A version of GeometryReader that behaves like a greedy ZStack with a default alignment of center.

GeometryReaderStack.swift

//
//  GeometryReaderStack.swift
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
//  of the Software, and to permit persons to whom the Software is furnished to do so,
//  subject to the following conditions:
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
//  INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
//  PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
//  COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
//  AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
//  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//
//  Copyright © 2020 Adam Fordyce. All rights reserved.
//

import SwiftUI

/**
 A version of GeometryReader that behaves like a greedy ZStack with a default alignment of center.
 */
public struct GeometryReaderStack<Content>: View where Content: View {
    
    public var content: (GeometryProxy) -> Content
    
    private let alignment: Alignment
    
    public init(alignment: Alignment = .center, @ViewBuilder content: @escaping (GeometryProxy) -> Content) {
        self.alignment = alignment
        self.content = content
    }
    
    public var body: some View {
        GeometryReader { (geo: GeometryProxy) in
            Color.clear
                .overlay(content(geo), alignment: alignment)
        }
    }
}

struct CenteredGeometryReader_Previews: PreviewProvider {
    struct CenteredGeometryReader_Harness: View {

        var body: some View {
            VStack {
                Group {
                    GeometryReader { (geo: GeometryProxy) in
                        VStack {
                            Text("In a GeometryReader,")
                            Text("I'm not centered")
                            Text("by default 😭")
                        }
                        .padding()
                        .background(Color.blue.opacity(0.5))
                    }
                    GeometryReaderStack { (geo: GeometryProxy) in
                        VStack {
                            Text("I'm centered by default")
                            Text("in a GeometryReaderStack! 🥳")
                        }
                        .padding()
                        .background(Color.blue.opacity(0.5))
                    }
                    GeometryReaderStack(alignment: .trailing) { (geo: GeometryProxy) in
                        VStack {
                            Text("I'm trailing edge aligned")
                            Text("in a GeometryReaderStack! 😎")
                        }
                        .padding()
                        .background(Color.blue.opacity(0.5))
                    }
                    GeometryReaderStack(alignment: .bottomTrailing) { (geo: GeometryProxy) in
                        VStack {
                            Text("I'm bottom trailing edge aligned")
                            Text("with content larger than the bounds of the frame").fixedSize()
                            Text("in a GeometryReaderStack! 🤔")
                        }
                        .padding()
                        .background(Color.blue.opacity(0.5))
                    }
                }
                .frame(350, 200)
                .border(Color.black)
            }
            .edgesIgnoringSafeArea(.all)
        }
    }

    static var previews: some View {
        CenteredGeometryReader_Harness()
            .padding(100)
            .previewSizeThatFits()
    }
}