WidthGetterDemo.swift

import SwiftUI

struct WidthPreferenceKey: PreferenceKey {
    typealias Value = CGFloat?

    static var defaultValue: CGFloat?

    static func reduce(value: inout CGFloat?, nextValue: () -> CGFloat?) {
        if let nextValue = nextValue(), value != nextValue {
            value = nextValue
        }
    }
}

struct WidthGetter: View {
    var body: some View {
        GeometryReader { geometry in
            Rectangle()
                .fill(Color.clear)
                .preference(
                    key: WidthPreferenceKey.self,
                    value: geometry.size.width
            )
        }
    }
}

// Usage

struct ContentView: View {
    @State var width: CGFloat?

    var body: some View {
        List {
            HStack {
                Rectangle()
                    .fill(Color.red)
                    .frame(width: 72, height: 72)
                Text("Measured width: \(self.width ?? 0)")
                Spacer()
            }
            .background(WidthGetter())
            .onPreferenceChange(WidthPreferenceKey.self) { value in
                self.width = value
            }
            .listRowInsets(EdgeInsets(top: 0, leading: 20, bottom: 0, trailing: 20))
        }
    }
}