Integrating PageFactory in a Prism Project

Intro

The Page Factory concept would be useless if does not work with MVVM frameworks. Here we are doing a POC of how the Page Pactory can work as along with Prism. We chose Prism because it is one of the biggest and most known frameworks , and because we use it usually.

Steps for Integration

Overriding NavigationService

We had to implement our own version of PageNavigationService where we override GetCurrentPageand CreatePage to be able to integrate the PageFactory. Notice that PageFactory is injected into the service.

public class PageFactoryNavigationService:PageNavigationService
{
	private readonly IPageFactory _pageFactory;

	public PageFactoryNavigationService(IPageFactory pageFactory, IContainerExtension container, IApplicationProvider applicationProvider, IPageBehaviorFactory pageBehaviorFactory, ILoggerFacade logger) : base(container, applicationProvider, pageBehaviorFactory, logger)
	{
		_pageFactory = pageFactory;
	}
    protected override Page GetCurrentPage()
    {
        return _applicationProvider?.MainPage?.Navigation?.NavigationStack[0]; 
    }
    protected override Page CreatePage(string segment)
    {
        if (Type.GetType(segment) != null) //in this case it should get the fully qualified assembly name
        {
            var type = Type.GetType(segment);   
            var page = _pageFactory.CreatePage(type);
            return page;
        }
        else
        {
            return base.CreatePage(segment);
        }
    }
}

Modification on Page

We have modified the PageFactory class so it takes the designated viewmodel from the container, this way the container will resolve all the dependencies injected in the constructor .

  public class PageFactory : IPageFactory
    {
        private readonly IContainerExtension _containerProvider;

        public PageFactory(IContainerExtension containerProvider)
        {
            _containerProvider = containerProvider;
        }
        public  Page CreatePage<T> () where T: INotifyPropertyChanged
        {         
            return CreatePage(typeof(T)); 
        }
        
        public  Page CreatePage (Type type)
        {
            var viewModel = _containerProvider.Resolve(type); 
            return CreatePage((ViewModelBase)viewModel, type); 
        }
         public Page CreatePage(INotifyPropertyChanged viewModel, Type type)
        {
        //........ The rest of the class 
     }

Registrations on App.cs

Here we simply Register PageFactory, PageFactoryNavigationService, the viewmodels , and the other pages of the app. Note: We do not need to register the ViewModels which are fed to the PageFactory as they are resolved by the container
Note: We need to register PageFactoryNavigationService twice; once with a named index and once without to make sure that Prism will always refer to our implementation.

 public partial class App : PrismApplication
    {
    	// Other Implementations and overrides 
    	 protected override async void OnInitialized()
        {
            InitializeComponent();
            await NavigationService.NavigateAsync($"NavigationPage/FirstPage"); 
        }

        protected override void RegisterTypes(IContainerRegistry containerRegistry)
        {
            /*INavigationService has to be registered in both modes, named and unnamed.*/
            containerRegistry.Register<INavigationService, PageFactoryNavigationService>(NavigationServiceName);
            containerRegistry.Register<INavigationService, PageFactoryNavigationService>();

            containerRegistry.Register<IPageFactory, PageFactory>();       

            containerRegistry.RegisterForNavigation<NavigationPage>();
            containerRegistry.RegisterForNavigation<FirstPage, FirstPageViewModel>();
            containerRegistry.RegisterForNavigation<Page3>();
        }
    }

Modifications on ViewModels

No much of modification needed here except that when navigating to another Page you can decide whether you want to use original implementation for NavigationService or ours. To use our implementation you have to pass the AssemblyQualifiedName of the designated ViewModel to the NavigtateAsync method.

  private async void DoPrismNavigation()
        {
            await NavigationService.NavigateAsync(nameof(Page3)); 
        }

        private async void DoPageFactoryNavigation()
        {
            await NavigationService.NavigateAsync(typeof(ProducedPage2ViewModel).AssemblyQualifiedName);
        }