Notify Activity of changes in viewModel - xamarin.forms

Question

I try to exit 'lock task mode' in Xamarin Android app. Here is what I am trying to achieve:
User taps on label (view in Xamarin.Forms) -> it cause change in ViewModel's boolean property to true
MainActivity (Xamarin.Android) observe that property has changed to true -> it makes application exit 'lock task mode'
My viewModel is placed in Xamarin.Forms 'App.xaml' class so it is accessible in Forms and Android part.
How Can I notify my Activity that property has changed so it can exit locked mode? I know this is propably very poor workaround, I would love to hear any advices and tips to make it more professional.
Thank you in advance!
EDIT
So the point is that I have got ViewModel with boolean property exitLockMode which indicates if app should be in lock mode or not:
public class AdminViewModel : BaseViewModel
{
//Number of taps to touch at main banner in 'MainPage' to open Admin Window
private int _tapsRequiredToAdmin = 5;
//Number of tolerance in miliseconds between next taps
private int _toleranceInMs = 1000;
private bool _exitLockMode = false;
public int ToleranceInMs { get => _toleranceInMs; }
public int TapsRequiredToAdmin { get => _tapsRequiredToAdmin; }
public bool ExitLockMode
{
get => _exitLockMode;
set => _exitLockMode=value;
}
}
AdminViewModel is created in 'App.xaml' class:
public partial class App : Application
{
private static AdminViewModel _adminViewModel;
public App()
{
InitializeComponent();
MainPage = new NavigationPage(new MainPage());
}
public static AdminViewModel AdminViewModel
{
get
{
if(_adminViewModel == null )
_adminViewModel = new AdminViewModel();
return _adminViewModel;
}
}
protected override void OnStart() { }
protected override void OnSleep() { }
protected override void OnResume() { }
}
In my main view (Xamarin.Forms) I have got label where admin want to tap few times in order to exit lock mode:
private DateTime? LastTap = null;
private byte NumberOfTaps = 0;
AdminViewModel adminViewModel = App.AdminViewModel;
**********************************************
//This is method binded to Label in <TapGestureRecognizer Tapped="OnLabelTapped">
private async void OnLabelTapped(object sender, EventArgs e)
{
if (LastTap == null || (DateTime.Now - LastTap.Value).TotalMilliseconds < adminViewModel.ToleranceInMs)
{
if (NumberOfTaps == (adminViewModel.TapsRequiredToAdmin - 1))
{
NumberOfTaps = 0;
LastTap = null;
adminViewModel.ExitLockMode = true;
return;
}
else
{
NumberOfTaps++;
LastTap = DateTime.Now;
}
}
else
{
NumberOfTaps = 1;
LastTap = DateTime.Now;
}
}
Now I want to achieve that when I turn 'ExitLockMode' bool to true, it notify my 'MainActivity' (Xamarin.Android) to fire 'StopLockTask()' method. I know that in native Android it could be handled by observing bool property, but I don't know how to do it here.
I am newbie so it could be very messy, every help appreciated.

Answer 1

As Jason said, you can use messagecenter.The Xamarin.Forms MessagingCenter class implements the publish-subscribe pattern, allowing message-based communication between components that are inconvenient to link by object and type references.
This mechanism allows publishers and subscribers to communicate without having a reference to each other, helping to reduce dependencies between them.
You can follow this document and the sample in it https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/messaging-center