Btukfyl

I need to pass data from ViewModel 1 to ViewModel 2 using Prism.

TodoItem is my Model with the string:

public TodoItem _todotItem { get; set; }



private readonly INavigationService _navigationService;

I assigned it in the constructor:

public MainPageViewModel(INavigationService navigationService, TodoItem todotItem)

{

    _navigationService = navigationService;

    _todotItem = todotItem;

}

And this is the code I use to navigate to next page (including the parameter):

NavigationParameters navParams = new NavigationParameters();

navParams.Add("PassedValue", _todoItem.name);



_navigationService.NavigateAsync("SecondPage", navParams);

When I set the breakpoint on _todoItem.name it says null. The data is fetched before I click the listview. What am i missing?

Edit:

This is how I navigate (click from listview):

private EventItem _selectedEvent { get; set; }

public EventItem SelectedEvent

{

    get { return _selectedEvent; }  

    set

    {

        if (_selectedEvent != value)

        {

            if (Device.RuntimePlatform == Device.iOS)

            {

                _selectedEvent = null;

            }

            else

            {

                _selectedEvent = value;

            }

            NavigationParameters navParams = new NavigationParameters();



            navParams.Add("PassedValue", _todoItem.name);





            _navigationService.NavigateAsync("SecondPage", navParams);

        }

    }

}

I do use the proper ways to catch the parameter on the second ViewModel (However, I get null in the first ViewModel):

public void OnNavigatingTo(INavigationParameters parameters)

{

    if (parameters.ContainsKey("PassedValue"))

    {

        _todo = (string)parameters["PassedValue"];



        OnPropertyChanged("Todo");

    }



}

I use HTTPClient to fetch data.

In order to capture the parameters on the second page, you must use Prism.Navigation.INavigationAware. Please, check the code below, maybe will help you out.

./Views/ItemView.xaml

<?xml version="1.0" encoding="utf-8" ?>

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 

    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"

    xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms" 

    prism:ViewModelLocator.AutowireViewModel="True" 

    x:Class="Project.Views.ItemView"

    Title="{Binding Title}">



    <StackLayout>

        <!-- List View -->

        <ListView ItemsSource="{Binding Items}" 

                  CachingStrategy="RecycleElement" 

                  HorizontalOptions="FillAndExpand"

                  VerticalOptions="FillAndExpand" 

                  HasUnevenRows="True" 

                  SelectionMode="None">

            <ListView.Behaviors>

                <b:EventToCommandBehavior EventName="ItemTapped" 

                                          Command="{Binding ItemTappedCommand}" 

                                          EventArgsParameterPath="Item" />

            </ListView.Behaviors>

            <ListView.ItemTemplate>

                <DataTemplate>

                    <!-- Data Template Cell -->

                </DataTemplate>

            </ListView.ItemTemplate>

        </ListView>

    </StackLayout>



</ContentPage>

./ViewModels/ViewModelBase.cs

using Prism;

using Prism.Mvvm;

using Prism.Navigation;

using System;



namespace Project.ViewModels

{

    // INavigationAware provides a way for objects involved in navigation to be notified of navigation activities.

    public class ViewModelBase : BindableBase, INavigationAware, IDestructible

    {

        // ...



        protected INavigationService NavigationService { get; private set; }



        public ViewModelBase(INavigationService navigationService) => NavigationService = navigationService;



        /// <summary>

        /// Called when the implementer is being navigated away from.

        /// </summary>

        public virtual void OnNavigatedFrom(INavigationParameters parameters)

        {



        }



        /// <summary>

        /// Called when the implementer has been navigated to.

        /// </summary>

        public virtual void OnNavigatedTo(INavigationParameters parameters)

        {



        }



        // ...

    }

}

./ViewModels/ItemViewModel.cs

using Prism.Navigation;

using System;

using System.Collections.ObjectModel;

using System.Threading.Tasks;

using System.Windows.Input;

using Xamarin.Forms;



namespace Project.ViewModels

{

    public class ItemViewModel : ViewModelBase

    {

        // ...



        private ObservableCollection<ItemList> _items;

        public ObservableCollection<ItemList> Items

        {

            get => _items;

            set => SetProperty(ref _items, value);

        }



        public ICommand ItemTappedCommand => new AsyncCommand(ItemTappedCommandAsync);



        public ItemViewModel(INavigationService navigationService)

            : base(navigationService)

        {

            Items = new ObservableCollection<ItemList>();

            // Load the data

        }



        // ...



        private async Task ItemTappedCommandAsync(object item)

        {

            var navParams = new NavigationParameters

            {

                { "ItemSelected", (Item)item }

            };



            await NavigationService.NavigateAsync(nameof(ItemDetailView), navParams);

        }

    }

}

./ViewModels/ItemDetailViewModel.cs

using Prism.Navigation;

using System;

using System.Collections.ObjectModel;

using System.Threading.Tasks;

using System.Windows.Input;

using Xamarin.Forms;



namespace Project.ViewModels

{

    public class ItemDetailViewModel : ViewModelBase

    {

        // ...



        public override void OnNavigatingTo(INavigationParameters parameters)

        {

            // Capture the parameter

            System.Diagnostics.Debug.WriteLine(parameters);

        }



        // ...

    }

}