struct AnimationObserverTest: View {
  @State private var offset = 0.0
  @State private var offsetSpan: ClosedRange<Double> = 0...1
  @State private var progressPercentage = 0.0
  @State private var isDone = false

   var body: some View {
     GeometryReader { geo in
       VStack {
         Text("Loading: \(progressPercentage)%")
         Rectangle()
           .foregroundColor(.blue)
           .frame(height: 50)
           .offset(x: offset)
           .animationObserver(for: offset) { progress in // HERE
             progressPercentage = 100 * abs(progress - offsetSpan.lowerBound) / (offsetSpan.upperBound - offsetSpan.lowerBound)
           } onComplete: {
             isDone = true
           }

         if isDone {
           Text("Done!")
         } else if progressPercentage >= 50 {
           Text("Woooooah, we're half way there...")
         }
         
         Button("Reload") {
           isDone = false
           offset = -geo.size.width
           offsetSpan = offset...0
           withAnimation(.easeIn(duration: 5)) {
             offset = 0
           }
         }
       }
     }
   }
}