androindindicatorPage.xaml.cs

using System;
using Xamarin.Forms;

namespace AndroindIndicator
{
	public partial class AndroindIndicatorPage : ContentPage
	{
		void Off_Clicked(object sender, System.EventArgs e)
		{
			this.IsBusy = false;
		}

		void On_Clicked(object sender, System.EventArgs e)
		{
			this.IsBusy = true;
		}

		public AndroindIndicatorPage()
		{
			InitializeComponent();
			On.Clicked += On_Clicked;
			Off.Clicked += Off_Clicked;
			Next.Clicked += NextOnClicked;
		}

		private void NextOnClicked(object sender, EventArgs eventArgs)
		{
			MessagingCenter.Send(this, "Next");
		}
	}
}

App.xaml.cs

using Xamarin.Forms;

namespace AndroindIndicator
{
	public partial class App : Application
	{
		public App()
		{
			InitializeComponent();

			MainPage = new NavigationPage(new AndroindIndicatorPage(){ Title = "Page1"});

			MessagingCenter.Subscribe<AndroindIndicatorPage>(this, "Next", o =>
			{
				var nav = MainPage as NavigationPage;
				nav?.PushAsync(new AndroindIndicatorPage() {Title = $"Page{nav.Navigation.NavigationStack.Count}"});

			});
		}

		protected override void OnStart()
		{
			// Handle when your app starts
		}

		protected override void OnSleep()
		{
			// Handle when your app sleeps
		}

		protected override void OnResume()
		{
			// Handle when your app resumes
		}
	}
}

MyNavigationRenderer.cs

using System.ComponentModel;
using androindindicator.Droid;
using Android.Support.V7.Widget;
using Android.Views;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Xamarin.Forms.Platform.Android.AppCompat;
using AProgressBar = Android.Widget.ProgressBar;
using View = Android.Views.View;

[assembly: ExportRenderer(typeof(NavigationPage), typeof(MyNavigationRenderer))]

namespace AndroindIndicator.Droid
{
	public class MyNavigationRenderer : NavigationPageRenderer
	{
		private AProgressBar _progress;

		protected override void OnLayout(bool changed, int l, int t, int r, int b)
		{
			base.OnLayout(changed, l, t, r, b);
			for (var i = 0; i < ChildCount; ++i) {
				var view = GetChildAt(i);
				if (view is Toolbar) {
					_progress?.SetZ(view.GetZ() + 1);
					_progress?.Layout(l, view.Bottom - 10, r, view.Bottom + 10);
				}
			}
		}

		protected override void OnElementChanged(ElementChangedEventArgs<NavigationPage> e)
		{
			base.OnElementChanged(e);
			if (_progress == null) {
				_progress = new AProgressBar(Context, null, Android.Resource.Attribute.ProgressBarStyleHorizontal)
				{
					Indeterminate = true
				};
				AddView(_progress);
				_progress.Visibility = ViewStates.Invisible;
			}

			if (e.OldElement != null) {
				e.OldElement.Pushed -= NewElementOnPushed;
				e.NewElement.Popped -= NewElement_Popped;
			}

			if (e.NewElement != null) {
				e.NewElement.Pushed += NewElementOnPushed;
				e.NewElement.Popped += NewElement_Popped;
				foreach (var page in e.NewElement.Navigation.NavigationStack) page.PropertyChanged += CurrentPageOnPropertyChanged;
			}
		}

		private void CurrentPageOnPropertyChanged(object sender, PropertyChangedEventArgs e)
		{
			if (e.PropertyName == nameof(Element.IsBusy)) {
				var page = (Page) sender;
				Element.IsBusy = page.IsBusy;
			}
		}

		private void NewElement_Popped(object sender, NavigationEventArgs e)
		{
			e.Page.PropertyChanged -= CurrentPageOnPropertyChanged;
		}

		private void NewElementOnPushed(object sender, NavigationEventArgs e)
		{
			e.Page.PropertyChanged += CurrentPageOnPropertyChanged;
		}

		protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
		{
			base.OnElementPropertyChanged(sender, e);
			if (e.PropertyName == nameof(Element.IsBusy)) {
				var page = (NavigationPage) sender;
				_progress.Visibility = page.IsBusy ? ViewStates.Visible : ViewStates.Invisible;
			}
		}
	}
}