/// <summary>
        /// Sets up the view model.
        /// </summary>
        /// <param name="sender">
        /// The source of the event; typically <see cref="Common.NavigationHelper"/>
        /// </param>
        /// <param name="e">Event data that provides both the navigation parameter passed to
        /// <see cref="Frame.Navigate(Type, Object)"/> when this page was initially requested and
        /// a dictionary of state preserved by this page during an earlier
        /// session. The state will be null the first time a page is visited.</param>
        private void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
            defaultViewModel = (TripListViewModel)this.DataContext;

            DefaultViewModel.SelectedTrip = null;

            Window.Current.CoreWindow.VisibilityChanged += CoreWindow_VisibilityChanged;
        }
        /// <summary>
        /// Populates the page with content passed during navigation. Any saved state is also
        /// provided when recreating a page from a prior session.
        /// Resets the state of the view model by passing in the parameters provided by the
        /// caller.
        /// </summary>
        /// <param name="sender">
        /// The source of the event; typically <see cref="Common.NavigationHelper"/>
        /// </param>
        /// <param name="e">Event data that provides both the navigation parameter passed to
        /// <see cref="Frame.Navigate(Type, Object)"/> when this page was initially requested and
        /// a dictionary of state preserved by this page during an earlier
        /// session. The state will be null the first time a page is visited.</param>
        private void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
            defaultViewModel = (TripViewModel)this.DataContext;
            if (e.NavigationParameter is Trip)
            {
                // Activated via selecting a trip in the main page's list of trips.
                DefaultViewModel.ShowTrip((Trip)e.NavigationParameter);
            }
            else if (e.NavigationParameter is TripVoiceCommand)
            {
                // look up destination, set trip.
                TripVoiceCommand voiceCommand = (TripVoiceCommand)e.NavigationParameter;
                DefaultViewModel.LoadTripFromStore(voiceCommand.destination);

                // artificially populate the page backstack so we have something to
                // go back to to get to the main page.
                PageStackEntry backEntry = new PageStackEntry(typeof(View.TripListView), null, null);
                this.Frame.BackStack.Add(backEntry);
            }
            else if(e.NavigationParameter is string)
            {
                // We've been URI Activated, possibly by a user clicking on a tile in a Cortana session,
                // we should see an argument like destination=<Location>. 
                // This should handle finding all of the destinations that match, but currently it only
                // finds the first one.
                string arguments = e.NavigationParameter as string;
                if(arguments != null)
                {
                    string[] args = arguments.Split('=');
                    if (args.Length == 2 && args[0].ToLowerInvariant() == "destination")
                    {
                        DefaultViewModel.LoadTripFromStore(args[1]);

                        // artificially populate the page backstack so we have something to
                        // go back to to get to the main page.
                        PageStackEntry backEntry = new PageStackEntry(typeof(View.TripListView), null, null);
                        this.Frame.BackStack.Add(backEntry);
                    }
                }
            }
            else
            {
                DefaultViewModel.NewTrip();
            }
        }