Prism navigation from within a Service only working when not navigating back to much

Question

I have a Prism Xamarin.Forms App created with the Prism Template Pack (2.0.7). All packages are up to date:

Xamarin.Forms v2.5.0.91635
Xamarin.Android.* v26.1.0.1
Prism.DryIoc.Forms v7.0.0.168-pre (Could not update to v7.0.0.269-pre due to a NullReferenceException in Android Renderer on startup, however my main application is using 269-pre without that problem)

I hosted the sample Application on https://github.com/dernippel/PrismNavApp

I have the following components:

Module "Attachments" (a Prism Module)
Module "Viewer" (a Prism Module)
Service "AttachmentService" registered with the Container with an Interface as Singleton

They should do this:

The AttachmentsPage (from the AttachmentsModule) lists some objects
Selecting one Attachment
AttachmentPageViewModel calls the "OpenAttachment"-Method of the Attachment Service
The Method determines the correct ViewerPage by type and uses the Prism-NavigationService to navigate directly to the Page (in the sample this is the "ImageViewerPage" of the ViewerModule)

This is only working when you do the following navigation:

MainPage -> AttachmentsPage -> ViewerPage -> (back Arrow) AttachmentsPage -> ViewerPage (and so on)

But if you navigate back to MainPage navigation to ViewerPage isn't working anymore:

MainPage -> AttachmentsPage -> ViewerPage -> (back Arrow) AttachmentsPage -> (back Arrow) MainPage -> AttachmentsPage -> (nothing happens anymore when tap on Button to navigate to ViewerPage)

The AttachmentsService gets the NavigationService via Constructor injection and navigates this way:

public AttachmentService(INavigationService navigationService)
    {
        this.navigationService = navigationService;
    }

    public async void OpenAttachmentWithViewer(object attachment)
    {
        // ToDo: handle parameter proberbly
        var attachmentType = "image";

        // select correct viewer
        if (attachmentType == "image")
        {
            // navigate to image viewer
            var navParams = new NavigationParameters();
            navParams.Add("object",attachment);
            var navTask = this.navigationService.NavigateAsync(
                "ImageViewerPage",
                navParams,
                useModalNavigation: false);
            await navTask;

            var result = navTask.Status;
            Debug.WriteLine($"Navigation State is {result}");
        }
    }

I tried to check the navigation Task result status, it is always "RanToCompletion".

Modifying the AttachmentsPageViewModel to navigate directly with the Prism NavigationService instead using the Service doesn't cause this behavior:

private void OnOpenAttachment()
    {
        // ToDo: get the selected attachment
        object selectedAttachment = null;

        // navigating inside a service -- not working when navigating back to MainPage
        //this.attachmentService.OpenAttachmentWithViewer(selectedAttachment);

        // navigation from view model -- working
        var navParams = new NavigationParameters();
        navParams.Add("object", selectedAttachment);
        this.navigationService.NavigateAsync("ImageViewerPage", navParams, useModalNavigation: false);
    }

Hint: I switched with my Main-Application from PCL-based to the new .NETStandard based solution and already had a similar functionality working successfully using Prism v6.3.0.1. This functionality is not even navigating once since the port.

Actually I don't know how to solve this.

Is it possible to have a look into the Prism NavigationService to determine why the navigation is not happen?

I didn't find any known bug in the Prism Repository yet.

Unknown Unknown · Accepted Answer · 2017-11-28T15:45:36

You can't use the NavigationService in another service, especially if that service is a singleton. Navigation in Xamarin.Forms is specific to a Page, and only works in context of an associated page. Instead, your service should return a result and you should navigate from your VM based on that result. Don't try to navigate from within a service.

Prism navigation from within a Service only working when not navigating back to much

1 Answers