Assign the size of a SwiftUI View to a Binding - Based on Federico Zanetello's `readSize`

View+Size.swift

import SwiftUI

/*
Example

struct MyView: View {
    @State private var childSize = CGSize.zero
    
    var body: some View {
        ChildView()
            .assignSize(to: $childSize)
    }
}
*/

extension View {
    func assignSize(to binding: Binding<CGSize>) -> some View {
        readSize { size in
            binding.wrappedValue = size
        }
    }
}

// From Federico's original work
// Gist: https://github.com/FiveStarsBlog/CodeSamples/blob/main/SwiftUI-read-a-view-size/View+readSize.swift
// Explainer blog post: https://fivestars.blog/articles/swiftui-share-layout-information/

struct SizePreferenceKey: PreferenceKey {
    static var defaultValue = CGSize.zero
    static func reduce(value: inout CGSize, nextValue: () -> CGSize) {}
}

extension View {
    func readSize(onChange: @escaping (CGSize) -> Void) -> some View {
        background(
            GeometryReader { geometryProxy in
                Color.clear
                    .preference(key: SizePreferenceKey.self, value: geometryProxy.size)
            }
        )
        .onPreferenceChange(SizePreferenceKey.self, perform: onChange)
    }
}

/*
Example:
var body: some View {
  childView
    .readSize { newSize in
      print("The new child size is: \(newSize)")
    }
}
*/