gistfile1.txt

Install LiveSharp extension: https://marketplace.visualstudio.com/items?itemName=ionoy.LiveSharp
Install LiveSharp NuGet package: https://www.nuget.org/packages/livesharp

1) add [assembly:LiveSharpInjectRuleBaseClass("Xamarin.Forms.ContentPage", "Xamarin.Forms.ContentView")] attribute to App.xaml.cs
2) add [LiveSharpStart] attribute to App constructor
3) add this code to the end of App constructor
 LiveSharpContext.AddUpdateHandler(ctx => {
    var instances = ctx.UpdatedMethods
        .SelectMany(method => method.Instances)
        .Distinct()
        .ToArray();

    Device.BeginInvokeOnMainThread(() => {
        foreach (var instance in instances) {
            try {
                var bindingFlags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static;
                if (instance is ContentPage page && instance.GetType().GetMethod("BuildContent", bindingFlags) is MethodInfo mi)
                    page.Content = (View)mi.Invoke(instance, null);
                else if (instance is ContentView view && instance.GetType().GetMethod("BuildContent", bindingFlags) is MethodInfo mi2)
                    view.Content = (View)mi2.Invoke(instance, null);
            } catch (TargetInvocationException e) {
                var inner = e.InnerException;
                while (inner is TargetInvocationException tie)
                    inner = tie.InnerException;
                if (inner != null)
                    throw inner;
                throw;
            }
        }
    });
});
4) Add public View BuildContent() method to your Page
5) Call Content = BuildContent(); from Page constructor
6) now you can define your UI in BuildContent method and update it in runtime