SizeReader.swift

public extension View {
    /**
     Read a view's size. The closure is called whenever the size changes.
     From https://stackoverflow.com/a/66822461/14351818
     */
    func sizeReader(size: @escaping (CGSize) -> Void) -> some View {
        return background(
            GeometryReader { geometry in
                Color.clear
                    .preference(key: ContentSizeReaderPreferenceKey.self, value: geometry.size)
                    .onPreferenceChange(ContentSizeReaderPreferenceKey.self) { newValue in
                        size(newValue)
                    }
            }
            .hidden()
        )
    }
}

public struct ContentSizeReaderPreferenceKey: PreferenceKey {
    public static var defaultValue: CGSize { return CGSize() }
    public static func reduce(value: inout CGSize, nextValue: () -> CGSize) { value = nextValue() }
}


/// Usage:

Text("Hi!")
    .sizeReader { size in
        print("Size: \(size)")
    }