prism null exception on Container.Resolve 2019-09-02 22:21发布 站内问答 / 移动开发 1293 3 4

Question

The line Resources.Add("eventAggregator", Container.Resolve()); raises Null exception.

UPDATE I've added all classes to explain more. As @Axemasta said, there is no need to register IEventAggregator and I removed registration. Now I don't how to connect the Listview EventAggregator behavior to the EventAggregator.

This is whole App.xaml code file.

public partial class App : PrismApplication
{
    /* 
     * The Xamarin Forms XAML Previewer in Visual Studio uses System.Activator.CreateInstance.
     * This imposes a limitation in which the App class must have a default constructor. 
     * App(IPlatformInitializer initializer = null) cannot be handled by the Activator.
     */
    public App() : this(null) { }


    public App(IPlatformInitializer initializer) : base(initializer) { }

    protected override async void OnInitialized()
    {
        InitializeComponent();

        Resources.Add("eventAggregator", Container.Resolve<IEventAggregator>());// Removed on update

        FlowListView.Init();

        await NavigationService.NavigateAsync("NavigationPage/MainPage");

    }

    protected override void RegisterTypes(IContainerRegistry containerRegistry)
    {            
        containerRegistry.RegisterForNavigation<NavigationPage>();
        containerRegistry.RegisterForNavigation<MainPage>();           
    }
}

}

The behavior class:

public class ScrollToMyModelBehavior : BehaviorBase<ListView>
{
    private IEventAggregator _eventAggregator;
    public IEventAggregator EventAggregator
    {
        get => _eventAggregator;
        set
        {
            if (!EqualityComparer<IEventAggregator>.Default.Equals(_eventAggregator, value))
            {
                _eventAggregator = value;
                _eventAggregator.GetEvent<ScrollToMyModelEvent>().Subscribe(OnScrollToEventPublished);
            }
        }
    }

    private void OnScrollToEventPublished(ListItem model)
    {
        AssociatedObject.ScrollTo(model, ScrollToPosition.Start, true);
    }

    protected override void OnDetachingFrom(ListView bindable)
    {
        base.OnDetachingFrom(bindable);
        // The Event Aggregator uses weak references so forgetting to do this
        // shouldn't create a problem, but it is a better practice.
        EventAggregator.GetEvent<ScrollToMyModelEvent>().Unsubscribe(OnScrollToEventPublished);
    }
}

The Event class:

public class ScrollToMyModelEvent : PubSubEvent<ListItem>
{
}

The page view model:

        public MainPageViewModel(INavigationService navigationService, IEventAggregator eventAggregator) 
        : base (navigationService)
    {
        Title = "صفحه اصلی";
        ListHeight = 100;
        ListWidth = 250;

        _eventAggregator = eventAggregator;

        Items items = new Items();
        ListViewItemSouce = items.GetItems();

        MyModels = items.GetItems();
        SelectedModel = ListViewItemSouce[3];
        _eventAggregator.GetEvent<ScrollToMyModelEvent>().Publish(SelectedModel);
    }

The page view:

<StackLayout HorizontalOptions="Center" VerticalOptions="Center" WidthRequest="{Binding ListWidth}" HeightRequest="{Binding ListHeight}"
                         Grid.Row="1" Grid.Column="1">
                <local:NativeListView x:Name="lst3" ItemsSource="{Binding ListViewItemSouce}" Margin="1" BackgroundColor="Transparent" RowHeight="47" HasUnevenRows="false">
                    <ListView.Behaviors>
                        <local:ScrollToMyModelBehavior EventAggregator="{StaticResource eventAggregator}" /> // Error raised that there is not such a static property
                    </ListView.Behaviors>
                    <ListView.ItemTemplate>
                        <DataTemplate>
                            <TextCell Text="{Binding Word}"  TextColor="Black"/>
                        </DataTemplate>
                    </ListView.ItemTemplate>
                </local:NativeListView>
            </StackLayout>

Answer 1

You do not need to register IEventAggregator when the app initialises, much like INavigationService or IPageDialog, you can use it straight out of the box!

To use EventAggregator you should do the following things:

Create an Event

You will first need to create an Event (using Prism) that you can pass to the EventAggregator. Your event should inherit from PubSubEvent, you can pass this an object (optional). So your event would look like this:

using System;
using Prism.Events;

namespace Company.App.Namespace.Events
{
    public class SampleEvent : PubSubEvent
    {
    }
}

Looking at a recent app, I most commonly use this when passing data between custom popup views (like a dictionary of params).

Subscribe to the Event

When IEventAggregator fires, anything that has subscribe to the event will execute any code specified. In the class you want to recieved the event you will have to do the following:

• Pass the class IEventAggregator through the constructor (prism does the DI afterall)
• Initialise a local IEventAggregator for use in this class
• Subscribe IEventAggregator to a handler method.

Here is what the code may look like:

public class TheClassListeningForAnEvent
{
    private readonly IEventAggregator _eventAggregator;

    public TheClassListeningForAnEvent(IEventAggregator eventAggregator)
    {
        _eventAggregator = eventAggregator;
        _eventAggregator.GetEvent<SampleEvent>().Subscribe(OnEventRecieved);
    }

    void OnEventRecieved()
    {
        //Do something here
    }
}

Fire the Event

Now you have registered for the event, you can fire the event. Pass the IEventAggregator into whatever class you want to fire the event from and use the Publish Method:

public class TheClassPublishingAnEvent
{
    private readonly IEventAggregator _eventAggregator;

    public TheClassListeningForAnEvent(IEventAggregator eventAggregator)
    {
        _eventAggregator = eventAggregator;

        _eventAggregator.GetEvent<SampleEvent>().Publish();
    }
}

Thats the long and the short of it. You could pass anything to the IEventAggregator, you would just need to handle for this in the methods you are subscribing.

Hopefully that is enough to get you going using IEventAggregator!

Answer 2

My guidance on this has evolved as features have been added to Prism to make this sort of thing even easier.

In the past the reason you would resolve and add the IEventAggregator as a StaticResource is that there was no way to inject this. We now have the ContainerProvider which amazingly allows you to add types in XAML that require Dependency Injection. To start you can refactor your ScrollToBehavior to use a DI Pattern by adding the IEventAggregator as a constructor parameter, removing the Bindable Property (if you choose).

public class ScrollToBehavior : BehaviorBase<ListView>
{
    private IEventAggregator _eventAggregator { get; }

    public ScrollToBehavior(IEventAggregator eventAggregator)
    {
        _eventAggregator = eventAggregator;
    }
}

As I mentioned you can use the ContainerProvider in XAML to resolve and provide a type that requires DI, as follows:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:ioc="clr-namespace:Prism.Ioc;assembly=Prism.Forms"
             xmlns:behavior="using:AwesomeProject.Behaviors
             x:Class="AwesomeProject.Views.ViewA">
  <ListView>
    <ListView.Behaviors>
      <ioc:ContainerProvider x:TypeArguments="behavior:ScrollToBehavior" />
    </ListView.Behaviors>
  </ListView>
</ContentPage>

Answer 3

Just verify if you are adding below code in IOS, Android and UWP projects.

IOS- Appdelegate

public class AppdelegateInitializer : IPlatformInitializer
    {
        public void RegisterTypes(IUnityContainer container)
        {

        }
    }

Android - MainActivity

public class MainActivityInitializer : IPlatformInitializer
    {
        public void RegisterTypes(IUnityContainer container)
        {

        }
    }

UWP-- MainPage.cs

public class UwpInitializer : IPlatformInitializer
    {
        public void RegisterTypes(IUnityContainer container)
        {

        }
    }