TBertuzzi

  services.AddTransient<IMainViewModel, MainViewModel>();
  services.AddTransient<ISomarViewModel, SomarViewModel>();
  services.AddTransient<MainPage>();
  services.AddTransient<SomarPage>();
  //Importante ser singleton
  services.AddSingleton<IEventAggregator, EventAggregator>();
  services.AddSingleton<App>();

TBertuzzi

using System;
using System.Threading.Tasks;
using MvvmHelpers;
using XamarinFormsEventAggregator.Events;

namespace XamarinFormsEventAggregator.ViewModels
{
    public class SomarViewModel : BaseViewModel
    {
        private readonly IEventAggregator _EventAggregator;

TBertuzzi

using System;
using System.Threading.Tasks;
using MvvmHelpers;
using MvvmHelpers.Commands;
using Xamarin.Forms;
using XamarinFormsEventAggregator.Events;

namespace XamarinFormsEventAggregator.ViewModels
{
    public class MainViewModel : BaseViewModel

TBertuzzi

using System;
namespace XamarinFormsEventAggregator.Events
{
    public class SomarMessage
    {
       public string Texto { get; set; }
    }
}

TBertuzzi

<?xml version="1.0" encoding="UTF-8" ?>
<ContentPage
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    x:Class="XamarinFormsEventAggregator.SomarPage">
    <ContentPage.Content>
        <StackLayout>
        <Frame BackgroundColor="#2196F3" Padding="24" CornerRadius="0">
            <Label Text="EventAggregator" HorizontalTextAlignment="Center" TextColor="White" FontSize="36"/>
        </Frame>

TBertuzzi

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             Padding="0,60,0,0"
             x:Class="XamarinFormsEventAggregator.MainPage">

    <StackLayout>
        <Frame BackgroundColor="#2196F3" Padding="24" CornerRadius="0">
            <Label Text="EventAggregator" HorizontalTextAlignment="Center" TextColor="White" FontSize="36"/>
        </Frame>

TBertuzzi

 private static EventAggregator instance;
        public static EventAggregator Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new EventAggregator();
                }
                return instance;

TBertuzzi

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace XamarinFormsEventAggregator.Events
{
    public class EventAggregator : IEventAggregator
    {

TBertuzzi

using System;
namespace XamarinFormsEventAggregator.Events
{
    public interface IEventAggregator
    {
        // Envia Mensagem e aguarda para ser processado na UI thread antes de retornar
        void SendMessage<T>(T message);


        // Publica a mensagem para processamento posterior na UI thread , retornando imediatamente.

TBertuzzi

public partial class AppDelegate : UIApplicationDelegate
	{
		NSObject _screenshotNotification = null;

		public override void OnActivated (UIApplication application)
		{
			 // Inicia a verificação
			    if (_screenshotNotification == null)
			    {
				_screenshotNotification = UIApplication.Notifications.ObserveUserDidTakeScreenshot((sender, args) =>