SwiftUI - Get size of child?

Updated and generalized @arsenius code. Now you can easily bind a parent view's state variable.

struct ChildSizeReader<Content: View>: View {
    @Binding var size: CGSize
    let content: () -> Content
    var body: some View {
        ZStack {
            content()
                .background(
                    GeometryReader { proxy in
                        Color.clear
                            .preference(key: SizePreferenceKey.self, value: proxy.size)
                    }
                )
        }
        .onPreferenceChange(SizePreferenceKey.self) { preferences in
            self.size = preferences
        }
    }
}

struct SizePreferenceKey: PreferenceKey {
    typealias Value = CGSize
    static var defaultValue: Value = .zero

    static func reduce(value _: inout Value, nextValue: () -> Value) {
        _ = nextValue()
    }
}

Usage:

struct ChildSizeReaderExample: View {
    @State var textSize: CGSize = .zero
    var body: some View {
        VStack {
            ChildSizeReader(size: $textSize) {
                Text("Hello I am some arbitrary text.")
            }
            Text("My size is \(textSize.debugDescription)!")
        }
    }
}

Basically, the answer at this point is to use a GeometryReader inside of the child's background(...) modifier.

// This won't be valid until the first layout pass is complete
@State var childSize: CGSize = .zero

var body: some View {
    ZStack {
        Text("Hello World!")
            .background(
                GeometryReader { proxy in
                    Color.clear
                       .preference(
                           key: SizePreferenceKey.self, 
                           value: proxy.size
                        )
                }
            )
      }
      .onPreferenceChange(SizePreferenceKey.self) { preferences in
          self.childSize = preferences
      }
}

struct SizePreferenceKey: PreferenceKey {
    typealias Value = CGSize
    static var defaultValue: Value = .zero

    static func reduce(value: inout Value, nextValue: () -> Value) {
        value = nextValue()
    }
}