/// <summary>
/// Raises the Closing event with a deferral to wait the event handlers to complete the execution.
/// </summary>
/// <returns></returns>
protected virtual Task OnClosingAsync()
{
if (!_closingInvoked)
{
_closingInvoked = true;
var e = new DeferralEventArgs();
Closing.RaiseEvent(this, e);
return(e.WaitForDeferralsAsync());
}
return(Task.FromResult <object>(null));
}