SwiftUI Animation with random variable

I would like to create an animation that repeats forever and depends on a random variable. Something like the following:

import SwiftUI

struct ContentView: View {
    @State var resize = 100
    
    var repeatingAnimation : Animation {        
        return Animation.default.repeatForever()
    }
    
    var body: some View {
        VStack {
            Spacer()
            RoundedRectangle(cornerRadius: 10)
                .frame(width: 40, height: CGFloat(resize))
                .onAppear() {
                    withAnimation(self.repeatingAnimation) {
                        self.resize = Int.random(in: 100 ... 400)
                    }
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

However the random in line 17 is only executed on initial load/appear. I was hoping that "resize" would change every time the animation repeats, so that the height of the RoundedRectangle would fluctuate/change.

How can this be done?

Kind regards,

Anders Sejersbøl