appBootstrapper.Router.Navigate.Execute(new ChildViewModel(HostScreen)).Subscribe();

ShowActivityIndicator = true;
appBootstrapper.Router.Navigate.Execute(new ChildViewModel(HostScreen)).Subscribe();
ShowActivityIndicator = false;

public void ShowChildView()
{
    ShowActivityIndicator = true;
    appBootstrapper.Router.Navigate.Execute(new ChildViewModel(HostScreen)).Subscribe();
    ChildViewModelShown = true;
}

public async void ViewOnAppearing()
{
    if (ChildViewModelShown) ShowActivityIndicator = false;
}

public async void OnAppearing()
{
    ViewModel.ViewOnAppearing();
}