MailFocusedViewModel.cs

using System;
using System.Collections.ObjectModel;
using Prism;
using Prism.Commands;
using SampleApp.Common;
using SampleApp.Common.Models;
using SampleApp.Services;

namespace SampleApp.Modules.Mail.ViewModels;

public class MailFocusedViewModel : ViewModelBase, IActiveAware
{
  private bool _isActive;
  private IMailService _mailService;

  public MailFocusedViewModel(IMailService mailService)
  {
    _mailService = mailService;
    MailMessages = new ObservableCollection<MailMessage>(_mailService.Messages);
  }

  public event EventHandler? IsActiveChanged;

  /// <summary>Xamarin.Forms only, this never gets hit.</summary>
  public bool IsActive { get => _isActive; set => SetProperty(ref _isActive, value, RaiseIsActiveChanged);}

  public DelegateCommand CmdTabClicked => new(() =>
  {
    System.Diagnostics.Debug.WriteLine("Tab clicked?");
  });

  public ObservableCollection<MailMessage> MailMessages { get; private set; }

  protected virtual void RaiseIsActiveChanged()
  {
    IsActiveChanged?.Invoke(this, EventArgs.Empty);
  }
}