aurora-sharp-desktop/Aurora/Design/Views/MainView/MainView.xaml.cs

using System;
using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.Linq;
using Aurora.Design.Components.NavigationMenu;
using Aurora.Design.Views.MainView;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Aurora.Design.Views.Main
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class MainView : MasterDetailPage
    {
        private Dictionary<int, BaseViewModel> _viewModels;
        private BaseViewModel _lastViewModel;

        public MainView()
        {
            InitializeComponent();
            BindingContext = new MainViewModel();
            _viewModels = new Dictionary<int, BaseViewModel>();
            MasterPage.ListView.ItemSelected += ListView_ItemSelected;

            Appearing += OnAppearing;
        }
        ~MainView()
        {
            Appearing -= OnAppearing;
        }

        private void ListView_ItemSelected(object sender, SelectedItemChangedEventArgs e)
        {
            var item = e.SelectedItem as NavigationItem;
            if (item == null)
                return;

            var view = (View)Activator.CreateInstance(item.TargetType);

            //Check if we have an instantiated viewModel
            BaseViewModel vm = new BaseViewModel();
            if (_viewModels.ContainsKey(item.Id))
            {
                _viewModels.TryGetValue(item.Id, out vm);
            }
            else
            {
                if (item.TargetViewModelType.BaseType != typeof(BaseViewModel))
                {
                    throw new InvalidOperationException("TargetViewModel field must be of type BaseViewModel");
                }

                //Instantiate new view model
                vm = (BaseViewModel)Activator.CreateInstance(item.TargetViewModelType);
                _viewModels.Add(item.Id, vm);
            }
            //Activate viewmodel
            vm.OnActive();

            //Deactivate last viewModel
            _lastViewModel.OnInactive();

            //Assign viewModel
            _lastViewModel = vm;
            view.BindingContext = vm;

            ContentPresenter viewContent = (ContentPresenter)ContentPage.Content.FindByName("ViewContent");
            viewContent.Content = view;

            MasterPage.ListView.SelectedItem = null;
        }

        /// <summary>
        /// Event handler for page appearing.
        /// </summary>
        /// <param name="sender">The object that fired the event.</param>
        /// <param name="args">The event arguments</param>
        private void OnAppearing(object sender, EventArgs args)
        {
            //Set initial view from first item in list
            ObservableCollection<NavigationGroupItem> screenList = (ObservableCollection<NavigationGroupItem>)MasterPage.ListView.ItemsSource;

            //Assign viewModel
            NavigationItem firstNavItem = screenList.FirstOrDefault().FirstOrDefault();
            var view = (View)Activator.CreateInstance(firstNavItem.TargetType);

            BaseViewModel vm = new BaseViewModel();
            if (_viewModels.ContainsKey(firstNavItem.Id))
            {
                _viewModels.TryGetValue(firstNavItem.Id, out vm);
            }
            else
            {
                //Instantiate new view model
                vm = (BaseViewModel)Activator.CreateInstance(firstNavItem.TargetViewModelType);
                _viewModels.Add(firstNavItem.Id, vm);
            }

            view.BindingContext = vm;
            _lastViewModel = vm;
            vm.OnActive();

            ContentPresenter viewContent = (ContentPresenter)ContentPage.Content.FindByName("ViewContent");
            viewContent.Content = view;

            MasterPage.ListView.SelectedItem = screenList.FirstOrDefault();
        }
    }

}