropDown())
controller.view.translatesAutoresizingMaskIntoConstraints = false
addSubview(controller.view)
controller.view.leadingAnchor.constraint(equalTo: refuelInfoView.actualUpliftVolume.trailingAnchor).isActive = true
controller.view.centerYAnchor.constraint(equalTo: refuelInfoView.actualUpliftVolume.centerYAnchor).isActive = true
}
But now when I expand, the whole menu shifts up:
Clearly not what I am looking for.
So I suppose my question is 2-fold:
1- Is there any way I can incorporate this element into an existing stack view without the stack increasing in height when the dropdown is expanded? (I guess not!)
2- If I have to add this to the main view rather than the stack, how can I stop the entire menu shifting up when it expands?
Update:
I have amended the code as follows:
if #available(iOS 14.0.0, *) {
let controller = UIHostingController(rootView: DataUnitDropDown())
controller.view.translatesAutoresizingMaskIntoConstraints = false
controller.view.clipsToBounds = false
controller.view.heightAnchor.constraint(equalToConstant: 22).isActive = true
controller.view.topAnchor.constraint(equalTo: topAnchor).isActive = true
stackSubviews = [inputField, controller.view]
}
Now, without the topAnchor constraint this works perfectly, but I do need this constraint. But I get an error due to the views. not being in the same hierarchy

Answer 1

I assume instead of
controller.view.centerYAnchor.constraint(equalTo:
refuelInfoView.actualUpliftVolume.centerYAnchor).isActive = true
you need
controller.view.topAnchor.constraint(equalTo:
refuelInfoView.actualUpliftVolume.bottomAnchor).isActive = true