Wednesday, June 03, 2009

Issues with the Silverlight Navigation architecture

by StefanOlson via Stefan Olson's Blog on 6/3/2009 3:56:40 PM

In my last post, I described some of the ways I was able to use the Silverlight URI mapping architecture to provide more customized routing.

However, there are a few problems with the navigation/URI mapping architecture that I haven't been able to find ways to work around.

Setting a title

One of the major problems I've encountered is getting the title (in the browser's title bar) to be set. There is a field in the Page class called title. So you would think would be relatively simple. It appears that there's some difficulty in setting the page title, unless it's set at a particular time during the navigation. Here's how I try and set it:

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    CachedCategory category = ClientStaticCache.GetCategoryById(
                         int.Parse(NavigationContext.QueryString["categoryid"]));

    Title = category.Name + " - " + "my site";
}

But the title never appears -  the URI Appears as the title. I started a thread on the Silverlight forum, but no one has been able to give me an answer as to when you can set the title so that it appears correctly in the title bar and the navigation history of the browser.

When you're working in an asynchronous world you often won't get the title at the time the navigation starts, there needs to be a way to set the title when the data has come down the line.

Asynchronous operation

If I want to go and just find out if a particular URI exists (e.g: a category), before navigating to it, I either have to have a cache of categories on my local Silverlight client, which is what I'm doing at the moment, or you need to go to the server and verify that that is a valid category. The problem with the URI mapping architecture is that it is synchronous, so it is not possible to go and ask the server and then come back to the URI mapper and say yes, this is where I want to go.

Null return from the URI mapper causes exception

As described in my previous post, if you return null from the URI mapper because you've handled the URI by displaying a dialog or something like that, an exception is caused. However, the exception doesn't happen if you change the URI in the address bar, only if you call it via Frame.Navigate.

Bug on connect

Reusing a page

Another scenario occurs when we have we have content on a page which is very slow to load. In my case the Virtual Tour Viewer has a page that contains floor plans. These are very slow to load, and I don't want to have to reload the floor plans as the page changes, because some pages display the floor plans and others don't.

Ideally, the Frame system would be able to come to you and say here's a page I'm asking for, would you like to give me the page. In this case, I could reuse the pages containing the floor plan and increase the speed of the application simply by handling an event on the frame.  e.g:

Page Frame_GetPageForUri(object sender, GetPageForUriEventArgs e)
{
    if (e.Uri.ToString() == "categorycontrol.xaml")
    {
        return CategoryControl;
    }
    return null;
}

private Page _CategoryControl;

private Page CategoryControl
{
    get
    {
        if (_CategoryControl == null)
        {
            _CategoryControl = new CategoryControl();
        }
        return _CategoryControl;
    }
}

The way I am looking to do this currently is to store the floor plan control outside of the page so that it doesn't get destroyed if the garbage collector tries to clean up when the page isn't displayed, but that is not very satisfactory.

Role-based pages

Another useful piece of the navigation architecture would be the ability to use an attribute to declare the required roles to display a particular page. If the user does not have that role, then a login dialog would be displayed, in the same way as is done with the ASP.net authentication system. Under Silverlight, you could do this in the same way as is done on domain service you have a requires authentication attribute and a requires role attribute:

[RequiresAuthentication]
public partial class CategoryPage : Page

WPF compatibility

The virtual tour viewer needs to work on both WPF and Silverlight. I've encountered a number of problems trying to share the code between WPF and Silverlight with regard to the navigation framework.  WPF 4 currently doesn't have the API improvements that have been made to the Silverlight navigation framework such as the URI mapper and a number of virtual functions in the page class.  This means I have had to create a whole set of classes just to create a level of compatibility, part of which is completely impossible because of the lack of the URI mapper!

It's not clear at this stage of this will be fixed by Beta 2 of the .net framework 4.

Overall the Silverlight navigation API is pretty good,  Hopefully some minor tweaks will make it a huge amount better.