NavigationService.cs

using Xamarin.Forms;
using System.Threading.Tasks;
using System;
using System.Collections.Generic;

namespace GreatQuotes.Services
{
    public class NavigationService
    {
        private static readonly Task TaskCompleted = Task.FromResult(0);
        readonly Dictionary<string, Func<Page>> registeredPages = new Dictionary<string, Func<Page>>();

       public void RegisterPage(string pageKey, Func<Page> creator)
       {
            if (string.IsNullOrEmpty(pageKey))
                throw new ArgumentNullException("pageKey");
           if (creator == null)
               throw new ArgumentNullException("creator");
           registeredPages.Add(pageKey, creator);
       }

       public void UnregisterPage(string pageKey)
       {
            if (string.IsNullOrEmpty(pageKey))
                throw new ArgumentNullException("pageKey");
            registeredPages.Remove(pageKey);
       }
        
        Page GetPageByKey(string pageKey)
        {
            if (string.IsNullOrEmpty(pageKey))
                throw new ArgumentNullException("key");

            Func<Page> creator;
            return registeredPages.TryGetValue(pageKey, out creator) ? creator.Invoke() : null;
        }

        public Task GotoPageAsync(string pageKey, object viewModel)
        {
            var page = GetPageByKey(pageKey);
            if (page == null)
                return TaskCompleted;

            if (viewModel != null)
                page.BindingContext = viewModel;

            return Application.Current.MainPage.Navigation.PushAsync(page);
        }

        public Task PopAsync()
        {
            // Ignore if nothing to pop. GotoPageAsync allows no page navigation cases for 
            // validation and or Master/Detail open/close
            return Application.Current.MainPage.Navigation.NavigationStack.Count == 1 
                ? TaskCompleted 
                : Application.Current.MainPage.Navigation.PopAsync();
        }

        public Task PushModalAsync(string pageKey, object viewModel = null)
        {
            var page = GetPageByKey(pageKey);
            if (page == null)
                throw new ArgumentException("Cannot navigate to unregistered page", "pageKey");

            if (viewModel != null)
                page.BindingContext = viewModel;

            return Application.Current.MainPage.Navigation.PushModalAsync(page);
        }

        public Task PopModalAsync()
        {
            return Application.Current.MainPage.Navigation.PopModalAsync();
        }
   }
}