Main.cs

namespace Main 
{

public partial class MainView : MasterDetailPage
    {
        public MainView()
        {
            InitializeComponent();
            Detail = new NavigationPage(new StartView())
            {
                BindingContext = new StartViewModel()
            };
            Master = new MainMenuView();
        }
    }
    
    public partial class StartView
    {
        public StartView()
        {
            InitializeComponent();
        }

        private void Button_OnClicked(object sender, EventArgs e)
        {
            var page = new ContentPage
            {
                Content = new StackLayout()
                {
                    Children =
                    {
                        new Label()
                        {
                            Text = "I am content page"
                        }
                    }
                },
                Title = "ContentPage"
            };
            Navigation.PushAsync(page, true);
        }
    }
    
    public interface IView 
    {
        string Title { get; }
        string Subtitle { get; }
    }
    
    public class StartViewModel : IView
    {

        public StartViewModel()
        {
            Title = "Start title";
            Subtitle = "Start sub title";
        }
    }
    }
    
    using System.ComponentModel;
using Clistr.Droid.Renderers;
using Clistr.Infrastructure;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Xamarin.Forms.Platform.Android.AppCompat.NavigationPageRenderer;
using Android.Support.V7.Widget.Toolbar;

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

namespace Main
{
    public class NavigationPageRenderer : NavigationRenderer
    {
        private Toolbar toolbar;

        protected override void OnElementChanged(ElementChangedEventArgs<NavigationPage> e)
        {
            base.OnElementChanged(e);
            SetCustomView(e.NewElement.CurrentPage.BindingContext as IView);
        }

        private void SetCustomView(IView view)
        {
            toolbar.Subtitle = view?.Subtitle ?? string.Empty;
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);
            if (e.PropertyName.Equals("CurrentPage"))
            {
                SetCustomView(((NavigationPage)sender).CurrentPage.BindingContext as IView);
            }
        }

        public override void OnViewAdded(Android.Views.View child)
        {
            base.OnViewAdded(child);
            if (child.GetType() == typeof(Toolbar))
            {
                toolbar = (Toolbar)child;
            }
        }
    }
}