Android Application Lifecycle

Question

I see a lot of answers concerning the Activity Lifecycle and that for most part is clear to me. What I'm looking for is the Application Lifecycle. I have the following case:

1. Application starts - Invalidate PIN
2. A PIN has to be entered if it is stale of invalid
3. Application is backgrounded (Android Home screen is visible) - Invalidate PIN
4. The applicatiion is foregrounded (App becomes visible again) - goto step 2

With the Activity Lifecycle it is hard if not impossible to achieve. Any suggestions?

Answer 1

Inspired by the solution Doomsknight pointed me to I constructed this solution without the Timer as proposed in the proposed answer. Here's the code from my mainapplication.cs:

    public void OnActivityPaused(Activity activity)
    {
        _lastActivity = DateTime.Now;
    }

    public void OnActivityResumed(Activity activity)
    {
        CrossCurrentActivity.Current.Activity = activity;

        DateTime now = DateTime.Now;
        TimeSpan span = now - _lastActivity;

        if (span.TotalMilliseconds > 2000)
        {
            Notifier.Classes.Settings.IsPinValid = false;
        }

        _lastActivity = now;
    }