SwiftUI: How to force a specific view in an HStack to be in the centre

Question

Given an HStack like the following:

        HStack{
            Text("View1")

            Text("Centre")

            Text("View2")

            Text("View3")
        }

How can I force the 'Centre' view to be in the centre?

Answer 1

Here is possible simple approach. Tested with Xcode 11.4 / iOS 13.4

struct DemoHStackOneInCenter: View {
    var body: some View {
        HStack{
            Spacer().overlay(Text("View1"))

            Text("Centre")

            Spacer().overlay(
                HStack {
                    Text("View2")
                    Text("View3")
                }
            )
        }
    }
}

The solution with additional alignments for left/right side views was provided in Position view relative to a another centered view

backup

Answer 2

the answer takes a handful of steps

1. wrap the HStack in a VStack. The VStack gets to control the horizontal alignment of it's children
2. Apply a custom alignment guide to the VStack
3. Create a subview of the VStack which takes the full width. Pin the custom alignment guide to the centre of this view. (This pins the alignment guide to the centre of the VStack)
4. align the centre of the 'Centre' view to the alignment guide

For the view which has to fill the VStack, I use a Geometry Reader. This automatically expands to take the size of the parent without otherwise disturbing the layout.

import SwiftUI


//Custom Alignment Guide
extension HorizontalAlignment {
    enum SubCenter: AlignmentID {
        static func defaultValue(in d: ViewDimensions) -> CGFloat {
            d[HorizontalAlignment.center]
        }
    }
    
    static let subCentre = HorizontalAlignment(SubCenter.self)
}

struct CentreSubviewOfHStack: View {
    var body: some View {
        //VStack Alignment set to the custom alignment
        VStack(alignment: .subCentre) {
            HStack{
                Text("View1")
                
                //Centre view aligned
                Text("Centre")
                .alignmentGuide(.subCentre) { d in d.width/2 }
                
                Text("View2")
                
                Text("View3")
            }
            
            //Geometry reader automatically fills the parent
            //this is aligned with the custom guide
            GeometryReader { geometry in
                EmptyView()
            }
            .alignmentGuide(.subCentre) { d in d.width/2 }
        }
    }
}

struct CentreSubviewOfHStack_Previews: PreviewProvider {
    static var previews: some View {
        CentreSubviewOfHStack()
            .previewLayout(CGSize.init(x: 250, y: 100))
    }
}

Edit: Note - this answer assumes that you can set a fixed height and width of the containing VStack. That stops the GeometryReader from 'pushing' too far out

In a different situation, I replaced the GeometryReader with a rectangle:

            //rectangle fills the width, then provides a centre for things to align to
            Rectangle()
                .frame(height:0)
                .frame(idealWidth:.infinity)
                .alignmentGuide(.colonCentre) { d in d.width/2 }

Note - this will still expand to maximum width unless constrained!